From 2e4914d0c5839ae105def9fbcbec7e979100174c Mon Sep 17 00:00:00 2001
From: odersky <odersky@gmail.com>
Date: Sat, 1 Nov 2025 13:53:28 +0100
Subject: [PATCH 1/7] Show the owner of the conflicting set in level errors

---
 compiler/src/dotty/tools/dotc/cc/CaptureSet.scala    |  4 +++-
 tests/neg-custom-args/captures/boundary.check        |  2 +-
 .../captures/effect-swaps-explicit.check             |  2 +-
 tests/neg-custom-args/captures/effect-swaps.check    |  2 +-
 tests/neg-custom-args/captures/filevar.check         |  2 +-
 tests/neg-custom-args/captures/heal-tparam-cs.check  |  2 +-
 tests/neg-custom-args/captures/i15923.check          |  2 +-
 tests/neg-custom-args/captures/i15923a.check         |  2 +-
 tests/neg-custom-args/captures/i15923b.check         |  2 +-
 tests/neg-custom-args/captures/i21920.check          |  2 +-
 .../captures/lazylists-exceptions.check              |  2 +-
 .../neg-custom-args/captures/leaking-iterators.check |  2 +-
 tests/neg-custom-args/captures/real-try.check        |  6 +++---
 tests/neg-custom-args/captures/reference-cc.check    |  4 ++--
 tests/neg-custom-args/captures/sep-curried-par.check |  2 +-
 tests/neg-custom-args/captures/simple-using.check    |  2 +-
 tests/neg-custom-args/captures/try.check             |  4 ++--
 tests/neg-custom-args/captures/usingLogFile.check    |  8 ++++----
 tests/neg-custom-args/captures/vars.check            | 12 ++++++------
 19 files changed, 33 insertions(+), 31 deletions(-)

diff --git a/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala b/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala
index d3ad7004d55e..4026e6f086ed 100644
--- a/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala
+++ b/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala
@@ -1338,7 +1338,9 @@ object CaptureSet:
             val levelStr = elem match
               case ref: TermRef => i", defined in ${ref.symbol.maybeOwner}\n"
               case _ => " "
-            i"""${elem.showAsCapability}${levelStr}cannot be included in outer capture set $cs"""
+            val ownerStr =
+              if cs.owner.exists then s" which is owned by ${cs.owner}" else ""
+            i"""${elem.showAsCapability}${levelStr}cannot be included in outer capture set $cs$ownerStr"""
           else if !elem.tryClassifyAs(cs.classifier) then
             i"""${elem.showAsCapability} is not classified as ${cs.classifier}, therefore it
                 |cannot be included in capture set $cs of ${cs.classifier.name} elements"""
diff --git a/tests/neg-custom-args/captures/boundary.check b/tests/neg-custom-args/captures/boundary.check
index b64d1edef8a6..62255507f7bd 100644
--- a/tests/neg-custom-args/captures/boundary.check
+++ b/tests/neg-custom-args/captures/boundary.check
@@ -5,7 +5,7 @@
    |  Found:    scala.util.boundary.Label[Object^'s1]
    |  Required: scala.util.boundary.Label[Object^]^²
    |
-   |  Note that capability cap cannot be included in outer capture set 's1.
+   |  Note that capability cap cannot be included in outer capture set 's1 which is owned by val local.
    |
    |  where:    ^ and cap refer to the universal root capability
    |            ^²        refers to a fresh root capability classified as Control in the type of value local
diff --git a/tests/neg-custom-args/captures/effect-swaps-explicit.check b/tests/neg-custom-args/captures/effect-swaps-explicit.check
index d46f9d3bf749..82a5a239807d 100644
--- a/tests/neg-custom-args/captures/effect-swaps-explicit.check
+++ b/tests/neg-custom-args/captures/effect-swaps-explicit.check
@@ -22,7 +22,7 @@
    |Found:    (contextual$9: boundary.Label[Result[Future[T^'s2]^'s3, E^'s4]^'s5]^'s6) ?->{fr, async} Future[T^'s7]^{fr, contextual$9}
    |Required: (boundary.Label[Result[Future[T^'s8]^'s9, E^'s10]]^) ?=> Future[T^'s8]^'s9
    |
-   |Note that capability contextual$9 cannot be included in outer capture set 's9.
+   |Note that capability contextual$9 cannot be included in outer capture set 's9 which is owned by method fail4.
    |
    |where:    ?=> refers to a fresh root capability created in method fail4 when checking argument to parameter body of method make
    |          ^   refers to the universal root capability
diff --git a/tests/neg-custom-args/captures/effect-swaps.check b/tests/neg-custom-args/captures/effect-swaps.check
index 2148de702501..4820318a9fb8 100644
--- a/tests/neg-custom-args/captures/effect-swaps.check
+++ b/tests/neg-custom-args/captures/effect-swaps.check
@@ -22,7 +22,7 @@
    |Found:    (contextual$9: boundary.Label[Result[Future[T^'s2]^'s3, E^'s4]^'s5]^'s6) ?->{fr, async} Future[T^'s7]^{fr, contextual$9}
    |Required: (boundary.Label[Result[Future[T^'s8]^'s9, E^'s10]]^) ?=> Future[T^'s8]^'s9
    |
-   |Note that capability contextual$9 cannot be included in outer capture set 's9.
+   |Note that capability contextual$9 cannot be included in outer capture set 's9 which is owned by method fail4.
    |
    |where:    ?=> refers to a fresh root capability created in method fail4 when checking argument to parameter body of method make
    |          ^   refers to the universal root capability
diff --git a/tests/neg-custom-args/captures/filevar.check b/tests/neg-custom-args/captures/filevar.check
index 41d749727a04..08f035cee2d0 100644
--- a/tests/neg-custom-args/captures/filevar.check
+++ b/tests/neg-custom-args/captures/filevar.check
@@ -4,7 +4,7 @@
    |Found:    (l: scala.caps.Capability^) ?->'s1 File^'s2 ->'s3 Unit
    |Required: (l: scala.caps.Capability^) ?-> (f: File^{l}) => Unit
    |
-   |Note that capability l cannot be included in outer capture set 's4 of parameter f.
+   |Note that capability l cannot be included in outer capture set 's4 of parameter f which is owned by method $anonfun.
    |
    |where:    => refers to a root capability associated with the result type of (using l: scala.caps.Capability^): (f: File^{l}) => Unit
    |          ^  refers to the universal root capability
diff --git a/tests/neg-custom-args/captures/heal-tparam-cs.check b/tests/neg-custom-args/captures/heal-tparam-cs.check
index eb910f007614..1e12956e20c1 100644
--- a/tests/neg-custom-args/captures/heal-tparam-cs.check
+++ b/tests/neg-custom-args/captures/heal-tparam-cs.check
@@ -4,7 +4,7 @@
    |Found:    (c: Capp^'s1) ->'s2 () ->{c} Unit
    |Required: (c: Capp^) => () ->'s3 Unit
    |
-   |Note that capability c cannot be included in outer capture set 's3.
+   |Note that capability c cannot be included in outer capture set 's3 which is owned by val test1.
    |
    |where:    => refers to a fresh root capability created in value test1 when checking argument to parameter op of method localCap
    |          ^  refers to the universal root capability
diff --git a/tests/neg-custom-args/captures/i15923.check b/tests/neg-custom-args/captures/i15923.check
index 6ae4cb76f4ac..d8204d62f836 100644
--- a/tests/neg-custom-args/captures/i15923.check
+++ b/tests/neg-custom-args/captures/i15923.check
@@ -4,7 +4,7 @@
    |Found:    (lcap: scala.caps.Capability^) ?->'s1 Cap^'s2 ->'s3 Id[Cap^'s4]^'s5
    |Required: (lcap: scala.caps.Capability^) ?-> Cap^{lcap} => Id[Cap^'s6]^'s7
    |
-   |Note that capability cap cannot be included in outer capture set 's6.
+   |Note that capability cap cannot be included in outer capture set 's6 which is owned by val leak.
    |
    |where:    =>  refers to a root capability associated with the result type of (using lcap: scala.caps.Capability^): Cap^{lcap} => Id[Cap^'s6]^'s7
    |          ^   refers to the universal root capability
diff --git a/tests/neg-custom-args/captures/i15923a.check b/tests/neg-custom-args/captures/i15923a.check
index bc2495391625..b25395e17ae6 100644
--- a/tests/neg-custom-args/captures/i15923a.check
+++ b/tests/neg-custom-args/captures/i15923a.check
@@ -4,7 +4,7 @@
   |Found:    (lcap: Cap^'s1) ->'s2 () ->'s3 Id[Cap^'s4]^'s5
   |Required: (lcap: Cap^) => () =>² Id[Cap^'s6]^'s7
   |
-  |Note that capability cap cannot be included in outer capture set 's6.
+  |Note that capability cap cannot be included in outer capture set 's6 which is owned by val leak.
   |
   |where:    =>  refers to a fresh root capability created in value leak when checking argument to parameter op of method withCap
   |          =>² refers to a root capability associated with the result type of (lcap: Cap^): () =>² Id[Cap^'s6]^'s7
diff --git a/tests/neg-custom-args/captures/i15923b.check b/tests/neg-custom-args/captures/i15923b.check
index 5078b97a66db..4a47cd8bcc7c 100644
--- a/tests/neg-custom-args/captures/i15923b.check
+++ b/tests/neg-custom-args/captures/i15923b.check
@@ -4,7 +4,7 @@
   |Found:    (x$0: Cap^) -> Id[Cap^{x$0}]
   |Required: (lcap: Cap^) => Id[Cap^'s1]^'s2
   |
-  |Note that capability lcap cannot be included in outer capture set 's1.
+  |Note that capability lcap cannot be included in outer capture set 's1 which is owned by val leak.
   |
   |where:    => refers to a fresh root capability created in value leak when checking argument to parameter op of method withCap
   |          ^  refers to the universal root capability
diff --git a/tests/neg-custom-args/captures/i21920.check b/tests/neg-custom-args/captures/i21920.check
index c9b1aa276d93..aca23e9da213 100644
--- a/tests/neg-custom-args/captures/i21920.check
+++ b/tests/neg-custom-args/captures/i21920.check
@@ -4,7 +4,7 @@
    |Found:    (f: File^'s1) ->'s2 Cell[File^'s3]{val head: () ->'s4 IterableOnce[File^'s5]^'s6}^'s7
    |Required: File^ => Cell[File^'s8]{val head: () ->'s9 IterableOnce[File^'s10]^'s11}^'s12
    |
-   |Note that capability cap cannot be included in outer capture set 's13.
+   |Note that capability cap cannot be included in outer capture set 's13 which is owned by method $anonfun.
    |
    |where:    =>  refers to a fresh root capability created in value cell when checking argument to parameter f of method open
    |          ^   refers to the universal root capability
diff --git a/tests/neg-custom-args/captures/lazylists-exceptions.check b/tests/neg-custom-args/captures/lazylists-exceptions.check
index 0f31ab58cca6..d74e019c3677 100644
--- a/tests/neg-custom-args/captures/lazylists-exceptions.check
+++ b/tests/neg-custom-args/captures/lazylists-exceptions.check
@@ -5,7 +5,7 @@
    |    Required: LazyList[Int]^'s1
    |
    |    Note that capability canThrow$1, defined in method try$1
-   |    cannot be included in outer capture set 's1 of method problem.
+   |    cannot be included in outer capture set 's1 of method problem which is owned by method problem.
 38 |      if i > 9 then throw Ex1()
 39 |      i * i
 40 |    }
diff --git a/tests/neg-custom-args/captures/leaking-iterators.check b/tests/neg-custom-args/captures/leaking-iterators.check
index b3b8e36fef79..e23d7684cac0 100644
--- a/tests/neg-custom-args/captures/leaking-iterators.check
+++ b/tests/neg-custom-args/captures/leaking-iterators.check
@@ -4,7 +4,7 @@
    |Found:    (log: java.io.FileOutputStream^'s1) ->'s2 cctest.Iterator[Int]^{log}
    |Required: java.io.FileOutputStream^ => cctest.Iterator[Int]^'s3
    |
-   |Note that capability log cannot be included in outer capture set 's3.
+   |Note that capability log cannot be included in outer capture set 's3 which is owned by method test.
    |
    |where:    => refers to a fresh root capability created in method test when checking argument to parameter op of method usingLogFile
    |          ^  refers to the universal root capability
diff --git a/tests/neg-custom-args/captures/real-try.check b/tests/neg-custom-args/captures/real-try.check
index 684dcb7c982d..d58adb1c71e4 100644
--- a/tests/neg-custom-args/captures/real-try.check
+++ b/tests/neg-custom-args/captures/real-try.check
@@ -23,7 +23,7 @@
    |    Required: () ->'s1 Unit
    |
    |    Note that capability canThrow$2, defined in method try$2
-   |    cannot be included in outer capture set 's1 of value x.
+   |    cannot be included in outer capture set 's1 of value x which is owned by val x.
    |
    | longer explanation available when compiling with `-explain`
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/real-try.scala:27:4 --------------------------------------
@@ -33,7 +33,7 @@
    |    Required: () ->'s3 Cell[Unit]^'s4
    |
    |    Note that capability canThrow$3, defined in method try$3
-   |    cannot be included in outer capture set 's3 of value y.
+   |    cannot be included in outer capture set 's3 of value y which is owned by val y.
    |
    | longer explanation available when compiling with `-explain`
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/real-try.scala:33:8 --------------------------------------
@@ -43,6 +43,6 @@
    |    Required: Cell[() ->'s5 Unit]^'s6
    |
    |    Note that capability canThrow$4, defined in method try$4
-   |    cannot be included in outer capture set 's5 of value b.
+   |    cannot be included in outer capture set 's5 of value b which is owned by val b.
    |
    | longer explanation available when compiling with `-explain`
diff --git a/tests/neg-custom-args/captures/reference-cc.check b/tests/neg-custom-args/captures/reference-cc.check
index 476cb2a09ceb..108d40ea1e37 100644
--- a/tests/neg-custom-args/captures/reference-cc.check
+++ b/tests/neg-custom-args/captures/reference-cc.check
@@ -4,7 +4,7 @@
    |Found:    (file: java.io.FileOutputStream^'s1) ->'s2 () ->{file} Unit
    |Required: java.io.FileOutputStream^ => () ->'s3 Unit
    |
-   |Note that capability file cannot be included in outer capture set 's3.
+   |Note that capability file cannot be included in outer capture set 's3 which is owned by val later.
    |
    |where:    => refers to a fresh root capability created in value later when checking argument to parameter op of method usingLogFile
    |          ^  refers to the universal root capability
@@ -16,7 +16,7 @@
    |Found:    (f: java.io.FileOutputStream^'s4) ->'s5 LzyList[Int]^{f}
    |Required: java.io.FileOutputStream^ => LzyList[Int]^'s6
    |
-   |Note that capability f cannot be included in outer capture set 's6.
+   |Note that capability f cannot be included in outer capture set 's6 which is owned by val xs.
    |
    |where:    => refers to a fresh root capability created in value xs when checking argument to parameter op of method usingLogFile
    |          ^  refers to the universal root capability
diff --git a/tests/neg-custom-args/captures/sep-curried-par.check b/tests/neg-custom-args/captures/sep-curried-par.check
index 04c052f9c7cc..be6f0e493adf 100644
--- a/tests/neg-custom-args/captures/sep-curried-par.check
+++ b/tests/neg-custom-args/captures/sep-curried-par.check
@@ -5,7 +5,7 @@
    |                                Required: (() ->'s1 Unit) ->'s2 Unit
    |
    |                                Note that capability p1, defined in method $anonfun
-   |                                cannot be included in outer capture set 's2.
+   |                                cannot be included in outer capture set 's2 which is owned by val bar.
    |
    |                                where:    cap is the universal root capability
    |
diff --git a/tests/neg-custom-args/captures/simple-using.check b/tests/neg-custom-args/captures/simple-using.check
index 54aa8a464ae4..e8e2948d349d 100644
--- a/tests/neg-custom-args/captures/simple-using.check
+++ b/tests/neg-custom-args/captures/simple-using.check
@@ -4,7 +4,7 @@
   |Found:    (f: java.io.FileOutputStream^'s1) ->'s2 () ->{f} Unit
   |Required: java.io.FileOutputStream^ => () ->'s3 Unit
   |
-  |Note that capability f cannot be included in outer capture set 's3.
+  |Note that capability f cannot be included in outer capture set 's3 which is owned by method test.
   |
   |where:    => refers to a fresh root capability created in method test when checking argument to parameter op of method usingLogFile
   |          ^  refers to the universal root capability
diff --git a/tests/neg-custom-args/captures/try.check b/tests/neg-custom-args/captures/try.check
index aa0137e1a08a..bfa2cafce460 100644
--- a/tests/neg-custom-args/captures/try.check
+++ b/tests/neg-custom-args/captures/try.check
@@ -37,7 +37,7 @@
    |Found:    (x: CT[Exception]^) ->'s3 () ->{x} Int
    |Required: CT[Exception]^ => () ->'s4 Int
    |
-   |Note that capability x cannot be included in outer capture set 's4.
+   |Note that capability x cannot be included in outer capture set 's4 which is owned by val xx.
    |
    |where:    => refers to a fresh root capability created in value xx when checking argument to parameter op of method handle
    |          ^  refers to the universal root capability
@@ -52,7 +52,7 @@
    |Found:    (x: CT[Exception]^) ->'s5 () ->{x} Int
    |Required: CT[Exception]^ => () ->'s6 Int
    |
-   |Note that capability x cannot be included in outer capture set 's6.
+   |Note that capability x cannot be included in outer capture set 's6 which is owned by val global.
    |
    |where:    => refers to a fresh root capability created in value global when checking argument to parameter op of method handle
    |          ^  refers to the universal root capability
diff --git a/tests/neg-custom-args/captures/usingLogFile.check b/tests/neg-custom-args/captures/usingLogFile.check
index b4ef40afd587..71750865a9e7 100644
--- a/tests/neg-custom-args/captures/usingLogFile.check
+++ b/tests/neg-custom-args/captures/usingLogFile.check
@@ -4,7 +4,7 @@
    |Found:    (f: java.io.FileOutputStream^'s1) ->'s2 () ->{f} Unit
    |Required: java.io.FileOutputStream^ => () ->'s3 Unit
    |
-   |Note that capability f cannot be included in outer capture set 's3.
+   |Note that capability f cannot be included in outer capture set 's3 which is owned by val later.
    |
    |where:    => refers to a fresh root capability created in value later when checking argument to parameter op of method usingLogFile
    |          ^  refers to the universal root capability
@@ -16,7 +16,7 @@
    |Found:    (f: java.io.FileOutputStream^'s4) ->'s5 Test2.Cell[() ->{f} Unit]^'s6
    |Required: java.io.FileOutputStream^ => Test2.Cell[() ->'s7 Unit]^'s8
    |
-   |Note that capability f cannot be included in outer capture set 's7.
+   |Note that capability f cannot be included in outer capture set 's7 which is owned by val later2.
    |
    |where:    => refers to a fresh root capability created in value later2 when checking argument to parameter op of method usingLogFile
    |          ^  refers to the universal root capability
@@ -28,7 +28,7 @@
    |Found:    (f: java.io.OutputStream^'s9) ->'s10 Int ->{f} Unit
    |Required: java.io.OutputStream^ => Int ->'s11 Unit
    |
-   |Note that capability f cannot be included in outer capture set 's11.
+   |Note that capability f cannot be included in outer capture set 's11 which is owned by val later.
    |
    |where:    => refers to a fresh root capability created in value later when checking argument to parameter op of method usingFile
    |          ^  refers to the universal root capability
@@ -40,7 +40,7 @@
    |Found:    (_$1: java.io.OutputStream^'s12) ->'s13 () ->{_$1} Unit
    |Required: java.io.OutputStream^ => () ->'s14 Unit
    |
-   |Note that capability _$1 cannot be included in outer capture set 's14.
+   |Note that capability _$1 cannot be included in outer capture set 's14 which is owned by val later.
    |
    |where:    => refers to a fresh root capability created in value later when checking argument to parameter op of method usingFile
    |          ^  refers to the universal root capability
diff --git a/tests/neg-custom-args/captures/vars.check b/tests/neg-custom-args/captures/vars.check
index c992228ca26d..399869e4d96f 100644
--- a/tests/neg-custom-args/captures/vars.check
+++ b/tests/neg-custom-args/captures/vars.check
@@ -28,14 +28,14 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/vars.scala:36:10 -----------------------------------------
 36 |  local { cap3 => // error
    |          ^
-   |          Found:    (cap3: CC^) ->'s1 String ->{cap3} String
-   |          Required: CC^ -> String ->{cap3²} String
+   |        Found:    (cap3: CC^) ->'s1 String ->{cap3} String
+   |        Required: CC^ -> String ->{cap3²} String
    |
-   |          Note that capability cap3 cannot be included in outer capture set {cap3²}.
+   |        Note that capability cap3 cannot be included in outer capture set {cap3²} which is owned by method test.
    |
-   |          where:    ^     refers to the universal root capability
-   |                    cap3  is a reference to a value parameter
-   |                    cap3² is a parameter in an anonymous function in method test
+   |        where:    ^     refers to the universal root capability
+   |                  cap3  is a reference to a value parameter
+   |                  cap3² is a parameter in an anonymous function in method test
 37 |    def g(x: String): String = if cap3 == cap3 then "" else "a"
 38 |    g
    |

