Don't classify capture sets as Mutable

odersky · odersky · commit d7666bf35a5b · 2025-11-14T18:59:14.000+01:00
We erroneously used Mutable as a classifier when calculating the implied
capture set of a constructor.
diff --git a/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala b/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
@@ -971,7 +971,7 @@ class CheckCaptures extends Recheck, SymTransformer:
               .getOrElse(cls, cls.info.decls.toList)           // pick all symbols in class scope for other classes
               .flatMap(classifiersOfFreshInType)
           if cls.typeRef.isMutableType then
-            fieldClassifiers = defn.Caps_Mutable :: fieldClassifiers
+            fieldClassifiers = defn.AnyClass :: fieldClassifiers
           val parentClassifiers =
             cls.parentSyms.map(impliedClassifiers).filter(_.nonEmpty)
           if fieldClassifiers.isEmpty && parentClassifiers.isEmpty
diff --git a/tests/neg-custom-args/captures/check-inferred.check b/tests/neg-custom-args/captures/check-inferred.check
@@ -58,4 +58,4 @@
    |           Fields capturing a root capability need to be given an explicit type unless the capability is already
    |           subsumed by the computed capability of the enclosing class.
    |
-   |           where:    ^ refers to a fresh root capability classified as Mutable in the type of value count
+   |           where:    ^ refers to a fresh root capability in the type of value count
diff --git a/tests/neg-custom-args/captures/i23726.check b/tests/neg-custom-args/captures/i23726.check
@@ -13,7 +13,7 @@
    |  Footprint set of function result      : {a}
    |  The two sets overlap at               : {a}
    |
-   |where:    ^  refers to a fresh root capability classified as Mutable in the type of value a
+   |where:    ^  refers to a fresh root capability in the type of value a
    |          ^² refers to a fresh root capability created in method test1 when checking argument to parameter x of method apply
 -- Error: tests/neg-custom-args/captures/i23726.scala:16:5 -------------------------------------------------------------
 16 |  f3(b)  // error
@@ -30,7 +30,7 @@
    |  Footprint set of function result      : {op, b}
    |  The two sets overlap at               : {b}
    |
-   |where:    ^  refers to a fresh root capability classified as Mutable in the type of value b
+   |where:    ^  refers to a fresh root capability in the type of value b
    |          ^² refers to a fresh root capability created in method test1 when checking argument to parameter x of method apply
 -- Error: tests/neg-custom-args/captures/i23726.scala:24:5 -------------------------------------------------------------
 24 |  f7(a)  // error
@@ -47,5 +47,5 @@
    |  Footprint set of function prefix      : {f7*, a, b}
    |  The two sets overlap at               : {a}
    |
-   |where:    ^  refers to a fresh root capability classified as Mutable in the type of value a
+   |where:    ^  refers to a fresh root capability in the type of value a
    |          ^² refers to a fresh root capability created in method test1 when checking argument to parameter x of method apply
diff --git a/tests/neg-custom-args/captures/i24335.check b/tests/neg-custom-args/captures/i24335.check
@@ -14,4 +14,4 @@
    |       Fields capturing a root capability need to be given an explicit type unless the capability is already
    |       subsumed by the computed capability of the enclosing class.
    |
-   |       where:    ^ refers to a fresh root capability classified as Mutable in the type of value r
+   |       where:    ^ refers to a fresh root capability in the type of value r
diff --git a/tests/neg-custom-args/captures/mut-iterator4.check b/tests/neg-custom-args/captures/mut-iterator4.check
@@ -13,7 +13,7 @@
   |
   |Note that capability cap is not included in capture set {Iterator.this, f}.
   |
-  |where:    cap is a fresh root capability classified as Mutable created in method map when constructing instance Object with (Iterator[U]^{cap².rd}) {...}
+  |where:    cap is a fresh root capability created in method map when constructing instance Object with (Iterator[U]^{cap².rd}) {...}
   |
   | longer explanation available when compiling with `-explain`
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/mut-iterator4.scala:23:34 --------------------------------
@@ -24,6 +24,6 @@
    |
    |Note that capability cap is not included in capture set {it, f}.
    |
-   |where:    cap is a fresh root capability classified as Mutable created in method mappedIterator when constructing instance Object with (Iterator[U]^{cap².rd}) {...}
+   |where:    cap is a fresh root capability created in method mappedIterator when constructing instance Object with (Iterator[U]^{cap².rd}) {...}
    |
    | longer explanation available when compiling with `-explain`
diff --git a/tests/neg-custom-args/captures/scope-extrude-mut.check b/tests/neg-custom-args/captures/scope-extrude-mut.check
@@ -7,7 +7,7 @@
    |        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:    ^          refers to a fresh root capability classified as Mutable in the type of value a1
+   |        where:    ^          refers to a fresh root capability in the type of value a1
    |                  ^² and cap refer to a fresh root capability in the type of variable a
    |
    | longer explanation available when compiling with `-explain`
diff --git a/tests/neg-custom-args/captures/sep-counter.check b/tests/neg-custom-args/captures/sep-counter.check
@@ -1,12 +1,12 @@
 -- Error: tests/neg-custom-args/captures/sep-counter.scala:12:19 -------------------------------------------------------
 12 |  def mkCounter(): Pair[Ref^, Ref^] =  // error
    |                   ^^^^^^^^^^^^^^^^
-   |Separation failure in method mkCounter's result type Pair[Ref^, Ref^²].
