From cd43a6e0d6a79264445ce14cdef8175c9768ba75 Mon Sep 17 00:00:00 2001
From: Yoonjae Jeon <yoonjae.jeon@devsisters.com>
Date: Sun, 17 Aug 2025 22:19:05 +0900
Subject: [PATCH] Add subtype-based fallback in inferPrefixMap

add new line

second approach

revert previous approach

address reviews

[Cherry-picked 4fd7a90f18ceec027cf6378951a3d71a8649de3c]
---
 .../src/dotty/tools/dotc/core/TypeOps.scala   |  9 +++++--
 tests/warn/i23369.scala                       | 26 +++++++++++++++++++
 2 files changed, 33 insertions(+), 2 deletions(-)
 create mode 100644 tests/warn/i23369.scala

diff --git a/compiler/src/dotty/tools/dotc/core/TypeOps.scala b/compiler/src/dotty/tools/dotc/core/TypeOps.scala
index cf03273b4805..f1936ff8cd01 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeOps.scala
+++ b/compiler/src/dotty/tools/dotc/core/TypeOps.scala
@@ -805,9 +805,13 @@ object TypeOps:
           prefixTVar.uncheckedNN
         case ThisType(tref) if !tref.symbol.isStaticOwner =>
           val symbol = tref.symbol
+          val compatibleSingleton = singletons.valuesIterator.find(_.underlying.derivesFrom(symbol))
           if singletons.contains(symbol) then
             prefixTVar = singletons(symbol) // e.g. tests/pos/i16785.scala, keep Outer.this
             prefixTVar.uncheckedNN
+          else if compatibleSingleton.isDefined then
+            prefixTVar = compatibleSingleton.get
+            prefixTVar.uncheckedNN
           else if symbol.is(Module) then
             TermRef(this(tref.prefix), symbol.sourceModule)
           else if (prefixTVar != null)
@@ -905,10 +909,11 @@ object TypeOps:
     }
 
     val inferThisMap = new InferPrefixMap
-    val tvars = tp1.etaExpand match
+    val prefixInferredTp = inferThisMap(tp1)
+    val tvars = prefixInferredTp.etaExpand match
       case eta: TypeLambda => constrained(eta)
       case _               => Nil
-    val protoTp1 = inferThisMap.apply(tp1).appliedTo(tvars)
+    val protoTp1 = prefixInferredTp.appliedTo(tvars)
 
     if gadtSyms.nonEmpty then
       ctx.gadtState.addToConstraint(gadtSyms)
diff --git a/tests/warn/i23369.scala b/tests/warn/i23369.scala
new file mode 100644
index 000000000000..5f4130ab96d3
--- /dev/null
+++ b/tests/warn/i23369.scala
@@ -0,0 +1,26 @@
+class Module {
+  type BarTy
+  sealed trait Adt[A]
+  case class Foo() extends Adt[String]
+  case class Bar[A <: BarTy](x: BarTy) extends Adt[A]
+}
+
+object Basic extends Module {
+  type BarTy = String
+}
+
+def test(a: Basic.Adt[String]) = {
+  a match { // warn: match may not be exhaustive
+    case Basic.Foo() =>
+  }
+}
+
+object Basic2 extends Module {
+  type BarTy = Int
+}
+
+def test2(a: Basic2.Adt[String]) = {
+  a match {
+    case Basic2.Foo() =>
+  }
+}