From 9744afe844fb90f21845a6a181b949b43453000a Mon Sep 17 00:00:00 2001
From: odersky <odersky@gmail.com>
Date: Sat, 1 Nov 2025 16:58:13 +0100
Subject: [PATCH 2/7] Refactoring: Replace Addenda by list of Notes

---
 .../src/dotty/tools/dotc/cc/Capability.scala  |  1 -
 .../src/dotty/tools/dotc/cc/CaptureSet.scala  |  1 -
 .../dotty/tools/dotc/cc/CheckCaptures.scala   | 45 ++++++++++---------
 .../dotty/tools/dotc/core/TypeComparer.scala  |  7 +--
 .../dotty/tools/dotc/reporting/messages.scala |  7 ++-
 .../dotty/tools/dotc/transform/Recheck.scala  |  6 +--
 .../tools/dotc/typer/ErrorReporting.scala     | 25 +++++------
 .../dotty/tools/dotc/typer/Implicits.scala    | 24 +++++-----
 .../src/dotty/tools/dotc/typer/Typer.scala    |  4 +-
 9 files changed, 56 insertions(+), 64 deletions(-)

diff --git a/compiler/src/dotty/tools/dotc/cc/Capability.scala b/compiler/src/dotty/tools/dotc/cc/Capability.scala
index 64ca777f1b76..a00f1a4151ac 100644
--- a/compiler/src/dotty/tools/dotc/cc/Capability.scala
+++ b/compiler/src/dotty/tools/dotc/cc/Capability.scala
@@ -5,7 +5,6 @@ package cc
 import core.*
 import Types.*, Symbols.*, Contexts.*, Decorators.*
 import util.{SimpleIdentitySet, EqHashMap}
-import typer.ErrorReporting.Addenda
 import util.common.alwaysTrue
 import scala.collection.mutable
 import CCState.*
diff --git a/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala b/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala
index 4026e6f086ed..9ec0961c2a3f 100644
--- a/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala
+++ b/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala
@@ -13,7 +13,6 @@ import reporting.trace
 import printing.{Showable, Printer}
 import printing.Texts.*
 import util.{SimpleIdentitySet, Property, EqHashMap}
-import typer.ErrorReporting.Addenda
 import scala.collection.{mutable, immutable}
 import TypeComparer.ErrorNote
 import CCState.*
diff --git a/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala b/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
index 061aabc03f44..a1d7870f15b7 100644
--- a/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
+++ b/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
@@ -14,7 +14,7 @@ import typer.ForceDegree
 import typer.Inferencing.isFullyDefined
 import typer.RefChecks.{checkAllOverrides, checkSelfAgainstParents, OverridingPairsChecker}
 import typer.Checking.{checkBounds, checkAppliedTypesIn}
-import typer.ErrorReporting.{Addenda, NothingToAdd, err}
+import typer.ErrorReporting.{Note, err}
 import typer.ProtoTypes.{LhsProto, WildcardSelectionProto, SelectionProto}
 import util.{SimpleIdentitySet, EqHashMap, EqHashSet, SrcPos, Property}
 import util.chaining.tap
@@ -399,7 +399,7 @@ class CheckCaptures extends Recheck, SymTransformer:
             case (fail: IncludeFailure) :: _ => fail.cs
             case _ => target
           def msg(provisional: Boolean) =
-            def toAdd: String = errorNotes(otherNotes).toAdd.mkString
+            def toAdd: String = errorNotes(otherNotes).map(_.render).mkString
             def descr: String =
               val d = realTarget.description
               if d.isEmpty then provenance else ""
@@ -1208,7 +1208,7 @@ class CheckCaptures extends Recheck, SymTransformer:
           // too annoying. This is a hole since a defualt getter's result type
           // might leak into a type variable.
 
-      def fail(tree: Tree, expected: Type, addenda: Addenda): Unit =
+      def fail(tree: Tree, expected: Type, notes: List[Note]): Unit =
         def maybeResult = if sym.is(Method) then " result" else ""
         report.error(
           em"""$sym needs an explicit$maybeResult type because the inferred type does not conform to
@@ -1218,7 +1218,7 @@ class CheckCaptures extends Recheck, SymTransformer:
               | Externally visible type: $expected""",
           tree.srcPos)
 
-      def addenda(expected: Type) = Addenda:
+      def addendum(expected: Type) = Note:
         def result = if tree.isInstanceOf[ValDef] then"" else " result"
         i"""
           |
@@ -1237,7 +1237,7 @@ class CheckCaptures extends Recheck, SymTransformer:
             val expected = tpt.tpe.dropAllRetains
             todoAtPostCheck += { () =>
               withCapAsRoot:
-                testAdapted(tp, expected, tree.rhs, addenda(expected))(fail)
+                testAdapted(tp, expected, tree.rhs, addendum(expected) :: Nil)(fail)
                 // The check that inferred <: expected is done after recheck so that it
                 // does not interfere with normal rechecking by constraining capture set variables.
             }
@@ -1444,34 +1444,34 @@ class CheckCaptures extends Recheck, SymTransformer:
 
     type BoxErrors = mutable.ListBuffer[Message] | Null
 
-    private def errorNotes(notes: List[TypeComparer.ErrorNote])(using Context): Addenda =
-      if notes.isEmpty then NothingToAdd
-      else new Addenda:
-        override def toAdd(using Context) = notes.map: note =>
+    private def errorNotes(notes: List[TypeComparer.ErrorNote])(using Context): List[Note] =
+      notes.map: note =>
+        Note:
           i"""
              |
              |Note that ${note.description}."""
 
     /** Addendas for error messages that show where we have under-approximated by
-     *  mapping a a capability in contravariant position to the empty set because
+     *  mapping of a capability in contravariant position to the empty set because
      *  the original result type of the map was not itself a capability.
      */
-    private def addApproxAddenda(using Context) =
-      new TypeAccumulator[Addenda]:
-        def apply(add: Addenda, t: Type) = t match
+    private def addApproxAddenda(using Context): TypeAccumulator[List[Note]] =
+      new TypeAccumulator:
+        def apply(notes: List[Note], t: Type) = t match
           case CapturingType(t, CaptureSet.EmptyWithProvenance(ref, mapped)) =>
             /* val (origCore, kind) = original match
               case tp @ AnnotatedType(parent, ann) if ann.hasSymbol(defn.ReachCapabilityAnnot) =>
                 (parent, " deep")
               case _ =>
                 (original, "")*/
-            add ++ Addenda:
+            Note:
                 i"""
                    |
                    |Note that a capability $ref in a capture set appearing in contravariant position
                    |was mapped to $mapped which is not a capability. Therefore, it was under-approximated to the empty set."""
+            :: notes
           case _ =>
-            foldOver(add, t)
+            foldOver(notes, t)
 
     /** Massage `actual` and `expected` types before checking conformance.
      *  Massaging is done by the methods following this one:
@@ -1480,8 +1480,8 @@ class CheckCaptures extends Recheck, SymTransformer:
      *  If the resulting types are not compatible, try again with an actual type
      *  where local capture roots are instantiated to root variables.
      */
-    override def checkConformsExpr(actual: Type, expected: Type, tree: Tree, addenda: Addenda)(using Context): Type =
-      try testAdapted(actual, expected, tree, addenda)(err.typeMismatch)
+    override def checkConformsExpr(actual: Type, expected: Type, tree: Tree, notes: List[Note])(using Context): Type =
+      try testAdapted(actual, expected, tree, notes: List[Note])(err.typeMismatch)
       catch case ex: AssertionError =>
         println(i"error while checking $tree: $actual against $expected")
         throw ex
@@ -1496,8 +1496,8 @@ class CheckCaptures extends Recheck, SymTransformer:
           case _ => NoType
       case _ => NoType
 
-    inline def testAdapted(actual: Type, expected: Type, tree: Tree, addenda: Addenda)
-        (fail: (Tree, Type, Addenda) => Unit)(using Context): Type =
+    inline def testAdapted(actual: Type, expected: Type, tree: Tree, notes: List[Note])
+        (fail: (Tree, Type, List[Note]) => Unit)(using Context): Type =
 
       var expected1 = alignDependentFunction(expected, actual.stripCapturing)
       val falseDeps = expected1 ne expected
@@ -1544,11 +1544,12 @@ class CheckCaptures extends Recheck, SymTransformer:
         }
 
       TypeComparer.compareResult(tryCurrentType || tryWidenNamed) match
-        case TypeComparer.CompareResult.Fail(notes) =>
+        case TypeComparer.CompareResult.Fail(errNotes) =>
           capt.println(i"conforms failed for ${tree}: $actual vs $expected")
           if falseDeps then expected1 = unalignFunction(expected1)
-          fail(tree.withType(actualBoxed), expected1,
-            addApproxAddenda(addenda ++ errorNotes(notes), expected1))
+          val toAdd0 = notes ++ errorNotes(errNotes)
+          val toAdd1 = addApproxAddenda(toAdd0, expected1)
+          fail(tree.withType(actualBoxed), expected1, toAdd1)
           actual
         case /*OK*/ _ =>
           if debugSuccesses then tree match
diff --git a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
index 1d1c497b2196..5b7f59cd2364 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
+++ b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
@@ -3327,7 +3327,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
    *  the same class as `note`.
    */
   def addErrorNote(note: ErrorNote): Unit =
-    if errorNotes.forall(_._2.kind != note.kind) then
+    if errorNotes.forall(_._2.getClass != note.getClass) then
       errorNotes = (recCount, note) :: errorNotes
       assert(maxErrorLevel <= recCount)
       maxErrorLevel = recCount
@@ -3357,11 +3357,6 @@ object TypeComparer {
 
   /** A base trait for data producing addenda to error messages */
   trait ErrorNote:
-    /** A disciminating kind. An error note is not added if it has the same kind
-     *  as an already existing error note.
-     */
-    def kind: Class[?] = getClass
-
     def description(using Context): String
   end ErrorNote
 
diff --git a/compiler/src/dotty/tools/dotc/reporting/messages.scala b/compiler/src/dotty/tools/dotc/reporting/messages.scala
index e86538eb8110..4ca9c18aa627 100644
--- a/compiler/src/dotty/tools/dotc/reporting/messages.scala
+++ b/compiler/src/dotty/tools/dotc/reporting/messages.scala
@@ -18,7 +18,7 @@ import ast.desugar
 import config.{Feature, MigrationVersion, ScalaVersion}
 import transform.patmat.Space
 import transform.patmat.SpaceEngine
-import typer.ErrorReporting.{err, matchReductionAddendum, substitutableTypeSymbolsInScope, Addenda, NothingToAdd}
+import typer.ErrorReporting.{err, matchReductionAddendum, substitutableTypeSymbolsInScope, Note}
 import typer.ProtoTypes.{ViewProto, SelectionProto, FunProto}
 import typer.Implicits.*
 import typer.Inferencing
@@ -298,7 +298,7 @@ extends NotFoundMsg(MissingIdentID) {
   }
 }
 
-class TypeMismatch(val found: Type, expected: Type, val inTree: Option[untpd.Tree], addenda: Addenda = NothingToAdd)(using Context)
+class TypeMismatch(val found: Type, expected: Type, val inTree: Option[untpd.Tree], notes: List[Note] = Nil)(using Context)
   extends TypeMismatchMsg(found, expected)(TypeMismatchID):
 
   private val shouldSuggestNN =
@@ -359,8 +359,7 @@ class TypeMismatch(val found: Type, expected: Type, val inTree: Option[untpd.Tre
     def importSuggestions =
       if expected.isTopType || found.isBottomType then ""
       else ctx.typer.importSuggestionAddendum(ViewProto(found.widen, expected))
-
-    addenda.toAdd.mkString ++ super.msgPostscript ++ importSuggestions
+    notes.map(_.render).mkString ++ super.msgPostscript ++ importSuggestions
 
   override def explain(using Context) =
     val treeStr = inTree.map(x => s"\nTree:\n\n${x.show}\n").getOrElse("")
diff --git a/compiler/src/dotty/tools/dotc/transform/Recheck.scala b/compiler/src/dotty/tools/dotc/transform/Recheck.scala
index 34e3773ba147..81cfdba22b2f 100644
--- a/compiler/src/dotty/tools/dotc/transform/Recheck.scala
+++ b/compiler/src/dotty/tools/dotc/transform/Recheck.scala
@@ -15,7 +15,7 @@ import typer.ErrorReporting.err
 import typer.ProtoTypes.{AnySelectionProto, LhsProto}
 import typer.TypeAssigner.seqLitType
 import typer.ConstFold
-import typer.ErrorReporting.{Addenda, NothingToAdd}
+import typer.ErrorReporting.Note
 import config.Printers.recheckr
 import util.Property
 import StdNames.nme
@@ -633,11 +633,11 @@ abstract class Recheck extends Phase, SymTransformer:
       println(i"fail while $actual iscompat $expected")
       throw ex
 
-    def checkConformsExpr(actual: Type, expected: Type, tree: Tree, addenda: Addenda = NothingToAdd)(using Context): Type =
+    def checkConformsExpr(actual: Type, expected: Type, tree: Tree, notes: List[Note] = Nil)(using Context): Type =
       //println(i"check conforms $actual <:< $expected")
       if !isCompatible(actual, expected) then
         recheckr.println(i"conforms failed for ${tree}: $actual vs $expected")
-        err.typeMismatch(tree.withType(actual), expected, addenda)
+        err.typeMismatch(tree.withType(actual), expected, notes)
       actual
 
     def checkUnit(unit: CompilationUnit)(using Context): Unit =
diff --git a/compiler/src/dotty/tools/dotc/typer/ErrorReporting.scala b/compiler/src/dotty/tools/dotc/typer/ErrorReporting.scala
index c3393d39ab05..8b96b2f6eb4c 100644
--- a/compiler/src/dotty/tools/dotc/typer/ErrorReporting.scala
+++ b/compiler/src/dotty/tools/dotc/typer/ErrorReporting.scala
@@ -71,18 +71,13 @@ object ErrorReporting {
           case _ => foldOver(s, tp)
     tps.foldLeft("")(collectMatchTrace)
 
-  /** A mixin trait that can produce added elements for an error message */
-  trait Addenda:
-    def toAdd(using Context): List[String]
-    def ++(follow: Addenda) = new Addenda:
-      def toAdd(using Context) = Addenda.this.toAdd ++ follow.toAdd
+  /** A note can produce an added string for an error message */
+  abstract class Note(val prefix: Boolean = false):
+    def render(using Context): String
 
-  object Addenda:
-    def apply(msg: Context ?=> String): Addenda = new Addenda:
-      def toAdd(using Context) = msg :: Nil
-
-  object NothingToAdd extends Addenda:
-    def toAdd(using Context): List[String] = Nil
+  object Note:
+    def apply(msg: Context ?=> String, prefix: Boolean = false) = new Note(prefix):
+      def render(using Context) = msg
 
   class Errors(using Context) {
 
@@ -180,7 +175,7 @@ object ErrorReporting {
 
     def patternConstrStr(tree: Tree): String = ???
 