-   |One part,  Ref^, hides capabilities  {c, cap, cap²}.
-   |Another part,  Ref^²,  captures capabilities  {c}.
-   |The two sets overlap at  {c}.
+   |                   Separation failure in method mkCounter's result type Pair[Ref^, Ref^²].
+   |                   One part,  Ref^, hides capabilities  {c, cap, cap²}.
+   |                   Another part,  Ref^²,  captures capabilities  {c}.
+   |                   The two sets overlap at  {c}.
    |
-   |where:    ^    refers to a fresh root capability in the result type of method mkCounter
-   |          ^²   refers to a fresh root capability in the result type of method mkCounter
-   |          cap  is a fresh root capability classified as Mutable in the type of value c
-   |          cap² is a fresh root capability classified as Mutable created in value c when constructing instance Ref
+   |                   where:    ^    refers to a fresh root capability in the result type of method mkCounter
+   |                             ^²   refers to a fresh root capability in the result type of method mkCounter
+   |                             cap  is a fresh root capability in the type of value c
+   |                             cap² is a fresh root capability created in value c when constructing instance Ref

Original file line number	Diff line number	Diff line change
`@@ -58,4 +58,4 @@`
`58`	`58`	`\| Fields capturing a root capability need to be given an explicit type unless the capability is already`
`59`	`59`	`\| subsumed by the computed capability of the enclosing class.`
`60`	`60`	`\|`
`61`		`- \| where: ^ refers to a fresh root capability classified as Mutable in the type of value count`
	`61`	`+ \| where: ^ refers to a fresh root capability in the type of value count`
Original file line number	Diff line number	Diff line change
`@@ -14,4 +14,4 @@`
`14`	`14`	`\| Fields capturing a root capability need to be given an explicit type unless the capability is already`
`15`	`15`	`\| subsumed by the computed capability of the enclosing class.`
`16`	`16`	`\|`
`17`		`- \| where: ^ refers to a fresh root capability classified as Mutable in the type of value r`
	`17`	`+ \| where: ^ refers to a fresh root capability in the type of value r`
Original file line number	Diff line number	Diff line change
`@@ -13,7 +13,7 @@`
`13`	`13`	`\|`
`14`	`14`	`\|Note that capability cap is not included in capture set {Iterator.this, f}.`
`15`	`15`	`\|`
`16`		`- \|where: cap is a fresh root capability classified as Mutable created in method map when constructing instance Object with (Iterator[U]^{cap².rd}) {...}`
	`16`	`+ \|where: cap is a fresh root capability created in method map when constructing instance Object with (Iterator[U]^{cap².rd}) {...}`
`17`	`17`	`\|`
`18`	`18`	\| longer explanation available when compiling with `-explain`
`19`	`19`	`-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/mut-iterator4.scala:23:34 --------------------------------`
`@@ -24,6 +24,6 @@`
`24`	`24`	`\|`
`25`	`25`	`\|Note that capability cap is not included in capture set {it, f}.`
`26`	`26`	`\|`
`27`		`- \|where: cap is a fresh root capability classified as Mutable created in method mappedIterator when constructing instance Object with (Iterator[U]^{cap².rd}) {...}`
	`27`	`+ \|where: cap is a fresh root capability created in method mappedIterator when constructing instance Object with (Iterator[U]^{cap².rd}) {...}`
`28`	`28`	`\|`
`29`	`29`	\| longer explanation available when compiling with `-explain`
Original file line number	Diff line number	Diff line change
`@@ -7,7 +7,7 @@`
`7`	`7`	`\| Note that capability a1 is not included in capture set {cap}`
`8`	`8`	`\| because (a1 : A^) in method b is not visible from cap in variable a.`
`9`	`9`	`\|`
`10`		`- \| where: ^ refers to a fresh root capability classified as Mutable in the type of value a1`
	`10`	`+ \| where: ^ refers to a fresh root capability in the type of value a1`
`11`	`11`	`\| ^² and cap refer to a fresh root capability in the type of variable a`
`12`	`12`	`\|`
`13`	`13`	\| longer explanation available when compiling with `-explain`