fix(patmat): Use childPrefix to resolve path-dependent sealed children

Author: nox213

compiler/src/dotty/tools/dotc/core/TypeOps.scala

@@ -748,7 +748,22 @@ object TypeOps:
     //
     if (child.name == tpnme.LOCAL_CHILD) return child.typeRef
 
-    val childTp = if (child.isTerm) child.termRef else child.typeRef
+    val childTp = {
+      val parentSym = parent.typeSymbol
+      val parentPre = parent.normalizedPrefix
+      val defaultRef = if (child.isTerm) child.termRef else child.typeRef
+
+      if (parentSym.isClass && parentPre.isInstanceOf[TermRef]) {
+        val newPrefix = childPrefix(parentPre, parentSym, child)
+        if (newPrefix.exists)
+          if (child.isTerm) TermRef(newPrefix, child.asTerm)
+          else TypeRef(newPrefix, child.asType)
+        else
+          defaultRef
+      }
+      else
+        defaultRef
+    }
 
     inContext(ctx.fresh.setExploreTyperState().setFreshGADTBounds.addMode(Mode.GadtConstraintInference)) {
       instantiateToSubType(childTp, parent, mixins).dealias

tests/warn/i23369.scala

@@ -0,0 +1,16 @@
+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() =>
+  }
+}