-    def typeMismatch(tree: Tree, pt: Type, addenda: Addenda = NothingToAdd): Tree = {
+    def typeMismatch(tree: Tree, pt: Type, notes: List[Note] = Nil): Tree = {
       val normTp = normalize(tree.tpe, pt)
       val normPt = normalize(pt, pt)
 
@@ -199,11 +194,11 @@ object ErrorReporting {
 
       def missingElse = tree match
         case If(_, _, elsep @ Literal(Constant(()))) if elsep.span.isSynthetic =>
-          Addenda("\nMaybe you are missing an else part for the conditional?")
+          Note("\nMaybe you are missing an else part for the conditional?") :: Nil
         case _ =>
-          NothingToAdd
+          Nil
 
-      errorTree(tree, TypeMismatch(treeTp, expectedTp, Some(tree), addenda ++ missingElse))
+      errorTree(tree, TypeMismatch(treeTp, expectedTp, Some(tree), notes ++ missingElse))
     }
 
     /** A subtype log explaining why `found` does not conform to `expected` */
diff --git a/compiler/src/dotty/tools/dotc/typer/Implicits.scala b/compiler/src/dotty/tools/dotc/typer/Implicits.scala
index f0569d850cb1..0a151d99cf1a 100644
--- a/compiler/src/dotty/tools/dotc/typer/Implicits.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Implicits.scala
@@ -467,7 +467,7 @@ object Implicits:
     }
   }
 
-  abstract class SearchFailureType extends ErrorType, Addenda {
+  abstract class SearchFailureType extends ErrorType {
     def expectedType: Type
     def argument: Tree
 
@@ -485,7 +485,7 @@ object Implicits:
         else i"convert from ${argument.tpe} to ${clarify(expectedType)}"
     }
 
-    def toAdd(using Context) = Nil
+    def notes(using Context): List[Note] = Nil
   }
 
   class NoMatchingImplicits(val expectedType: Type, val argument: Tree, constraint: Constraint = OrderingConstraint.empty)
@@ -540,10 +540,12 @@ object Implicits:
   /** A failure value indicating that an implicit search for a conversion was not tried */
   case class TooUnspecific(target: Type) extends NoMatchingImplicits(NoType, EmptyTree, OrderingConstraint.empty):
 
-    override def toAdd(using Context) =
-      i"""
-         |Note that implicit conversions were not tried because the result of an implicit conversion
-         |must be more specific than $target""" :: Nil
+    override def notes(using Context) =
+      Note:
+        i"""
+          |Note that implicit conversions were not tried because the result of an implicit conversion
+          |must be more specific than $target"""
+      :: Nil
 
     override def msg(using Context) =
       super.msg.append(i"\nThe expected type $target is not specific enough, so no search was attempted")
@@ -567,14 +569,16 @@ object Implicits:
         str2 = alt2.ref.showRef
       em"both $str1 and $str2 $qualify".withoutDisambiguation()
 
-    override def toAdd(using Context) =
+    override def notes(using Context) =
       if !argument.isEmpty && argument.tpe.widen.isRef(defn.NothingClass) then
         Nil
       else
         val what = if (expectedType.isInstanceOf[SelectionProto]) "extension methods" else "conversions"
-        i"""
-           |Note that implicit $what cannot be applied because they are ambiguous;
-           |$explanation""" :: Nil
+        Note:
+          i"""
+             |Note that implicit $what cannot be applied because they are ambiguous;
+             |$explanation"""
+        :: Nil
 
     def asNested = if nested then this else AmbiguousImplicits(alt1, alt2, expectedType, argument, nested = true)
   end AmbiguousImplicits
diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
index fc71c0e43034..3294c1d58ce6 100644
--- a/compiler/src/dotty/tools/dotc/typer/Typer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Typer.scala
@@ -4218,7 +4218,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
                 then
                   tryExtensionOrConversion(tree, pt, mbrProto, qual, locked, compat, inSelect)
                 else
-                  err.typeMismatch(qual, selProto, failure.reason) // TODO: report NotAMember instead, but need to be aware of failure
+                  err.typeMismatch(qual, selProto, failure.reason.notes) // TODO: report NotAMember instead, but need to be aware of failure
             rememberSearchFailure(qual, failure)
       catch case ex: TypeError => nestedFailure(ex)
 
@@ -4889,7 +4889,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         else
           val tree1 = healAdapt(tree, pt)
           if tree1 ne tree then readapt(tree1)
-          else err.typeMismatch(tree, pt, failure)
+          else err.typeMismatch(tree, pt, failure.notes)
 
       pt match
         case _: SelectionProto =>

From ec74abb736d1da3c1da06f9ef88b35cfd89aa19e Mon Sep 17 00:00:00 2001
From: odersky <odersky@gmail.com>
Date: Sat, 1 Nov 2025 20:04:00 +0100
Subject: [PATCH 3/7] Refactoring: Merge TypeComparer.ErrorNote and
 ErrorReporting.Note

The merged class is reporting.Message.Note.
---
 .../src/dotty/tools/dotc/cc/CaptureSet.scala  | 33 ++++++++++++-------
 .../dotty/tools/dotc/cc/CheckCaptures.scala   | 16 +++------
 .../dotty/tools/dotc/core/TypeComparer.scala  | 20 +++++------
 .../dotty/tools/dotc/reporting/Message.scala  |  8 +++++
 .../dotty/tools/dotc/reporting/messages.scala |  4 +--
 .../dotty/tools/dotc/transform/Recheck.scala  |  2 +-
 .../tools/dotc/typer/ErrorReporting.scala     | 10 +-----
 .../dotty/tools/dotc/typer/Implicits.scala    |  1 +
 .../captures/heal-tparam-cs.check             |  2 +-
 tests/neg-custom-args/captures/i16226.check   |  2 +-
 10 files changed, 49 insertions(+), 49 deletions(-)

diff --git a/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala b/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala
index 9ec0961c2a3f..14048213e733 100644
--- a/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala
+++ b/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala
@@ -10,11 +10,11 @@ import annotation.threadUnsafe
 import annotation.constructorOnly
 import annotation.internal.sharable
 import reporting.trace
+import reporting.Message.Note
 import printing.{Showable, Printer}
 import printing.Texts.*
 import util.{SimpleIdentitySet, Property, EqHashMap}
 import scala.collection.{mutable, immutable}
-import TypeComparer.ErrorNote
 import CCState.*
 import TypeOps.AvoidMap
 import compiletime.uninitialized
@@ -160,8 +160,8 @@ sealed abstract class CaptureSet extends Showable:
 
   final def keepAlways: Boolean = this.isInstanceOf[EmptyWithProvenance]
 
-  def failWith(fail: TypeComparer.ErrorNote)(using Context): false =
-    TypeComparer.addErrorNote(fail)
+  def failWith(note: Note)(using Context): false =
+    TypeComparer.addErrorNote(note)
     false
 
   /** Try to include an element in this capture set.
@@ -1304,16 +1304,18 @@ object CaptureSet:
    *  when a subsumes check decides that an existential variable `ex` cannot be
    *  instantiated to the other capability `other`.
    */
-  case class ExistentialSubsumesFailure(val ex: ResultCap, val other: Capability) extends ErrorNote:
-    def description(using Context): String =
+  case class ExistentialSubsumesFailure(val ex: ResultCap, val other: Capability) extends Note:
+    def render(using Context): String =
       def reason =
         if other.isTerminalCapability then ""
         else " since that capability is not a `Sharable` capability"
-      i"""the existential capture root in ${ex.originalBinder.resType}
+      i"""
+         |
+         |Note that the existential capture root in ${ex.originalBinder.resType}
          |cannot subsume the capability $other$reason."""
 
   /** Failure indicating that `elem` cannot be included in `cs` */
-  case class IncludeFailure(cs: CaptureSet, elem: Capability, levelError: Boolean = false) extends ErrorNote, Showable:
+  case class IncludeFailure(cs: CaptureSet, elem: Capability, levelError: Boolean = false) extends Note, Showable:
     private var myTrace: List[CaptureSet] = cs :: Nil
 
     def trace: List[CaptureSet] = myTrace
@@ -1322,7 +1324,12 @@ object CaptureSet:
       res.myTrace = cs1 :: this.myTrace
       res
 
-    def description(using Context): String =
+    def render(using Context) =
+      i"""
+         |
+         |Note that $description."""
+
+    private def description(using Context): String =
       def why =
         val reasons = cs.elems.toList.collect:
           case c: FreshCap if !c.acceptsLevelOf(elem) =>
@@ -1371,11 +1378,13 @@ object CaptureSet:
    *  @param  lo    the lower type of the orginal type comparison, or NoType if not known
    *  @param  hi    the upper type of the orginal type comparison, or NoType if not known
    */
-  case class MutAdaptFailure(cs: CaptureSet, lo: Type = NoType, hi: Type = NoType) extends ErrorNote:
-    def description(using Context): String =
+  case class MutAdaptFailure(cs: CaptureSet, lo: Type = NoType, hi: Type = NoType) extends Note:
+    def render(using Context): String =
       def ofType(tp: Type) = if tp.exists then i"of the mutable type $tp" else "of a mutable type"
-      i"""$cs is an exclusive capture set ${ofType(hi)},
-          |it cannot subsume a read-only capture set ${ofType(lo)}"""
+      i"""
+         |
+         |Note that $cs is an exclusive capture set ${ofType(hi)},
+         |it cannot subsume a read-only capture set ${ofType(lo)}."""
 
   /** A VarState serves as a snapshot mechanism that can undo
    *  additions of elements or super sets if an operation fails
diff --git a/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala b/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
index a1d7870f15b7..b5b8803a4015 100644
--- a/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
+++ b/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
@@ -14,7 +14,7 @@ import typer.ForceDegree
 import typer.Inferencing.isFullyDefined
 import typer.RefChecks.{checkAllOverrides, checkSelfAgainstParents, OverridingPairsChecker}
 import typer.Checking.{checkBounds, checkAppliedTypesIn}
-import typer.ErrorReporting.{Note, err}
+import typer.ErrorReporting.err
 import typer.ProtoTypes.{LhsProto, WildcardSelectionProto, SelectionProto}
 import util.{SimpleIdentitySet, EqHashMap, EqHashSet, SrcPos, Property}
 import util.chaining.tap
@@ -26,6 +26,7 @@ import CCState.*
 import StdNames.nme
 import NameKinds.{DefaultGetterName, WildcardParamName, UniqueNameKind}
 import reporting.{trace, Message, OverrideError}
+import reporting.Message.Note
 import Annotations.Annotation
 import Capabilities.*
 import Mutability.*
@@ -399,7 +400,7 @@ class CheckCaptures extends Recheck, SymTransformer:
             case (fail: IncludeFailure) :: _ => fail.cs
             case _ => target
           def msg(provisional: Boolean) =
-            def toAdd: String = errorNotes(otherNotes).map(_.render).mkString
+            def toAdd: String = otherNotes.map(_.render).mkString
             def descr: String =
               val d = realTarget.description
               if d.isEmpty then provenance else ""
@@ -1444,13 +1445,6 @@ class CheckCaptures extends Recheck, SymTransformer:
 
     type BoxErrors = mutable.ListBuffer[Message] | Null
 
-    private def errorNotes(notes: List[TypeComparer.ErrorNote])(using Context): List[Note] =
-      notes.map: note =>
-        Note:
-          i"""
-             |
-             |Note that ${note.description}."""
-
     /** Addendas for error messages that show where we have under-approximated by
      *  mapping of a capability in contravariant position to the empty set because
      *  the original result type of the map was not itself a capability.
@@ -1544,10 +1538,10 @@ class CheckCaptures extends Recheck, SymTransformer:
         }
 
       TypeComparer.compareResult(tryCurrentType || tryWidenNamed) match
-        case TypeComparer.CompareResult.Fail(errNotes) =>
+        case TypeComparer.CompareResult.Fail(cmpNotes) =>
           capt.println(i"conforms failed for ${tree}: $actual vs $expected")
           if falseDeps then expected1 = unalignFunction(expected1)
-          val toAdd0 = notes ++ errorNotes(errNotes)
+          val toAdd0 = notes ++ cmpNotes
           val toAdd1 = addApproxAddenda(toAdd0, expected1)
           fail(tree.withType(actualBoxed), expected1, toAdd1)
           actual
diff --git a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
index 5b7f59cd2364..e5292968b249 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
+++ b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
@@ -26,6 +26,7 @@ import cc.*
 import Capabilities.Capability
 import NameKinds.WildcardParamName
 import MatchTypes.isConcrete
+import reporting.Message.Note
 import scala.util.boundary, boundary.break
 
 /** Provides methods to compare types.
@@ -61,7 +62,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
   private var monitored = false
 
   private var maxErrorLevel: Int = -1
-  protected var errorNotes: List[(Int, ErrorNote)] = Nil
+  protected var errorNotes: List[(Int, Note)] = Nil
 
   val undoLog = mutable.ArrayBuffer[() => Unit]()
 
@@ -3323,10 +3324,10 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
   def reduceMatchWith[T](op: MatchReducer => T)(using Context): T =
     inSubComparer(matchReducer)(op)
 
-  /** Add given ErrorNote note, provided there is not yet an error note with
+  /** Add given note, provided there is not yet an error note with
    *  the same class as `note`.
    */
-  def addErrorNote(note: ErrorNote): Unit =
+  def addErrorNote(note: Note): Unit =
     if errorNotes.forall(_._2.getClass != note.getClass) then
       errorNotes = (recCount, note) :: errorNotes
       assert(maxErrorLevel <= recCount)
@@ -3355,15 +3356,10 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
 
 object TypeComparer {
 
-  /** A base trait for data producing addenda to error messages */
-  trait ErrorNote:
-    def description(using Context): String
-  end ErrorNote
-
   /** A richer compare result, returned by `testSubType` and `test`. */
   enum CompareResult:
     case OK, OKwithGADTUsed, OKwithOpaquesUsed
-    case Fail(errorNotes: List[ErrorNote])
+    case Fail(errorNotes: List[Note])
 
   /** Class for unification variables used in `natValue`. */
   private class AnyConstantType extends UncachedGroundType with ValueType {
@@ -3541,10 +3537,10 @@ object TypeComparer {
   def inNestedLevel(op: => Boolean)(using Context): Boolean =
     currentComparer.inNestedLevel(op)
 
-  def addErrorNote(note: ErrorNote)(using Context): Unit =
+  def addErrorNote(note: Note)(using Context): Unit =
     currentComparer.addErrorNote(note)
 
-  def updateErrorNotes(f: PartialFunction[ErrorNote, ErrorNote])(using Context): Unit =
+  def updateErrorNotes(f: PartialFunction[Note, Note])(using Context): Unit =
     currentComparer.errorNotes = currentComparer.errorNotes.mapConserve: p =>
       val (level, note) = p
       if f.isDefinedAt(note) then (level, f(note)) else p
@@ -3958,7 +3954,7 @@ class ExplainingTypeComparer(initctx: Context, short: Boolean) extends TypeCompa
   private val b = new StringBuilder
   private var lastForwardGoal: String | Null = null
 
