Don't check for matchability if TypeTest is used

Author: odersky

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -771,13 +771,17 @@ class Typer extends Namer
       def typedTpt = checkSimpleKinded(typedType(tree.tpt))
       def handlePattern: Tree = {
         val tpt1 = typedTpt
-        if (!ctx.isAfterTyper && pt != defn.ImplicitScrutineeTypeRef)
-          checkMatchable(pt, tree.srcPos, pattern = true)
+        if !ctx.isAfterTyper && pt != defn.ImplicitScrutineeTypeRef then
           withMode(Mode.GadtConstraintInference) {
             TypeComparer.constrainPatternType(tpt1.tpe, pt)
           }
+        val matched = ascription(tpt1, isWildcard = true)
         // special case for an abstract type that comes with a class tag
-        tryWithTypeTest(ascription(tpt1, isWildcard = true), pt)
+        val result = tryWithTypeTest(ascribed, pt)
+        if (result eq matched) && pt != defn.ImplicitScrutineeTypeRef then
+          // no check for matchability if TestTest was applied
+          checkMatchable(pt, tree.srcPos, pattern = true)
+        result
       }
       cases(
         ifPat = handlePattern,

tests/neg-custom-args/fatal-warnings/type-test-matchable.scala

@@ -0,0 +1,13 @@
+import scala.language.`3.1-migration`
+import scala.reflect.TypeTest
+
+trait Foo:
+  type X
+  type Y <: X
+  def x: X
+  given TypeTest[X, Y] = ???
+
+object Test:
+  def test(foo: Foo): Unit =
+    foo.x match
+      case x: foo.Y =>