Merge pull request #11522 from Linyxus/fix-gadt-strip-refinement

Fix refinement stripping in GADT constrainer

Author: abgruszecki

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

@@ -140,8 +140,8 @@ trait PatternTypeConstrainer { self: TypeComparer =>
           either(constrainPatternType(pat1, scrut), constrainPatternType(pat2, scrut))
         case AndType(pat1, pat2) =>
           constrainPatternType(pat1, scrut) && constrainPatternType(pat2, scrut)
-        case scrut: RefinedOrRecType =>
-          constrainPatternType(stripRefinement(scrut), pat)
+        case pat: RefinedOrRecType =>
+          constrainPatternType(stripRefinement(pat), scrut)
         case pat =>
           constrainSimplePatternType(pat, scrut) || classesMayBeCompatible && constrainUpcasted(scrut)
       }

tests/pos/gadt-strip-refinement.scala

@@ -0,0 +1,6 @@
+trait Tag[T] { type X }
+class IntTag extends Tag[Int]
+
+def foo[T](x: Tag[T]): T = x match {
+  case _: IntTag { type X = String } => 0
+}