-  private def appendFailure(notes: List[ErrorNote]) =
+  private def appendFailure(notes: List[Note]) =
     if lastForwardGoal != null then  // last was deepest goal that failed
       b.append(s"  = false")
       for case note: printing.Showable <- notes do
diff --git a/compiler/src/dotty/tools/dotc/reporting/Message.scala b/compiler/src/dotty/tools/dotc/reporting/Message.scala
index 32c9bf91f919..47f405636a2b 100644
--- a/compiler/src/dotty/tools/dotc/reporting/Message.scala
+++ b/compiler/src/dotty/tools/dotc/reporting/Message.scala
@@ -41,6 +41,14 @@ object Message:
       i"\n$what can be rewritten automatically under -rewrite $optionStr."
     else ""
 
+  /** A note can produce an added string for an error message */
+  abstract class Note(val prefix: Boolean = false):
+    def render(using Context): String
+
+  object Note:
+    def apply(msg: Context ?=> String, prefix: Boolean = false) = new Note(prefix):
+      def render(using Context) = msg
+
   enum Disambiguation:
     case All
     case AllExcept(strs: List[String])
diff --git a/compiler/src/dotty/tools/dotc/reporting/messages.scala b/compiler/src/dotty/tools/dotc/reporting/messages.scala
index 4ca9c18aa627..f79857b020a3 100644
--- a/compiler/src/dotty/tools/dotc/reporting/messages.scala
+++ b/compiler/src/dotty/tools/dotc/reporting/messages.scala
@@ -18,7 +18,7 @@ import ast.desugar
 import config.{Feature, MigrationVersion, ScalaVersion}
 import transform.patmat.Space
 import transform.patmat.SpaceEngine
-import typer.ErrorReporting.{err, matchReductionAddendum, substitutableTypeSymbolsInScope, Note}
+import typer.ErrorReporting.{err, matchReductionAddendum, substitutableTypeSymbolsInScope}
 import typer.ProtoTypes.{ViewProto, SelectionProto, FunProto}
 import typer.Implicits.*
 import typer.Inferencing
@@ -38,7 +38,7 @@ import scala.jdk.CollectionConverters.*
 import dotty.tools.dotc.util.SourceFile
 import dotty.tools.dotc.config.SourceVersion
 import DidYouMean.*
-import Message.Disambiguation
+import Message.{Disambiguation, Note}
 
 /**  Messages
   *  ========
diff --git a/compiler/src/dotty/tools/dotc/transform/Recheck.scala b/compiler/src/dotty/tools/dotc/transform/Recheck.scala
index 81cfdba22b2f..63b9525a88f4 100644
--- a/compiler/src/dotty/tools/dotc/transform/Recheck.scala
+++ b/compiler/src/dotty/tools/dotc/transform/Recheck.scala
@@ -15,7 +15,7 @@ import typer.ErrorReporting.err
 import typer.ProtoTypes.{AnySelectionProto, LhsProto}
 import typer.TypeAssigner.seqLitType
 import typer.ConstFold
-import typer.ErrorReporting.Note
+import reporting.Message.Note
 import config.Printers.recheckr
 import util.Property
 import StdNames.nme
diff --git a/compiler/src/dotty/tools/dotc/typer/ErrorReporting.scala b/compiler/src/dotty/tools/dotc/typer/ErrorReporting.scala
index 8b96b2f6eb4c..76e55cda279f 100644
--- a/compiler/src/dotty/tools/dotc/typer/ErrorReporting.scala
+++ b/compiler/src/dotty/tools/dotc/typer/ErrorReporting.scala
@@ -12,9 +12,9 @@ import util.Spans.NoSpan
 import util.SrcPos
 import config.Feature
 import reporting.*
+import Message.Note
 import collection.mutable
 
-
 object ErrorReporting {
 
   import tpd.*
@@ -71,14 +71,6 @@ object ErrorReporting {
           case _ => foldOver(s, tp)
     tps.foldLeft("")(collectMatchTrace)
 
-  /** A note can produce an added string for an error message */
-  abstract class Note(val prefix: Boolean = false):
-    def render(using Context): String
-
-  object Note:
-    def apply(msg: Context ?=> String, prefix: Boolean = false) = new Note(prefix):
-      def render(using Context) = msg
-
   class Errors(using Context) {
 
     /** An explanatory note to be added to error messages
diff --git a/compiler/src/dotty/tools/dotc/typer/Implicits.scala b/compiler/src/dotty/tools/dotc/typer/Implicits.scala
index 0a151d99cf1a..d3e3a0d06bd8 100644
--- a/compiler/src/dotty/tools/dotc/typer/Implicits.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Implicits.scala
@@ -31,6 +31,7 @@ import Feature.{migrateTo3, sourceVersion}
 import config.Printers.{implicits, implicitsDetailed}
 import collection.mutable
 import reporting.*
+import Message.Note
 import transform.Splicer
 import annotation.tailrec
 
diff --git a/tests/neg-custom-args/captures/heal-tparam-cs.check b/tests/neg-custom-args/captures/heal-tparam-cs.check
index 1e12956e20c1..9ceeee3d7164 100644
--- a/tests/neg-custom-args/captures/heal-tparam-cs.check
+++ b/tests/neg-custom-args/captures/heal-tparam-cs.check
@@ -20,7 +20,7 @@
    |    Note that capability x$0 is not included in capture set {cap}.
    |
    |    Note that the existential capture root in () =>² Unit
-   |    cannot subsume the capability (x$0 : Capp^'s4) since that capability is not a `Sharable` capability..
+   |    cannot subsume the capability (x$0 : Capp^'s4) since that capability is not a `Sharable` capability.
    |
    |    where:    =>        refers to a root capability associated with the result type of (c: Capp^): () => Unit
    |              =>² and ^ refer to the universal root capability
diff --git a/tests/neg-custom-args/captures/i16226.check b/tests/neg-custom-args/captures/i16226.check
index d48897126600..479219de5100 100644
--- a/tests/neg-custom-args/captures/i16226.check
+++ b/tests/neg-custom-args/captures/i16226.check
@@ -25,7 +25,7 @@
    |Note that capability f1 is not included in capture set {cap}.
    |
    |Note that the existential capture root in LazyRef[B]^²
-   |cannot subsume the capability (f1 : A^'s15 ->'s16 B^'s17) since that capability is not a `Sharable` capability..
+   |cannot subsume the capability (f1 : A^'s15 ->'s16 B^'s17) since that capability is not a `Sharable` capability.
    |
    |where:    => and cap refer to a root capability associated with the result type of (ref1: LazyRef[A^'s11]{val elem: () ->'s12 A^'s13}^'s14, f1: A^'s15 ->'s16 B^'s17):
    |  LazyRef[B^'s19]{val elem: () => B^'s20}^{f1, ref1}

From e5706502adefb73d92cc1b970c5e118abc09a403 Mon Sep 17 00:00:00 2001
From: odersky <odersky@gmail.com>
Date: Sun, 2 Nov 2025 11:45:00 +0100
Subject: [PATCH 4/7] Show level errors before type mismatches

---
 .../src/dotty/tools/dotc/cc/Capability.scala  |  2 +-
 .../src/dotty/tools/dotc/cc/CaptureOps.scala  |  9 +++-
 .../src/dotty/tools/dotc/cc/CaptureSet.scala  | 49 +++++++++++++------
 .../dotty/tools/dotc/cc/CheckCaptures.scala   |  3 --
 .../dotty/tools/dotc/reporting/Message.scala  | 11 ++++-
 .../dotty/tools/dotc/reporting/messages.scala |  6 +--
 tests/neg-custom-args/captures/boundary.check |  5 +-
 .../captures/effect-swaps-explicit.check      |  7 +--
 .../captures/effect-swaps.check               |  7 +--
 tests/neg-custom-args/captures/filevar.check  |  7 +--
 .../captures/heal-tparam-cs.check             |  7 +--
 tests/neg-custom-args/captures/i15923.check   | 11 +++--
 tests/neg-custom-args/captures/i15923a.check  | 11 +++--
 tests/neg-custom-args/captures/i15923b.check  |  5 +-
 tests/neg-custom-args/captures/i21920.check   |  9 ++--
 tests/neg-custom-args/captures/i23431.check   |  4 +-
 .../captures/lazylists-exceptions.check       |  7 +--
 .../captures/leaking-iterators.check          |  7 +--
 tests/neg-custom-args/captures/reaches.check  |  2 +-
 tests/neg-custom-args/captures/real-try.check | 23 +++++----
 .../captures/reference-cc.check               | 16 +++---
 .../captures/scoped-caps.check                |  2 +-
 .../captures/scoped-caps2.check               |  2 +-
 .../captures/sep-curried-par.check            |  9 ++--
 .../captures/simple-using.check               |  7 +--
 tests/neg-custom-args/captures/try.check      | 14 +++---
 .../captures/usingLogFile.check               | 28 ++++++-----
 tests/neg-custom-args/captures/vars.check     | 13 ++---
 28 files changed, 168 insertions(+), 115 deletions(-)

diff --git a/compiler/src/dotty/tools/dotc/cc/Capability.scala b/compiler/src/dotty/tools/dotc/cc/Capability.scala
index a00f1a4151ac..fb6a8cff96e8 100644
--- a/compiler/src/dotty/tools/dotc/cc/Capability.scala
+++ b/compiler/src/dotty/tools/dotc/cc/Capability.scala
@@ -899,7 +899,7 @@ object Capabilities:
           case _ => c1
 
     def showAsCapability(using Context) =
-      i"capability ${ctx.printer.toTextCapability(this).show}"
+      i"${ctx.printer.toTextCapability(this).show}"
 
     def toText(printer: Printer): Text = printer.toTextCapability(this)
   end Capability
diff --git a/compiler/src/dotty/tools/dotc/cc/CaptureOps.scala b/compiler/src/dotty/tools/dotc/cc/CaptureOps.scala
index 125569c16033..c4659ea9fa96 100644
--- a/compiler/src/dotty/tools/dotc/cc/CaptureOps.scala
+++ b/compiler/src/dotty/tools/dotc/cc/CaptureOps.scala
@@ -15,6 +15,7 @@ import CaptureSet.VarState
 import Capabilities.*
 import StdNames.nme
 import config.Feature
+import dotty.tools.dotc.core.NameKinds.TryOwnerName
 
 /** Attachment key for capturing type trees */
 private val Captures: Key[CaptureSet] = Key()
@@ -624,9 +625,13 @@ extension (sym: Symbol)
     || sym.info.hasAnnotation(defn.ConsumeAnnot)
 
   def qualString(prefix: String)(using Context): String =
+    if !sym.exists then "" else i" $prefix ${sym.ownerString}"
+
+  def ownerString(using Context): String =
     if !sym.exists then ""
-    else if sym.isAnonymousFunction then i" $prefix enclosing function"
-    else i" $prefix $sym"
+    else if sym.isAnonymousFunction then i"an enclosing function"
+    else if sym.name.is(TryOwnerName) then i"an enclosing try expression"
+    else sym.show
 
 extension (tp: AnnotatedType)
   /** Is this a boxed capturing type? */
diff --git a/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala b/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala
index 14048213e733..b5c13cedfe80 100644
--- a/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala
+++ b/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala
@@ -1324,12 +1324,25 @@ object CaptureSet:
       res.myTrace = cs1 :: this.myTrace
       res
 
-    def render(using Context) =
+    override def prefix(using Context) = cs match
+      case cs: Var =>
+        !cs.levelOK(elem)
+        || cs.isBadRoot(elem) && elem.isInstanceOf[FreshCap]
+      case _ =>
+        false
+
+    def trailing(msg: String)(using Context): String =
       i"""
          |
-         |Note that $description."""
+         |Note that $msg."""
 
-    private def description(using Context): String =
+    def leading(msg: String)(using Context): String =
+      i"""$msg.
+         |The leakage occurred when trying to match the following types:
+         |
+         |"""
+
+    def render(using Context): String =
       def why =
         val reasons = cs.elems.toList.collect:
           case c: FreshCap if !c.acceptsLevelOf(elem) =>
@@ -1340,27 +1353,33 @@ object CaptureSet:
         else reasons.mkString("\nbecause ", "\nand ", "")
       cs match
         case cs: Var =>
+          def ownerStr =
+            if !cs.description.isEmpty then "" else cs.owner.qualString("which is owned by")
           if !cs.levelOK(elem) then
-            val levelStr = elem match
-              case ref: TermRef => i", defined in ${ref.symbol.maybeOwner}\n"
-              case _ => " "
-            val ownerStr =
-              if cs.owner.exists then s" which is owned by ${cs.owner}" else ""
-            i"""${elem.showAsCapability}${levelStr}cannot be included in outer capture set $cs$ownerStr"""
+            val outlivesStr = elem match
+              case ref: TermRef => i"${ref.symbol.maybeOwner.qualString("defined in")} outlives its scope:\n"
+              case _ => " outlives its scope: "
+            leading:
+              i"""Capability ${elem.showAsCapability}${outlivesStr}it leaks into outer capture set $cs$ownerStr"""
           else if !elem.tryClassifyAs(cs.classifier) then
-            i"""${elem.showAsCapability} is not classified as ${cs.classifier}, therefore it
+            trailing:
+              i"""capability ${elem.showAsCapability} is not classified as ${cs.classifier}, therefore it
                 |cannot be included in capture set $cs of ${cs.classifier.name} elements"""
           else if cs.isBadRoot(elem) then
             elem match
               case elem: FreshCap =>
-                i"""local ${elem.showAsCapability} created in ${elem.ccOwner}
-                  |cannot be included in outer capture set $cs"""
+                leading:
+                  i"""Local capability ${elem.showAsCapability} created in ${elem.ccOwner} outlives its scope:
+                     |It leaks into outer capture set $cs$ownerStr"""
               case _ =>
-                i"universal ${elem.showAsCapability} cannot be included in capture set $cs"
+                trailing:
+                  i"universal capability ${elem.showAsCapability} cannot be included in capture set $cs"
           else
-            i"${elem.showAsCapability} cannot be included in capture set $cs"
+            trailing:
+              i"capability ${elem.showAsCapability} cannot be included in capture set $cs"
         case _ =>
-          i"${elem.showAsCapability} is not included in capture set $cs$why"
+          trailing:
+            i"capability ${elem.showAsCapability} is not included in capture set $cs$why"
 
     override def toText(printer: Printer): Text =
       inContext(printer.printerContext):
diff --git a/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala b/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
index b5b8803a4015..5464f9ea2df7 100644
--- a/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
+++ b/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
@@ -201,9 +201,6 @@ object CheckCaptures:
     && !sym.isOneOf(DeferredOrTermParamOrAccessor)
     && !sym.hasAnnotation(defn.UntrackedCapturesAnnot)
 
-  private def ownerStr(owner: Symbol)(using Context): String =
-    if owner.isAnonymousFunction then "enclosing function" else owner.show
-
   trait CheckerAPI:
     /** Complete symbol info of a val or a def */
     def completeDef(tree: ValOrDefDef, sym: Symbol, completer: LazyType)(using Context): Type
diff --git a/compiler/src/dotty/tools/dotc/reporting/Message.scala b/compiler/src/dotty/tools/dotc/reporting/Message.scala
index 47f405636a2b..33ce11f45425 100644
--- a/compiler/src/dotty/tools/dotc/reporting/Message.scala
+++ b/compiler/src/dotty/tools/dotc/reporting/Message.scala
@@ -42,11 +42,18 @@ object Message:
     else ""
 
   /** A note can produce an added string for an error message */
-  abstract class Note(val prefix: Boolean = false):
+  abstract class Note:
+  
+  	/** Should the note be shown before the actual message or after?
+  	 *  Default is after.
+  	 */
+    def prefix(using Context): Boolean = false
+
+    /** The note rendered as part of an error message */
     def render(using Context): String
 
   object Note:
-    def apply(msg: Context ?=> String, prefix: Boolean = false) = new Note(prefix):
+    def apply(msg: Context ?=> String) = new Note:
       def render(using Context) = msg
 
   enum Disambiguation:
diff --git a/compiler/src/dotty/tools/dotc/reporting/messages.scala b/compiler/src/dotty/tools/dotc/reporting/messages.scala
index f79857b020a3..3aebf257b5c5 100644
--- a/compiler/src/dotty/tools/dotc/reporting/messages.scala
+++ b/compiler/src/dotty/tools/dotc/reporting/messages.scala
@@ -343,7 +343,7 @@ class TypeMismatch(val found: Type, expected: Type, val inTree: Option[untpd.Tre
           mapOver(tp)
         case _ =>
           mapOver(tp)
-
+    val preface = notes.filter(_.prefix).map(_.render).mkString
     val found1 = reported(found)
     reported.setVariance(-1)
     val expected1 = reported(expected)
@@ -351,7 +351,7 @@ class TypeMismatch(val found: Type, expected: Type, val inTree: Option[untpd.Tre
       if (found1 frozen_<:< expected1) || reported.fbounded then (found, expected)
       else (found1, expected1)
     val (foundStr, expectedStr) = Formatting.typeDiff(found2.normalized, expected2.normalized)
-    i"""|Found:    $foundStr
+    i"""|${preface}Found:    $foundStr
         |Required: $expectedStr${reported.notes}"""
   end msg
 
@@ -359,7 +359,7 @@ class TypeMismatch(val found: Type, expected: Type, val inTree: Option[untpd.Tre
     def importSuggestions =
       if expected.isTopType || found.isBottomType then ""
       else ctx.typer.importSuggestionAddendum(ViewProto(found.widen, expected))
-    notes.map(_.render).mkString ++ super.msgPostscript ++ importSuggestions
+    notes.filter(!_.prefix).map(_.render).mkString ++ super.msgPostscript ++ importSuggestions
 
   override def explain(using Context) =
     val treeStr = inTree.map(x => s"\nTree:\n\n${x.show}\n").getOrElse("")
diff --git a/tests/neg-custom-args/captures/boundary.check b/tests/neg-custom-args/captures/boundary.check
index 62255507f7bd..0c440125a403 100644
--- a/tests/neg-custom-args/captures/boundary.check
+++ b/tests/neg-custom-args/captures/boundary.check
@@ -2,11 +2,12 @@
  4 |  boundary[AnyRef^]:
  5 |    l1 ?=> // error // error
    |  ^
+   |  Capability cap outlives its scope: it leaks into outer capture set 's1 which is owned by value local.
+   |  The leakage occurred when trying to match the following types:
+   |
    |  Found:    scala.util.boundary.Label[Object^'s1]
    |  Required: scala.util.boundary.Label[Object^]^²
    |
-   |  Note that capability cap cannot be included in outer capture set 's1 which is owned by val local.
-   |
    |  where:    ^ and cap refer to the universal root capability
    |            ^²        refers to a fresh root capability classified as Control in the type of value local
  6 |      boundary[Unit]: l2 ?=>
diff --git a/tests/neg-custom-args/captures/effect-swaps-explicit.check b/tests/neg-custom-args/captures/effect-swaps-explicit.check
index 82a5a239807d..098054bc31ed 100644
--- a/tests/neg-custom-args/captures/effect-swaps-explicit.check
+++ b/tests/neg-custom-args/captures/effect-swaps-explicit.check
@@ -19,10 +19,11 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/effect-swaps-explicit.scala:69:10 ------------------------
 69 |          Future: fut ?=> // error, type mismatch
    |          ^
-   |Found:    (contextual$9: boundary.Label[Result[Future[T^'s2]^'s3, E^'s4]^'s5]^'s6) ?->{fr, async} Future[T^'s7]^{fr, contextual$9}
-   |Required: (boundary.Label[Result[Future[T^'s8]^'s9, E^'s10]]^) ?=> Future[T^'s8]^'s9
+   |Capability contextual$9 outlives its scope: it leaks into outer capture set 's2 which is owned by method fail4.
+   |The leakage occurred when trying to match the following types:
    |
-   |Note that capability contextual$9 cannot be included in outer capture set 's9 which is owned by method fail4.
+   |Found:    (contextual$9: boundary.Label[Result[Future[T^'s3]^'s4, E^'s5]^'s6]^'s7) ?->{fr, async} Future[T^'s8]^{fr, contextual$9}
+   |Required: (boundary.Label[Result[Future[T^'s9]^'s2, E^'s10]]^) ?=> Future[T^'s9]^'s2
    |
    |where:    ?=> refers to a fresh root capability created in method fail4 when checking argument to parameter body of method make
    |          ^   refers to the universal root capability
diff --git a/tests/neg-custom-args/captures/effect-swaps.check b/tests/neg-custom-args/captures/effect-swaps.check
index 4820318a9fb8..fdf00220fa4e 100644
--- a/tests/neg-custom-args/captures/effect-swaps.check
+++ b/tests/neg-custom-args/captures/effect-swaps.check
@@ -19,10 +19,11 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/effect-swaps.scala:69:10 ---------------------------------
 69 |          Future: fut ?=> // error, type mismatch
    |          ^
-   |Found:    (contextual$9: boundary.Label[Result[Future[T^'s2]^'s3, E^'s4]^'s5]^'s6) ?->{fr, async} Future[T^'s7]^{fr, contextual$9}
-   |Required: (boundary.Label[Result[Future[T^'s8]^'s9, E^'s10]]^) ?=> Future[T^'s8]^'s9
+   |Capability contextual$9 outlives its scope: it leaks into outer capture set 's2 which is owned by method fail4.
+   |The leakage occurred when trying to match the following types:
    |
-   |Note that capability contextual$9 cannot be included in outer capture set 's9 which is owned by method fail4.
+   |Found:    (contextual$9: boundary.Label[Result[Future[T^'s3]^'s4, E^'s5]^'s6]^'s7) ?->{fr, async} Future[T^'s8]^{fr, contextual$9}
+   |Required: (boundary.Label[Result[Future[T^'s9]^'s2, E^'s10]]^) ?=> Future[T^'s9]^'s2
    |
    |where:    ?=> refers to a fresh root capability created in method fail4 when checking argument to parameter body of method make
    |          ^   refers to the universal root capability
diff --git a/tests/neg-custom-args/captures/filevar.check b/tests/neg-custom-args/captures/filevar.check
index 08f035cee2d0..ed3996b1e4d8 100644
--- a/tests/neg-custom-args/captures/filevar.check
+++ b/tests/neg-custom-args/captures/filevar.check
@@ -1,10 +1,11 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/filevar.scala:15:12 --------------------------------------
 15 |  withFile: f =>       // error with level checking, was OK under both schemes before
    |            ^
-   |Found:    (l: scala.caps.Capability^) ?->'s1 File^'s2 ->'s3 Unit
-   |Required: (l: scala.caps.Capability^) ?-> (f: File^{l}) => Unit
+   |Capability l outlives its scope: it leaks into outer capture set 's1 of parameter f.
+   |The leakage occurred when trying to match the following types:
    |
-   |Note that capability l cannot be included in outer capture set 's4 of parameter f which is owned by method $anonfun.
+   |Found:    (l: scala.caps.Capability^) ?->'s2 File^'s3 ->'s4 Unit
+   |Required: (l: scala.caps.Capability^) ?-> (f: File^{l}) => Unit
    |
    |where:    => refers to a root capability associated with the result type of (using l: scala.caps.Capability^): (f: File^{l}) => Unit
    |          ^  refers to the universal root capability
diff --git a/tests/neg-custom-args/captures/heal-tparam-cs.check b/tests/neg-custom-args/captures/heal-tparam-cs.check
index 9ceeee3d7164..7a36f9654c57 100644
--- a/tests/neg-custom-args/captures/heal-tparam-cs.check
+++ b/tests/neg-custom-args/captures/heal-tparam-cs.check
@@ -1,10 +1,11 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/heal-tparam-cs.scala:10:25 -------------------------------
 10 |  val test1 = localCap { c => // error
    |                         ^
-   |Found:    (c: Capp^'s1) ->'s2 () ->{c} Unit
-   |Required: (c: Capp^) => () ->'s3 Unit
+   |Capability c outlives its scope: it leaks into outer capture set 's1 which is owned by value test1.
+   |The leakage occurred when trying to match the following types:
    |
-   |Note that capability c cannot be included in outer capture set 's3 which is owned by val test1.
+   |Found:    (c: Capp^'s2) ->'s3 () ->{c} Unit
+   |Required: (c: Capp^) => () ->'s1 Unit
    |
    |where:    => refers to a fresh root capability created in value test1 when checking argument to parameter op of method localCap
    |          ^  refers to the universal root capability
diff --git a/tests/neg-custom-args/captures/i15923.check b/tests/neg-custom-args/captures/i15923.check
index d8204d62f836..44c918ff57be 100644
--- a/tests/neg-custom-args/captures/i15923.check
+++ b/tests/neg-custom-args/captures/i15923.check
@@ -1,13 +1,14 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/i15923.scala:12:21 ---------------------------------------
 12 |  val leak = withCap(cap => mkId(cap)) // error
    |                     ^^^^^^^^^^^^^^^^
-   |Found:    (lcap: scala.caps.Capability^) ?->'s1 Cap^'s2 ->'s3 Id[Cap^'s4]^'s5
-   |Required: (lcap: scala.caps.Capability^) ?-> Cap^{lcap} => Id[Cap^'s6]^'s7
+   |Capability cap outlives its scope: it leaks into outer capture set 's1 which is owned by value leak.
+   |The leakage occurred when trying to match the following types:
    |
-   |Note that capability cap cannot be included in outer capture set 's6 which is owned by val leak.
+   |Found:    (lcap: scala.caps.Capability^) ?->'s2 Cap^'s3 ->'s4 Id[Cap^'s5]^'s6
+   |Required: (lcap: scala.caps.Capability^) ?-> Cap^{lcap} => Id[Cap^'s1]^'s7
    |
-   |where:    =>  refers to a root capability associated with the result type of (using lcap: scala.caps.Capability^): Cap^{lcap} => Id[Cap^'s6]^'s7
+   |where:    =>  refers to a root capability associated with the result type of (using lcap: scala.caps.Capability^): Cap^{lcap} => Id[Cap^'s1]^'s7
    |          ^   refers to the universal root capability
-   |          cap is a root capability associated with the result type of (x$0: Cap^'s2): Id[Cap^'s4]^'s5
+   |          cap is a root capability associated with the result type of (x$0: Cap^'s3): Id[Cap^'s5]^'s6
    |
    | longer explanation available when compiling with `-explain`
diff --git a/tests/neg-custom-args/captures/i15923a.check b/tests/neg-custom-args/captures/i15923a.check
index b25395e17ae6..d21ce5cfd04b 100644
--- a/tests/neg-custom-args/captures/i15923a.check
+++ b/tests/neg-custom-args/captures/i15923a.check
@@ -1,14 +1,15 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/i15923a.scala:7:21 ---------------------------------------
 7 |  val leak = withCap(lcap => () => mkId(lcap)) // error
   |                     ^^^^^^^^^^^^^^^^^^^^^^^^
-  |Found:    (lcap: Cap^'s1) ->'s2 () ->'s3 Id[Cap^'s4]^'s5
-  |Required: (lcap: Cap^) => () =>² Id[Cap^'s6]^'s7
+  |Capability cap outlives its scope: it leaks into outer capture set 's1 which is owned by value leak.
+  |The leakage occurred when trying to match the following types:
   |
-  |Note that capability cap cannot be included in outer capture set 's6 which is owned by val leak.
+  |Found:    (lcap: Cap^'s2) ->'s3 () ->'s4 Id[Cap^'s5]^'s6
+  |Required: (lcap: Cap^) => () =>² Id[Cap^'s1]^'s7
   |
   |where:    =>  refers to a fresh root capability created in value leak when checking argument to parameter op of method withCap
-  |          =>² refers to a root capability associated with the result type of (lcap: Cap^): () =>² Id[Cap^'s6]^'s7
+  |          =>² refers to a root capability associated with the result type of (lcap: Cap^): () =>² Id[Cap^'s1]^'s7
   |          ^   refers to the universal root capability
-  |          cap is a root capability associated with the result type of (): Id[Cap^'s4]^'s5
+  |          cap is a root capability associated with the result type of (): Id[Cap^'s5]^'s6
   |
   | longer explanation available when compiling with `-explain`
diff --git a/tests/neg-custom-args/captures/i15923b.check b/tests/neg-custom-args/captures/i15923b.check
index 4a47cd8bcc7c..ef2e1ed7e16f 100644
--- a/tests/neg-custom-args/captures/i15923b.check
+++ b/tests/neg-custom-args/captures/i15923b.check
@@ -1,11 +1,12 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/i15923b.scala:8:21 ---------------------------------------
 8 |  val leak = withCap(f) // error
   |                     ^
+  |Capability lcap outlives its scope: it leaks into outer capture set 's1 which is owned by value leak.
+  |The leakage occurred when trying to match the following types:
+  |
   |Found:    (x$0: Cap^) -> Id[Cap^{x$0}]
   |Required: (lcap: Cap^) => Id[Cap^'s1]^'s2
   |
-  |Note that capability lcap cannot be included in outer capture set 's1 which is owned by val leak.
-  |
   |where:    => refers to a fresh root capability created in value leak when checking argument to parameter op of method withCap
   |          ^  refers to the universal root capability
   |
diff --git a/tests/neg-custom-args/captures/i21920.check b/tests/neg-custom-args/captures/i21920.check
index aca23e9da213..6479386622fa 100644
--- a/tests/neg-custom-args/captures/i21920.check
+++ b/tests/neg-custom-args/captures/i21920.check
@@ -1,13 +1,14 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/i21920.scala:34:35 ---------------------------------------
 34 |  val cell: Cell[File] = File.open(f => Cell(() => Seq(f))) // error
    |                                   ^^^^^^^^^^^^^^^^^^^^^^^
-   |Found:    (f: File^'s1) ->'s2 Cell[File^'s3]{val head: () ->'s4 IterableOnce[File^'s5]^'s6}^'s7
-   |Required: File^ => Cell[File^'s8]{val head: () ->'s9 IterableOnce[File^'s10]^'s11}^'s12
+   |Capability cap outlives its scope: it leaks into outer capture set 's1 which is owned by an enclosing function.
+   |The leakage occurred when trying to match the following types:
    |
-   |Note that capability cap cannot be included in outer capture set 's13 which is owned by method $anonfun.
+   |Found:    (f: File^'s2) ->'s3 Cell[File^'s4]{val head: () ->'s5 IterableOnce[File^'s6]^'s7}^'s8
+   |Required: File^ => Cell[File^'s9]{val head: () ->'s10 IterableOnce[File^'s11]^'s12}^'s13
    |
    |where:    =>  refers to a fresh root capability created in value cell when checking argument to parameter f of method open
    |          ^   refers to the universal root capability
-   |          cap is a root capability associated with the result type of (): IterableOnce[File^'s13]^²
+   |          cap is a root capability associated with the result type of (): IterableOnce[File^'s1]^²
    |
    | longer explanation available when compiling with `-explain`
diff --git a/tests/neg-custom-args/captures/i23431.check b/tests/neg-custom-args/captures/i23431.check
index 5bbcb2b720f0..10885339334d 100644
--- a/tests/neg-custom-args/captures/i23431.check
+++ b/tests/neg-custom-args/captures/i23431.check
@@ -18,7 +18,7 @@
    |             Required: IO^²
    |
    |             Note that capability cap is not included in capture set {cap²}
-   |             because cap in enclosing function is not visible from cap² in variable myIO.
+   |             because cap in an enclosing function is not visible from cap² in variable myIO.
    |
    |             where:    ^ and cap   refer to a fresh root capability in the type of parameter io2
    |                       ^² and cap² refer to a fresh root capability in the type of variable myIO
@@ -31,7 +31,7 @@
    |Required: IO^ => Unit
    |
    |Note that capability cap is not included in capture set {cap²}
-   |because cap in enclosing function is not visible from cap² in variable myIO.
+   |because cap in an enclosing function is not visible from cap² in variable myIO.
    |
    |where:    =>   refers to a fresh root capability created in anonymous function of type (io1: IO^): Unit when checking argument to parameter op of method withIO
    |          ^    refers to the universal root capability
diff --git a/tests/neg-custom-args/captures/lazylists-exceptions.check b/tests/neg-custom-args/captures/lazylists-exceptions.check
index d74e019c3677..c0ca2bd3b258 100644
--- a/tests/neg-custom-args/captures/lazylists-exceptions.check
+++ b/tests/neg-custom-args/captures/lazylists-exceptions.check
@@ -1,11 +1,12 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/lazylists-exceptions.scala:37:17 -------------------------
 37 |    tabulate(10) { i =>  // error
    |    ^
+   |    Capability canThrow$1 defined in an enclosing try expression outlives its scope:
+   |    it leaks into outer capture set 's1 of method problem.
+   |    The leakage occurred when trying to match the following types:
+   |
    |    Found:    LazyList[Int]^{canThrow$1}
    |    Required: LazyList[Int]^'s1
-   |
-   |    Note that capability canThrow$1, defined in method try$1
-   |    cannot be included in outer capture set 's1 of method problem which is owned by method problem.
 38 |      if i > 9 then throw Ex1()
 39 |      i * i
 40 |    }
diff --git a/tests/neg-custom-args/captures/leaking-iterators.check b/tests/neg-custom-args/captures/leaking-iterators.check
index e23d7684cac0..f867fc5d4a79 100644
--- a/tests/neg-custom-args/captures/leaking-iterators.check
+++ b/tests/neg-custom-args/captures/leaking-iterators.check
@@ -1,10 +1,11 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/leaking-iterators.scala:56:16 ----------------------------
 56 |  usingLogFile: log => // error
    |                ^
-   |Found:    (log: java.io.FileOutputStream^'s1) ->'s2 cctest.Iterator[Int]^{log}
-   |Required: java.io.FileOutputStream^ => cctest.Iterator[Int]^'s3
+   |Capability log outlives its scope: it leaks into outer capture set 's1 which is owned by method test.
+   |The leakage occurred when trying to match the following types:
    |
-   |Note that capability log cannot be included in outer capture set 's3 which is owned by method test.
+   |Found:    (log: java.io.FileOutputStream^'s2) ->'s3 cctest.Iterator[Int]^{log}
+   |Required: java.io.FileOutputStream^ => cctest.Iterator[Int]^'s1
    |
    |where:    => refers to a fresh root capability created in method test when checking argument to parameter op of method usingLogFile
    |          ^  refers to the universal root capability
diff --git a/tests/neg-custom-args/captures/reaches.check b/tests/neg-custom-args/captures/reaches.check
index e20a318064bb..8c099c520d60 100644
--- a/tests/neg-custom-args/captures/reaches.check
+++ b/tests/neg-custom-args/captures/reaches.check
@@ -124,7 +124,7 @@
 -- Error: tests/neg-custom-args/captures/reaches.scala:60:36 -----------------------------------------------------------
 60 |  val leaked = usingFile[File^{id*}]: f => // error: separation
    |               ^
-   |        Local cap created in type of parameter x leaks into capture scope of enclosing function
+   |        Local cap created in type of parameter x leaks into capture scope of an enclosing function
    |
    |        where:    cap is a fresh root capability created in value id of parameter parameter x of method $anonfun
 61 |    val f1: File^{id*} = id(f)
diff --git a/tests/neg-custom-args/captures/real-try.check b/tests/neg-custom-args/captures/real-try.check
index d58adb1c71e4..7f574644f7ca 100644
--- a/tests/neg-custom-args/captures/real-try.check
+++ b/tests/neg-custom-args/captures/real-try.check
@@ -19,30 +19,33 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/real-try.scala:21:4 --------------------------------------
 21 |    () => foo(1) // error
    |    ^^^^^^^^^^^^
+   |    Capability canThrow$2 defined in an enclosing try expression outlives its scope:
+   |    it leaks into outer capture set 's1 of value x.
+   |    The leakage occurred when trying to match the following types:
+   |
    |    Found:    () ->{canThrow$2} Unit
    |    Required: () ->'s1 Unit
    |
-   |    Note that capability canThrow$2, defined in method try$2
-   |    cannot be included in outer capture set 's1 of value x which is owned by val x.
-   |
    | longer explanation available when compiling with `-explain`
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/real-try.scala:27:4 --------------------------------------
 27 |    () => Cell(foo(1)) // error
    |    ^^^^^^^^^^^^^^^^^^
-   |    Found:    () ->{canThrow$3} Cell[Unit]^'s2
-   |    Required: () ->'s3 Cell[Unit]^'s4
+   |    Capability canThrow$3 defined in an enclosing try expression outlives its scope:
+   |    it leaks into outer capture set 's2 of value y.
+   |    The leakage occurred when trying to match the following types:
    |
-   |    Note that capability canThrow$3, defined in method try$3
-   |    cannot be included in outer capture set 's3 of value y which is owned by val y.
+   |    Found:    () ->{canThrow$3} Cell[Unit]^'s3
+   |    Required: () ->'s2 Cell[Unit]^'s4
    |
    | longer explanation available when compiling with `-explain`
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/real-try.scala:33:8 --------------------------------------
 33 |    Cell(() => foo(1)) // error
    |    ^^^^^^^^^^^^^^^^^^
+   |    Capability canThrow$4 defined in an enclosing try expression outlives its scope:
+   |    it leaks into outer capture set 's5 of value b.
+   |    The leakage occurred when trying to match the following types:
+   |
    |    Found:    Cell[() ->{canThrow$4} Unit]
    |    Required: Cell[() ->'s5 Unit]^'s6
    |
-   |    Note that capability canThrow$4, defined in method try$4
-   |    cannot be included in outer capture set 's5 of value b which is owned by val b.
-   |
    | longer explanation available when compiling with `-explain`
diff --git a/tests/neg-custom-args/captures/reference-cc.check b/tests/neg-custom-args/captures/reference-cc.check
index 108d40ea1e37..df4567a5c173 100644
--- a/tests/neg-custom-args/captures/reference-cc.check
+++ b/tests/neg-custom-args/captures/reference-cc.check
@@ -1,10 +1,11 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/reference-cc.scala:44:29 ---------------------------------
 44 |  val later = usingLogFile { file => () => file.write(0) } // error
    |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |Found:    (file: java.io.FileOutputStream^'s1) ->'s2 () ->{file} Unit
-   |Required: java.io.FileOutputStream^ => () ->'s3 Unit
+   |Capability file outlives its scope: it leaks into outer capture set 's1 which is owned by value later.
+   |The leakage occurred when trying to match the following types:
    |
-   |Note that capability file cannot be included in outer capture set 's3 which is owned by val later.
+   |Found:    (file: java.io.FileOutputStream^'s2) ->'s3 () ->{file} Unit
+   |Required: java.io.FileOutputStream^ => () ->'s1 Unit
    |
    |where:    => refers to a fresh root capability created in value later when checking argument to parameter op of method usingLogFile
    |          ^  refers to the universal root capability
@@ -13,10 +14,11 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/reference-cc.scala:47:25 ---------------------------------
 47 |  val xs = usingLogFile: f => // error
    |                         ^
-   |Found:    (f: java.io.FileOutputStream^'s4) ->'s5 LzyList[Int]^{f}
-   |Required: java.io.FileOutputStream^ => LzyList[Int]^'s6
+   |Capability f outlives its scope: it leaks into outer capture set 's4 which is owned by value xs.
+   |The leakage occurred when trying to match the following types:
    |
-   |Note that capability f cannot be included in outer capture set 's6 which is owned by val xs.
+   |Found:    (f: java.io.FileOutputStream^'s5) ->'s6 LzyList[Int]^{f}
+   |Required: java.io.FileOutputStream^ => LzyList[Int]^'s4
    |
    |where:    => refers to a fresh root capability created in value xs when checking argument to parameter op of method usingLogFile
    |          ^  refers to the universal root capability
@@ -30,7 +32,7 @@
    |Required: () => Double
    |
    |Note that capability canThrow$1 is not included in capture set {cap}
-   |because (canThrow$1 : CanThrow[LimitExceeded]) in method try$1 is not visible from cap in enclosing function.
+   |because (canThrow$1 : CanThrow[LimitExceeded]) in an enclosing try expression is not visible from cap in an enclosing function.
    |
    |where:    => and cap refer to a fresh root capability created in anonymous function of type (using erased x$1: CanThrow[LimitExceeded]): () => Double when instantiating expected result type () ->{cap²} Double of function literal
    |
diff --git a/tests/neg-custom-args/captures/scoped-caps.check b/tests/neg-custom-args/captures/scoped-caps.check
index 4c738997c213..d01b55ad0747 100644
--- a/tests/neg-custom-args/captures/scoped-caps.check
+++ b/tests/neg-custom-args/captures/scoped-caps.check
@@ -85,7 +85,7 @@
    |Required: B^²
    |
    |Note that capability cap is not included in capture set {cap²}
-   |because cap in enclosing function is not visible from cap² in value _$14.
+   |because cap in an enclosing function is not visible from cap² in value _$14.
    |
    |where:    ^ and cap   refer to a fresh root capability created in anonymous function of type (x: S): B^³ when instantiating method apply's type (x: S^³): B^⁴
    |          ^² and cap² refer to a fresh root capability in the type of value _$14
diff --git a/tests/neg-custom-args/captures/scoped-caps2.check b/tests/neg-custom-args/captures/scoped-caps2.check
index 21867171cb6b..cd739e80bc20 100644
--- a/tests/neg-custom-args/captures/scoped-caps2.check
+++ b/tests/neg-custom-args/captures/scoped-caps2.check
@@ -61,7 +61,7 @@
    |Required: C^²
    |
    |Note that capability cap is not included in capture set {cap²}
-   |because cap in enclosing function is not visible from cap² in value _$8.
+   |because cap in an enclosing function is not visible from cap² in value _$8.
    |
    |where:    ^ and cap   refer to a fresh root capability classified as SharedCapability created in anonymous function of type (x: C): C when instantiating method apply's type (x: C^³): C^⁴
    |          ^² and cap² refer to a fresh root capability classified as SharedCapability in the type of value _$8
diff --git a/tests/neg-custom-args/captures/sep-curried-par.check b/tests/neg-custom-args/captures/sep-curried-par.check
index be6f0e493adf..746b5d041338 100644
--- a/tests/neg-custom-args/captures/sep-curried-par.check
+++ b/tests/neg-custom-args/captures/sep-curried-par.check
@@ -1,11 +1,12 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/sep-curried-par.scala:23:32 ------------------------------
 23 |  val bar = (p1: () => Unit) => (p2: () ->{p1, cap} Unit) => par(p1, p2) // error, but error message could be better
    |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |                                Found:    (p2: () ->{p1, cap} Unit) ->{p1} Unit
-   |                                Required: (() ->'s1 Unit) ->'s2 Unit
+   |                                Capability p1 defined in an enclosing function outlives its scope:
+   |                                it leaks into outer capture set 's1 which is owned by value bar.
+   |                                The leakage occurred when trying to match the following types:
    |
-   |                                Note that capability p1, defined in method $anonfun
-   |                                cannot be included in outer capture set 's2 which is owned by val bar.
+   |                                Found:    (p2: () ->{p1, cap} Unit) ->{p1} Unit
+   |                                Required: (() ->'s2 Unit) ->'s1 Unit
    |
    |                                where:    cap is the universal root capability
    |
diff --git a/tests/neg-custom-args/captures/simple-using.check b/tests/neg-custom-args/captures/simple-using.check
index e8e2948d349d..84206a8f78ce 100644
--- a/tests/neg-custom-args/captures/simple-using.check
+++ b/tests/neg-custom-args/captures/simple-using.check
@@ -1,10 +1,11 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/simple-using.scala:8:17 ----------------------------------
 8 |  usingLogFile { f => () => f.write(2) } // error
   |                 ^^^^^^^^^^^^^^^^^^^^^
-  |Found:    (f: java.io.FileOutputStream^'s1) ->'s2 () ->{f} Unit
-  |Required: java.io.FileOutputStream^ => () ->'s3 Unit
+  |Capability f outlives its scope: it leaks into outer capture set 's1 which is owned by method test.
+  |The leakage occurred when trying to match the following types:
   |
-  |Note that capability f cannot be included in outer capture set 's3 which is owned by method test.
+  |Found:    (f: java.io.FileOutputStream^'s2) ->'s3 () ->{f} Unit
+  |Required: java.io.FileOutputStream^ => () ->'s1 Unit
   |
   |where:    => refers to a fresh root capability created in method test when checking argument to parameter op of method usingLogFile
   |          ^  refers to the universal root capability
diff --git a/tests/neg-custom-args/captures/try.check b/tests/neg-custom-args/captures/try.check
index bfa2cafce460..33b69e0e3fd6 100644
--- a/tests/neg-custom-args/captures/try.check
+++ b/tests/neg-custom-args/captures/try.check
@@ -34,10 +34,11 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/try.scala:36:4 -------------------------------------------
 36 |    (x: CanThrow[Exception]) => // error
    |    ^
-   |Found:    (x: CT[Exception]^) ->'s3 () ->{x} Int
-   |Required: CT[Exception]^ => () ->'s4 Int
+   |Capability x outlives its scope: it leaks into outer capture set 's3 which is owned by value xx.
+   |The leakage occurred when trying to match the following types:
    |
-   |Note that capability x cannot be included in outer capture set 's4 which is owned by val xx.
+   |Found:    (x: CT[Exception]^) ->'s4 () ->{x} Int
+   |Required: CT[Exception]^ => () ->'s3 Int
    |
    |where:    => refers to a fresh root capability created in value xx when checking argument to parameter op of method handle
    |          ^  refers to the universal root capability
@@ -49,10 +50,11 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/try.scala:48:2 -------------------------------------------
 48 |  (x: CanThrow[Exception]) =>  // error
    |  ^
-   |Found:    (x: CT[Exception]^) ->'s5 () ->{x} Int
-   |Required: CT[Exception]^ => () ->'s6 Int
+   |Capability x outlives its scope: it leaks into outer capture set 's5 which is owned by value global.
+   |The leakage occurred when trying to match the following types:
    |
-   |Note that capability x cannot be included in outer capture set 's6 which is owned by val global.
+   |Found:    (x: CT[Exception]^) ->'s6 () ->{x} Int
+   |Required: CT[Exception]^ => () ->'s5 Int
    |
    |where:    => refers to a fresh root capability created in value global when checking argument to parameter op of method handle
    |          ^  refers to the universal root capability
diff --git a/tests/neg-custom-args/captures/usingLogFile.check b/tests/neg-custom-args/captures/usingLogFile.check
index 71750865a9e7..51cbe5b334d1 100644
--- a/tests/neg-custom-args/captures/usingLogFile.check
+++ b/tests/neg-custom-args/captures/usingLogFile.check
@@ -1,10 +1,11 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/usingLogFile.scala:22:29 ---------------------------------
 22 |  val later = usingLogFile { f => () => f.write(0) } // error
    |                             ^^^^^^^^^^^^^^^^^^^^^
-   |Found:    (f: java.io.FileOutputStream^'s1) ->'s2 () ->{f} Unit
-   |Required: java.io.FileOutputStream^ => () ->'s3 Unit
+   |Capability f outlives its scope: it leaks into outer capture set 's1 which is owned by value later.
+   |The leakage occurred when trying to match the following types:
    |
-   |Note that capability f cannot be included in outer capture set 's3 which is owned by val later.
+   |Found:    (f: java.io.FileOutputStream^'s2) ->'s3 () ->{f} Unit
+   |Required: java.io.FileOutputStream^ => () ->'s1 Unit
    |
    |where:    => refers to a fresh root capability created in value later when checking argument to parameter op of method usingLogFile
    |          ^  refers to the universal root capability
@@ -13,10 +14,11 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/usingLogFile.scala:27:38 ---------------------------------
 27 |  private val later2 = usingLogFile { f => Cell(() => f.write(0)) } // error
    |                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |Found:    (f: java.io.FileOutputStream^'s4) ->'s5 Test2.Cell[() ->{f} Unit]^'s6
-   |Required: java.io.FileOutputStream^ => Test2.Cell[() ->'s7 Unit]^'s8
+   |Capability f outlives its scope: it leaks into outer capture set 's4 which is owned by value later2.
+   |The leakage occurred when trying to match the following types:
    |
-   |Note that capability f cannot be included in outer capture set 's7 which is owned by val later2.
+   |Found:    (f: java.io.FileOutputStream^'s5) ->'s6 Test2.Cell[() ->{f} Unit]^'s7
+   |Required: java.io.FileOutputStream^ => Test2.Cell[() ->'s4 Unit]^'s8
    |
    |where:    => refers to a fresh root capability created in value later2 when checking argument to parameter op of method usingLogFile
    |          ^  refers to the universal root capability
@@ -25,10 +27,11 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/usingLogFile.scala:43:33 ---------------------------------
 43 |    val later = usingFile("out", f => (y: Int) => xs.foreach(x => f.write(x + y))) // error
    |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |Found:    (f: java.io.OutputStream^'s9) ->'s10 Int ->{f} Unit
-   |Required: java.io.OutputStream^ => Int ->'s11 Unit
+   |Capability f outlives its scope: it leaks into outer capture set 's9 which is owned by value later.
+   |The leakage occurred when trying to match the following types:
    |
-   |Note that capability f cannot be included in outer capture set 's11 which is owned by val later.
+   |Found:    (f: java.io.OutputStream^'s10) ->'s11 Int ->{f} Unit
+   |Required: java.io.OutputStream^ => Int ->'s9 Unit
    |
    |where:    => refers to a fresh root capability created in value later when checking argument to parameter op of method usingFile
    |          ^  refers to the universal root capability
@@ -37,10 +40,11 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/usingLogFile.scala:52:6 ----------------------------------
 52 |      usingLogger(_, l => () => l.log("test")))  // error after checking mapping scheme
    |      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |Found:    (_$1: java.io.OutputStream^'s12) ->'s13 () ->{_$1} Unit
-   |Required: java.io.OutputStream^ => () ->'s14 Unit
+   |Capability _$1 outlives its scope: it leaks into outer capture set 's12 which is owned by value later.
+   |The leakage occurred when trying to match the following types:
    |
-   |Note that capability _$1 cannot be included in outer capture set 's14 which is owned by val later.
+   |Found:    (_$1: java.io.OutputStream^'s13) ->'s14 () ->{_$1} Unit
+   |Required: java.io.OutputStream^ => () ->'s12 Unit
    |
    |where:    => refers to a fresh root capability created in value later when checking argument to parameter op of method usingFile
    |          ^  refers to the universal root capability
diff --git a/tests/neg-custom-args/captures/vars.check b/tests/neg-custom-args/captures/vars.check
index 399869e4d96f..58a6708dc869 100644
--- a/tests/neg-custom-args/captures/vars.check
+++ b/tests/neg-custom-args/captures/vars.check
@@ -28,14 +28,15 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/vars.scala:36:10 -----------------------------------------
 36 |  local { cap3 => // error
    |          ^
-   |        Found:    (cap3: CC^) ->'s1 String ->{cap3} String
-   |        Required: CC^ -> String ->{cap3²} String
+   |      Capability cap3 outlives its scope: it leaks into outer capture set {cap3²} which is owned by method test.
+   |      The leakage occurred when trying to match the following types:
    |
-   |        Note that capability cap3 cannot be included in outer capture set {cap3²} which is owned by method test.
+   |      Found:    (cap3: CC^) ->'s1 String ->{cap3} String
+   |      Required: CC^ -> String ->{cap3²} String
    |
-   |        where:    ^     refers to the universal root capability
-   |                  cap3  is a reference to a value parameter
-   |                  cap3² is a parameter in an anonymous function in method test
+   |      where:    ^     refers to the universal root capability
+   |                cap3  is a reference to a value parameter
+   |                cap3² is a parameter in an anonymous function in method test
 37 |    def g(x: String): String = if cap3 == cap3 then "" else "a"
 38 |    g
    |

From 7ca78b14d1645b2dbcb7fbd9f7ba32472cc94960 Mon Sep 17 00:00:00 2001
From: odersky <odersky@gmail.com>
Date: Sun, 2 Nov 2025 18:26:06 +0100
Subject: [PATCH 5/7] Drop ExistentialSubsumesFailure

Present the information as part of explaining an IncludeFailure instead.
---
 .../src/dotty/tools/dotc/cc/Capability.scala  |  5 +-
 .../src/dotty/tools/dotc/cc/CaptureSet.scala  | 91 ++++++++-----------
 .../dotty/tools/dotc/cc/CheckCaptures.scala   |  2 +-
 .../captures/heal-tparam-cs.check             | 16 ++--
 tests/neg-custom-args/captures/i16226.check   |  6 +-
 5 files changed, 51 insertions(+), 69 deletions(-)

diff --git a/compiler/src/dotty/tools/dotc/cc/Capability.scala b/compiler/src/dotty/tools/dotc/cc/Capability.scala
index fb6a8cff96e8..53e272a19ef4 100644
--- a/compiler/src/dotty/tools/dotc/cc/Capability.scala
+++ b/compiler/src/dotty/tools/dotc/cc/Capability.scala
@@ -786,12 +786,9 @@ object Capabilities:
               && prefixAllowsAddHidden
               && vs.addHidden(x.hiddenSet, y)
         case x: ResultCap =>
-          val result = y match
+          y match
             case y: ResultCap => vs.unify(x, y)
             case _ => y.derivesFromShared
-          if !result then
-            TypeComparer.addErrorNote(CaptureSet.ExistentialSubsumesFailure(x, y))
-          result
         case GlobalCap =>
           y match
             case GlobalCap => true
diff --git a/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala b/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala
index b5c13cedfe80..51883ea305b8 100644
--- a/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala
+++ b/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala
@@ -1300,20 +1300,6 @@ object CaptureSet:
   /** A TypeMap that is the identity on capabilities */
   trait IdentityCaptRefMap extends TypeMap
 
-  /** A value of this class is produced and added as a note to ccState
-   *  when a subsumes check decides that an existential variable `ex` cannot be
-   *  instantiated to the other capability `other`.
-   */
-  case class ExistentialSubsumesFailure(val ex: ResultCap, val other: Capability) extends Note:
-    def render(using Context): String =
-      def reason =
-        if other.isTerminalCapability then ""
-        else " since that capability is not a `Sharable` capability"
-      i"""
-         |
-         |Note that the existential capture root in ${ex.originalBinder.resType}
-         |cannot subsume the capability $other$reason."""
-
   /** Failure indicating that `elem` cannot be included in `cs` */
   case class IncludeFailure(cs: CaptureSet, elem: Capability, levelError: Boolean = false) extends Note, Showable:
     private var myTrace: List[CaptureSet] = cs :: Nil
@@ -1342,44 +1328,47 @@ object CaptureSet:
          |
          |"""
 
-    def render(using Context): String =
-      def why =
-        val reasons = cs.elems.toList.collect:
-          case c: FreshCap if !c.acceptsLevelOf(elem) =>
-            i"$elem${elem.levelOwner.qualString("in")} is not visible from $c${c.ccOwner.qualString("in")}"
-          case c: FreshCap if !elem.tryClassifyAs(c.hiddenSet.classifier) =>
-            i"$c is classified as ${c.hiddenSet.classifier} but $elem is not"
-        if reasons.isEmpty then ""
-        else reasons.mkString("\nbecause ", "\nand ", "")
-      cs match
-        case cs: Var =>
-          def ownerStr =
-            if !cs.description.isEmpty then "" else cs.owner.qualString("which is owned by")
-          if !cs.levelOK(elem) then
-            val outlivesStr = elem match
-              case ref: TermRef => i"${ref.symbol.maybeOwner.qualString("defined in")} outlives its scope:\n"
-              case _ => " outlives its scope: "
-            leading:
-              i"""Capability ${elem.showAsCapability}${outlivesStr}it leaks into outer capture set $cs$ownerStr"""
-          else if !elem.tryClassifyAs(cs.classifier) then
-            trailing:
-              i"""capability ${elem.showAsCapability} is not classified as ${cs.classifier}, therefore it
-                |cannot be included in capture set $cs of ${cs.classifier.name} elements"""
-          else if cs.isBadRoot(elem) then
-            elem match
-              case elem: FreshCap =>
-                leading:
-                  i"""Local capability ${elem.showAsCapability} created in ${elem.ccOwner} outlives its scope:
-                     |It leaks into outer capture set $cs$ownerStr"""
-              case _ =>
-                trailing:
-                  i"universal capability ${elem.showAsCapability} cannot be included in capture set $cs"
-          else
-            trailing:
-              i"capability ${elem.showAsCapability} cannot be included in capture set $cs"
-        case _ =>
+    def render(using Context): String = cs match
+      case cs: Var =>
+        def ownerStr =
+          if !cs.description.isEmpty then "" else cs.owner.qualString("which is owned by")
+        if !cs.levelOK(elem) then
+          val outlivesStr = elem match
+            case ref: TermRef => i"${ref.symbol.maybeOwner.qualString("defined in")} outlives its scope:\n"
+            case _ => " outlives its scope: "
+          leading:
+            i"""Capability ${elem.showAsCapability}${outlivesStr}it leaks into outer capture set $cs$ownerStr"""
+        else if !elem.tryClassifyAs(cs.classifier) then
+          trailing:
+            i"""capability ${elem.showAsCapability} is not classified as ${cs.classifier}, therefore it
+              |cannot be included in capture set $cs of ${cs.classifier.name} elements"""
+        else if cs.isBadRoot(elem) then
+          elem match
+            case elem: FreshCap =>
+              leading:
+                i"""Local capability ${elem.showAsCapability} created in ${elem.ccOwner} outlives its scope:
+                    |It leaks into outer capture set $cs$ownerStr"""
+            case _ =>
+              trailing:
+                i"universal capability ${elem.showAsCapability} cannot be included in capture set $cs"
+        else
           trailing:
-            i"capability ${elem.showAsCapability} is not included in capture set $cs$why"
+            i"capability ${elem.showAsCapability} cannot be included in capture set $cs"
+      case _ =>
+        def why =
+          val reasons = cs.elems.toList.collect:
+            case c: FreshCap if !c.acceptsLevelOf(elem) =>
+              i"$elem${elem.levelOwner.qualString("in")} is not visible from $c${c.ccOwner.qualString("in")}"
+            case c: FreshCap if !elem.tryClassifyAs(c.hiddenSet.classifier) =>
+              i"$c is classified as ${c.hiddenSet.classifier} but ${elem.showAsCapability} is not"
+            case c: ResultCap if !c.subsumes(elem) =>
+              val toAdd = if elem.isTerminalCapability then "" else " since that capability is not a SharedCapability"
+              i"$c, which is existentially bound in ${c.originalBinder.resType}, cannot subsume ${elem.showAsCapability}$toAdd"
+          if reasons.isEmpty then ""
+          else reasons.mkString("\nbecause ", "\nand ", "")
+
+        trailing:
+          i"capability ${elem.showAsCapability} is not included in capture set $cs$why"
 
     override def toText(printer: Printer): Text =
       inContext(printer.printerContext):
diff --git a/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala b/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
index 5464f9ea2df7..a08e47b9995d 100644
--- a/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
+++ b/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
@@ -21,7 +21,7 @@ import util.chaining.tap
 import transform.{Recheck, PreRecheck, CapturedVars}
 import Recheck.*
 import scala.collection.mutable
-import CaptureSet.{withCaptureSetsExplained, IncludeFailure, ExistentialSubsumesFailure, MutAdaptFailure}
+import CaptureSet.{withCaptureSetsExplained, IncludeFailure, MutAdaptFailure}
 import CCState.*
 import StdNames.nme
 import NameKinds.{DefaultGetterName, WildcardParamName, UniqueNameKind}
diff --git a/tests/neg-custom-args/captures/heal-tparam-cs.check b/tests/neg-custom-args/captures/heal-tparam-cs.check
index 7a36f9654c57..d4077b27e882 100644
--- a/tests/neg-custom-args/captures/heal-tparam-cs.check
+++ b/tests/neg-custom-args/captures/heal-tparam-cs.check
@@ -15,17 +15,15 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/heal-tparam-cs.scala:15:13 -------------------------------
 15 |    localCap { c =>  // error
    |    ^
-   |    Found:    (x$0: Capp^'s4) ->'s5 () ->{x$0} Unit
-   |    Required: (c: Capp^) -> () => Unit
+   |Found:    (x$0: Capp^'s4) ->'s5 () ->{x$0} Unit
+   |Required: (c: Capp^) -> () => Unit
    |
-   |    Note that capability x$0 is not included in capture set {cap}.
+   |Note that capability x$0 is not included in capture set {cap}
+   |because cap, which is existentially bound in () =>² Unit, cannot subsume x$0 since that capability is not a SharedCapability.
    |
-   |    Note that the existential capture root in () =>² Unit
-   |    cannot subsume the capability (x$0 : Capp^'s4) since that capability is not a `Sharable` capability.
-   |
-   |    where:    =>        refers to a root capability associated with the result type of (c: Capp^): () => Unit
-   |              =>² and ^ refer to the universal root capability
-   |              cap       is a root capability associated with the result type of (x$0: Capp^'s4): () ->{x$0} Unit
+   |where:    =>        refers to a root capability associated with the result type of (c: Capp^): () => Unit
+   |          =>² and ^ refer to the universal root capability
+   |          cap       is a root capability associated with the result type of (x$0: Capp^'s4): () ->{x$0} Unit
 16 |      (c1: Capp^) => () => { c1.use() }
 17 |    }
    |
diff --git a/tests/neg-custom-args/captures/i16226.check b/tests/neg-custom-args/captures/i16226.check
index 479219de5100..860f80ade5d5 100644
--- a/tests/neg-custom-args/captures/i16226.check
+++ b/tests/neg-custom-args/captures/i16226.check
@@ -22,10 +22,8 @@
    |  LazyRef[B^'s19]{val elem: () => B^'s20}^{f1, ref1}
    |Required: (ref: LazyRef[A]^{io}, f: A =>² B) =>³ LazyRef[B]^
    |
-   |Note that capability f1 is not included in capture set {cap}.
-   |
-   |Note that the existential capture root in LazyRef[B]^²
-   |cannot subsume the capability (f1 : A^'s15 ->'s16 B^'s17) since that capability is not a `Sharable` capability.
+   |Note that capability f1 is not included in capture set {cap}
+   |because cap, which is existentially bound in LazyRef[B]^², cannot subsume f1 since that capability is not a SharedCapability.
    |
    |where:    => and cap refer to a root capability associated with the result type of (ref1: LazyRef[A^'s11]{val elem: () ->'s12 A^'s13}^'s14, f1: A^'s15 ->'s16 B^'s17):
    |  LazyRef[B^'s19]{val elem: () => B^'s20}^{f1, ref1}

From 744a3d41604306b51760fcfc0de6aeae1952e971 Mon Sep 17 00:00:00 2001
From: odersky <odersky@gmail.com>
Date: Mon, 3 Nov 2025 11:38:21 +0100
Subject: [PATCH 6/7] Try to pick the most informative note to display

---
 .../src/dotty/tools/dotc/cc/CaptureSet.scala  |  18 ++
 .../dotty/tools/dotc/core/TypeComparer.scala  |  10 +-
 .../dotty/tools/dotc/reporting/Message.scala  |   9 +-
 .../neg-custom-args/captures/cc-poly-2.check  |   6 +-
 tests/neg-custom-args/captures/i23431.check   |  16 +-
 tests/neg-custom-args/captures/i24137.check   |   2 +-
 .../neg-custom-args/captures/outer-var.check  |  12 +-
 .../captures/scope-extrude-mut.check          |   8 +-
 .../captures/scope-extrusions.check           | 163 ++++++++++++++++++
 .../captures/scope-extrusions.scala           |  33 ++++
 10 files changed, 249 insertions(+), 28 deletions(-)
 create mode 100644 tests/neg-custom-args/captures/scope-extrusions.check
 create mode 100644 tests/neg-custom-args/captures/scope-extrusions.scala

diff --git a/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala b/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala
index 51883ea305b8..78af0e6c57c8 100644
--- a/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala
+++ b/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala
@@ -1317,6 +1317,18 @@ object CaptureSet:
       case _ =>
         false
 
+    /** An include failure F1 covers another include failure F2 unless F2
+     *  strictly subsumes F1, which means they describe the same capture sets
+     *  and the element in F2 is more specific than the element in F1.
+     */
+    override def covers(other: Note)(using Context) = other match
+      case other @ IncludeFailure(cs1, elem1, _) =>
+        val strictlySubsumes =
+          cs.elems == cs1.elems
+          && elem1.singletonCaptureSet.mightSubcapture(elem.singletonCaptureSet)
+        !strictlySubsumes
+      case _ => false
+
     def trailing(msg: String)(using Context): String =
       i"""
          |
@@ -1387,6 +1399,7 @@ object CaptureSet:
    *  @param  hi    the upper type of the orginal type comparison, or NoType if not known
    */
   case class MutAdaptFailure(cs: CaptureSet, lo: Type = NoType, hi: Type = NoType) extends Note:
+
     def render(using Context): String =
       def ofType(tp: Type) = if tp.exists then i"of the mutable type $tp" else "of a mutable type"
       i"""
@@ -1394,6 +1407,11 @@ object CaptureSet:
          |Note that $cs is an exclusive capture set ${ofType(hi)},
          |it cannot subsume a read-only capture set ${ofType(lo)}."""
 
+    // Show only one failure of this kind
+    override def covers(other: Note)(using Context) =
+      other.isInstanceOf[MutAdaptFailure]
+  end MutAdaptFailure
+
   /** A VarState serves as a snapshot mechanism that can undo
    *  additions of elements or super sets if an operation fails
    */
diff --git a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
index e5292968b249..77fdc24a01cc 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
+++ b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
@@ -3324,12 +3324,12 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
   def reduceMatchWith[T](op: MatchReducer => T)(using Context): T =
     inSubComparer(matchReducer)(op)
 
-  /** Add given note, provided there is not yet an error note with
-   *  the same class as `note`.
+  /** Add given note, provided there is not yet an error note that covers `note`
+   *  If the new note is added, any existing note covered by it is removed first.
    */
-  def addErrorNote(note: Note): Unit =
-    if errorNotes.forall(_._2.getClass != note.getClass) then
-      errorNotes = (recCount, note) :: errorNotes
+  def addErrorNote(note: Note)(using Context): Unit =
+    if !errorNotes.exists(_._2.covers(note)) then
+      errorNotes = (recCount, note) :: errorNotes.filterConserve(n => !note.covers(n._2))
       assert(maxErrorLevel <= recCount)
       maxErrorLevel = recCount
 
diff --git a/compiler/src/dotty/tools/dotc/reporting/Message.scala b/compiler/src/dotty/tools/dotc/reporting/Message.scala
index 33ce11f45425..73e9412830c4 100644
--- a/compiler/src/dotty/tools/dotc/reporting/Message.scala
+++ b/compiler/src/dotty/tools/dotc/reporting/Message.scala
@@ -43,7 +43,7 @@ object Message:
 
   /** A note can produce an added string for an error message */
   abstract class Note:
-  
+
   	/** Should the note be shown before the actual message or after?
   	 *  Default is after.
   	 */
@@ -52,6 +52,13 @@ object Message:
     /** The note rendered as part of an error message */
     def render(using Context): String
 
+    /** If note N1 covers note N2 then N1 and N2 won't be shown together in
+     *  an error message. Instead we show the note that's strictly better in terms
+     *  of the "covers" partial ordering, or, if there's no strict wionner, the first
+     *  added note.
+     */
+    def covers(other: Note)(using Context): Boolean = false
+
   object Note:
     def apply(msg: Context ?=> String) = new Note:
       def render(using Context) = msg
diff --git a/tests/neg-custom-args/captures/cc-poly-2.check b/tests/neg-custom-args/captures/cc-poly-2.check
index 77cb6e369c2e..62be3ba87085 100644
--- a/tests/neg-custom-args/captures/cc-poly-2.check
+++ b/tests/neg-custom-args/captures/cc-poly-2.check
@@ -4,9 +4,9 @@
    |                   Found:    (d : Test.D^)
    |                   Required: Test.D^{c1}
    |
-   |                   Note that capability cap is not included in capture set {c1}.
+   |                   Note that capability d is not included in capture set {c1}.
    |
-   |                   where:    ^ and cap refer to a fresh root capability in the type of value d
+   |                   where:    ^ refers to a fresh root capability in the type of value d
    |
    | longer explanation available when compiling with `-explain`
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/cc-poly-2.scala:16:20 ------------------------------------
@@ -15,6 +15,6 @@
    |                    Found:    (x : Test.D^{d})
    |                    Required: Test.D^{c1}
    |
-   |                    Note that capability d is not included in capture set {c1}.
+   |                    Note that capability x is not included in capture set {c1}.
    |
    | longer explanation available when compiling with `-explain`
diff --git a/tests/neg-custom-args/captures/i23431.check b/tests/neg-custom-args/captures/i23431.check
index 10885339334d..3c8f09cdc1b9 100644
--- a/tests/neg-custom-args/captures/i23431.check
+++ b/tests/neg-custom-args/captures/i23431.check
@@ -4,11 +4,11 @@
   |             Found:    (io : IO^)
   |             Required: IO^²
   |
-  |             Note that capability cap is not included in capture set {cap²}
-  |             because cap in method setIO is not visible from cap² in variable myIO.
+  |             Note that capability io is not included in capture set {cap}
+  |             because (io : IO^) in method setIO is not visible from cap in variable myIO.
   |
-  |             where:    ^ and cap   refer to a fresh root capability in the type of parameter io
-  |                       ^² and cap² refer to a fresh root capability in the type of variable myIO
+  |             where:    ^          refers to a fresh root capability in the type of parameter io
+  |                       ^² and cap refer to a fresh root capability in the type of variable myIO
   |
   | longer explanation available when compiling with `-explain`
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/i23431.scala:11:13 ---------------------------------------
@@ -17,11 +17,11 @@
    |             Found:    (io2 : IO^)
    |             Required: IO^²
    |
-   |             Note that capability cap is not included in capture set {cap²}
-   |             because cap in an enclosing function is not visible from cap² in variable myIO.
+   |             Note that capability io2 is not included in capture set {cap}
+   |             because (io2 : IO^) in an enclosing function is not visible from cap in variable myIO.
    |
-   |             where:    ^ and cap   refer to a fresh root capability in the type of parameter io2
-   |                       ^² and cap² refer to a fresh root capability in the type of variable myIO
+   |             where:    ^          refers to a fresh root capability in the type of parameter io2
+   |                       ^² and cap refer to a fresh root capability in the type of variable myIO
    |
    | longer explanation available when compiling with `-explain`
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/i23431.scala:12:12 ---------------------------------------
diff --git a/tests/neg-custom-args/captures/i24137.check b/tests/neg-custom-args/captures/i24137.check
index 3f7d14144c32..60919d7b7591 100644
--- a/tests/neg-custom-args/captures/i24137.check
+++ b/tests/neg-custom-args/captures/i24137.check
@@ -4,7 +4,7 @@
    |                     Found:    (b : B{val elem1: A^{a}; val elem2: A^{a}}^{cap, a})
    |                     Required: B^{async}
    |
-   |                     Note that capability cap is not included in capture set {async}.
+   |                     Note that capability b is not included in capture set {async}.
    |
    |                     where:    cap is a fresh root capability in the type of value b
    |
diff --git a/tests/neg-custom-args/captures/outer-var.check b/tests/neg-custom-args/captures/outer-var.check
index 45489ab0ff5b..ae9f6f3619a0 100644
--- a/tests/neg-custom-args/captures/outer-var.check
+++ b/tests/neg-custom-args/captures/outer-var.check
@@ -4,11 +4,11 @@
    |        Found:    (q : () => Unit)
    |        Required: () ->{p, q²} Unit
    |
-   |        Note that capability cap is not included in capture set {p, q²}.
+   |        Note that capability q is not included in capture set {p, q²}.
    |
-   |        where:    => and cap refer to a fresh root capability in the type of parameter q
-   |                  q          is a parameter in method inner
-   |                  q²         is a parameter in method test
+   |        where:    => refers to a fresh root capability in the type of parameter q
+   |                  q  is a parameter in method inner
+   |                  q² is a parameter in method test
    |
    | longer explanation available when compiling with `-explain`
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/outer-var.scala:13:9 -------------------------------------
@@ -39,9 +39,9 @@
    |        Found:    (q : () => Unit)
    |        Required: () ->{p} Unit
    |
-   |        Note that capability cap cannot be included in capture set {p} of variable y.
+   |        Note that capability q cannot be included in capture set {p} of variable y.
    |
-   |        where:    => and cap refer to a fresh root capability in the type of parameter q
+   |        where:    => refers to a fresh root capability in the type of parameter q
    |
    | longer explanation available when compiling with `-explain`
 -- Error: tests/neg-custom-args/captures/outer-var.scala:17:57 ---------------------------------------------------------
diff --git a/tests/neg-custom-args/captures/scope-extrude-mut.check b/tests/neg-custom-args/captures/scope-extrude-mut.check
index 5f1671b19a47..905becd123e4 100644
--- a/tests/neg-custom-args/captures/scope-extrude-mut.check
+++ b/tests/neg-custom-args/captures/scope-extrude-mut.check
@@ -4,10 +4,10 @@
    |        Found:    (a1 : A^)
    |        Required: A^²
    |
-   |        Note that capability cap is not included in capture set {cap²}
-   |        because cap in method b is not visible from cap² in variable a.
+   |        Note that capability a1 is not included in capture set {cap}
+   |        because (a1 : A^) in method b is not visible from cap in variable a.
    |
-   |        where:    ^ and cap   refer to a fresh root capability classified as Mutable in the type of value a1
-   |                  ^² and cap² refer to a fresh root capability classified as Mutable in the type of variable a
+   |        where:    ^          refers to a fresh root capability classified as Mutable in the type of value a1
+   |                  ^² and cap refer to a fresh root capability classified as Mutable in the type of variable a
    |
    | longer explanation available when compiling with `-explain`
diff --git a/tests/neg-custom-args/captures/scope-extrusions.check b/tests/neg-custom-args/captures/scope-extrusions.check
new file mode 100644
index 000000000000..e20aa8995047
--- /dev/null
+++ b/tests/neg-custom-args/captures/scope-extrusions.check
@@ -0,0 +1,163 @@
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/scope-extrusions.scala:9:8 -------------------------------
+9 |    v = x  // error
+  |        ^
+  |     Found:    (x : IO)
+  |     Required: IO^
+  |
+  |     Note that capability x is not included in capture set {cap}
+  |     because (x : IO) in method f1 is not visible from cap in variable v.
+  |
+  |     where:    ^ and cap refer to a fresh root capability classified as SharedCapability in the type of variable v
+  |
+  | longer explanation available when compiling with `-explain`
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/scope-extrusions.scala:10:8 ------------------------------
+10 |    w = g  // error
+   |        ^
+   |        Found:    () ->{x} Unit
+   |        Required: () => Unit
+   |
+   |        Note that capability x is not included in capture set {cap}
+   |        because (x : IO) in method f1 is not visible from cap in variable w.
+   |
+   |        where:    => and cap refer to a fresh root capability in the type of variable w
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/scope-extrusions.scala:19:11 -----------------------------
+19 |  withFile(io => io)         // error
+   |           ^^^^^^^^
+   |Capability io outlives its scope: it leaks into outer capture set 's1 which is owned by method test.
+   |The leakage occurred when trying to match the following types:
+   |
+   |Found:    (io: IO^'s2) ->'s3 IO^{io}
+   |Required: IO^ => IO^'s1
+   |
+   |where:    => refers to a fresh root capability created in method test when checking argument to parameter op of method withFile
+   |          ^  refers to the universal root capability
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/scope-extrusions.scala:20:11 -----------------------------
+20 |  withFile(id)               // error
+   |           ^^
+   |Capability x outlives its scope: it leaks into outer capture set 's4 which is owned by method test.
+   |The leakage occurred when trying to match the following types:
+   |
+   |Found:    (x: IO^) ->'s5 IO^{x}
+   |Required: IO^ => IO^'s4
+   |
+   |where:    => refers to a fresh root capability created in method test when checking argument to parameter op of method withFile
+   |          ^  refers to the universal root capability
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/scope-extrusions.scala:21:11 -----------------------------
+21 |  withFile(x => id(x))       // error
+   |           ^^^^^^^^^^
+   |Capability x outlives its scope: it leaks into outer capture set 's6 which is owned by method test.
+   |The leakage occurred when trying to match the following types:
+   |
+   |Found:    (x: IO^'s7) ->'s8 IO^{x}
+   |Required: IO^ => IO^'s6
+   |
+   |where:    => refers to a fresh root capability created in method test when checking argument to parameter op of method withFile
+   |          ^  refers to the universal root capability
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/scope-extrusions.scala:22:11 -----------------------------
+22 |  withFile(id2)              // error, note mentions cap since we never have a more specific include failure
+   |           ^^^
+   |Capability cap outlives its scope: it leaks into outer capture set 's9 which is owned by method test.
+   |The leakage occurred when trying to match the following types:
+   |
+   |Found:    (x: IO^) ->'s10 IO^²
+   |Required: IO^ => IO^'s9
+   |
+   |where:    =>  refers to a fresh root capability created in method test when checking argument to parameter op of method withFile
+   |          ^   refers to the universal root capability
+   |          ^²  refers to a root capability associated with the result type of (x: IO^): IO^²
+   |          cap is a root capability associated with the result type of (x: IO^): IO^'s9
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/scope-extrusions.scala:23:11 -----------------------------
+23 |  withFile(x => id2(x))      // error, note mentions cap since we never have a more specific include failure
+   |           ^^^^^^^^^^^
+   |Capability cap outlives its scope: it leaks into outer capture set 's11 which is owned by method test.
+   |The leakage occurred when trying to match the following types:
+   |
+   |Found:    (x: IO^'s12) ->'s13 IO^
+   |Required: IO^² => IO^'s11
+   |
+   |where:    =>  refers to a fresh root capability created in method test when checking argument to parameter op of method withFile
+   |          ^   refers to a root capability associated with the result type of (x: IO^'s12): IO^
+   |          ^²  refers to the universal root capability
+   |          cap is a root capability associated with the result type of (x: IO^²): IO^'s11
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/scope-extrusions.scala:24:11 -----------------------------
+24 |  withFile(identity)         // error, note mentions cap since we never have a more specific include failure
+   |           ^^^^^^^^
+   |Capability cap outlives its scope: it leaks into outer capture set 's14 which is owned by method test.
+   |The leakage occurred when trying to match the following types:
+   |
+   |Found:    (x: IO^'s15) ->'s16 IO^'s17
+   |Required: IO^ => IO^'s14
+   |
+   |where:    =>  refers to a fresh root capability created in method test when checking argument to parameter op of method withFile
+   |          ^   refers to the universal root capability
+   |          cap is a root capability associated with the result type of (x: IO^): IO^'s14
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/scope-extrusions.scala:25:11 -----------------------------
+25 |  withFile(x => identity(x)) // error, note mentions cap since we never have a more specific include failure
+   |           ^^^^^^^^^^^^^^^^
+   |Capability cap outlives its scope: it leaks into outer capture set 's18 which is owned by method test.
+   |The leakage occurred when trying to match the following types:
+   |
+   |Found:    (x: IO^'s19) ->'s20 IO^'s21
+   |Required: IO^ => IO^'s18
+   |
+   |where:    =>  refers to a fresh root capability created in method test when checking argument to parameter op of method withFile
+   |          ^   refers to the universal root capability
+   |          cap is a root capability associated with the result type of (x: IO^): IO^'s18
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/scope-extrusions.scala:27:12 -----------------------------
+27 |  withFile: io =>     // error
+   |            ^
+   |Capability io outlives its scope: it leaks into outer capture set 's22 which is owned by method test.
+   |The leakage occurred when trying to match the following types:
+   |
+   |Found:    (io: IO^'s23) ->'s24 () ->{io} Unit
+   |Required: IO^ => () ->'s22 Unit
+   |
+   |where:    => refers to a fresh root capability created in method test when checking argument to parameter op of method withFile
+   |          ^  refers to the universal root capability
+28 |    () => println(io)
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/scope-extrusions.scala:30:21 -----------------------------
+30 |  val f2: IO => IO = (x: IO) => x  // error
+   |                     ^^^^^^^^^^^^
+   |    Found:    (x: IO^) ->'s25 IO^{x}
+   |    Required: IO^ => IO^²
+   |
+   |    Note that capability x is not included in capture set {cap}
+   |    because (x : IO) is not visible from cap in value f2.
+   |
+   |    where:    =>         refers to a fresh root capability in the type of value f2
+   |              ^          refers to the universal root capability
+   |              ^² and cap refer to a fresh root capability classified as SharedCapability in the type of value f2
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/scope-extrusions.scala:32:21 -----------------------------
+32 |  val f4: IO => IO = f3  // error
+   |                     ^^
+   |    Found:    (f3 : (x$0: IO) ->{} IO^{x$0})
+   |    Required: IO^ => IO^²
+   |
+   |    Note that capability x$0 is not included in capture set {cap}
+   |    because (x$0 : IO) is not visible from cap in value f4.
+   |
+   |    where:    =>         refers to a fresh root capability in the type of value f4
+   |              ^          refers to the universal root capability
+   |              ^² and cap refer to a fresh root capability classified as SharedCapability in the type of value f4
+   |
+   | longer explanation available when compiling with `-explain`
diff --git a/tests/neg-custom-args/captures/scope-extrusions.scala b/tests/neg-custom-args/captures/scope-extrusions.scala
new file mode 100644
index 000000000000..277a8df6cfbb
--- /dev/null
+++ b/tests/neg-custom-args/captures/scope-extrusions.scala
@@ -0,0 +1,33 @@
+// Tests quality of error messages
+class IO extends caps.SharedCapability
+
+def test(io: IO): Unit =
+  var v: IO = io
+  var w: () => Unit = () => ()
+  def f1(x: IO) =
+    def g() = println(x)
+    v = x  // error
+    w = g  // error
+
+  def withFile[T](op: IO => T): T =
+    val io = IO()
+    op(io)
+
+  def id(x: IO): x.type = x
+  def id2(x: IO): IO = x
+
+  withFile(io => io)         // error
+  withFile(id)               // error
+  withFile(x => id(x))       // error
+  withFile(id2)              // error, note mentions cap since we never have a more specific include failure
+  withFile(x => id2(x))      // error, note mentions cap since we never have a more specific include failure
+  withFile(identity)         // error, note mentions cap since we never have a more specific include failure
+  withFile(x => identity(x)) // error, note mentions cap since we never have a more specific include failure
+
+  withFile: io =>     // error
+    () => println(io)
+
+  val f2: IO => IO = (x: IO) => x  // error
+  val f3 = (x: IO) => x
+  val f4: IO => IO = f3  // error
+

From 2ea4ecd1f2223032e1bd751d874dbbd852826805 Mon Sep 17 00:00:00 2001
From: odersky <odersky@gmail.com>
Date: Tue, 4 Nov 2025 17:01:00 +0100
Subject: [PATCH 7/7] Rename prefix --> showAsPrefix

---
 compiler/src/dotty/tools/dotc/cc/CaptureSet.scala      | 2 +-
 compiler/src/dotty/tools/dotc/reporting/Message.scala  | 2 +-
 compiler/src/dotty/tools/dotc/reporting/messages.scala | 4 ++--
 3 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala b/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala
index 78af0e6c57c8..8cc30b03a389 100644
--- a/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala
+++ b/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala
@@ -1310,7 +1310,7 @@ object CaptureSet:
       res.myTrace = cs1 :: this.myTrace
       res
 
-    override def prefix(using Context) = cs match
+    override def showAsPrefix(using Context) = cs match
       case cs: Var =>
         !cs.levelOK(elem)
         || cs.isBadRoot(elem) && elem.isInstanceOf[FreshCap]
diff --git a/compiler/src/dotty/tools/dotc/reporting/Message.scala b/compiler/src/dotty/tools/dotc/reporting/Message.scala
index 73e9412830c4..f0bb57652fd9 100644
--- a/compiler/src/dotty/tools/dotc/reporting/Message.scala
+++ b/compiler/src/dotty/tools/dotc/reporting/Message.scala
@@ -47,7 +47,7 @@ object Message:
   	/** Should the note be shown before the actual message or after?
   	 *  Default is after.
   	 */
-    def prefix(using Context): Boolean = false
+    def showAsPrefix(using Context): Boolean = false
 
     /** The note rendered as part of an error message */
     def render(using Context): String
diff --git a/compiler/src/dotty/tools/dotc/reporting/messages.scala b/compiler/src/dotty/tools/dotc/reporting/messages.scala
index 3aebf257b5c5..c281cbab544e 100644
--- a/compiler/src/dotty/tools/dotc/reporting/messages.scala
+++ b/compiler/src/dotty/tools/dotc/reporting/messages.scala
@@ -343,7 +343,7 @@ class TypeMismatch(val found: Type, expected: Type, val inTree: Option[untpd.Tre
           mapOver(tp)
         case _ =>
           mapOver(tp)
-    val preface = notes.filter(_.prefix).map(_.render).mkString
+    val preface = notes.filter(_.showAsPrefix).map(_.render).mkString
     val found1 = reported(found)
     reported.setVariance(-1)
     val expected1 = reported(expected)
@@ -359,7 +359,7 @@ class TypeMismatch(val found: Type, expected: Type, val inTree: Option[untpd.Tre
     def importSuggestions =
       if expected.isTopType || found.isBottomType then ""
       else ctx.typer.importSuggestionAddendum(ViewProto(found.widen, expected))
-    notes.filter(!_.prefix).map(_.render).mkString ++ super.msgPostscript ++ importSuggestions
+    notes.filter(!_.showAsPrefix).map(_.render).mkString ++ super.msgPostscript ++ importSuggestions
 
   override def explain(using Context) =
     val treeStr = inTree.map(x => s"\nTree:\n\n${x.show}\n").getOrElse("")