Fix #6443: Widen typerefs to underlying lambdas in TypeTestCasts

compiler/src/dotty/tools/dotc/transform/TypeTestsCasts.scala

@@ -92,7 +92,12 @@ object TypeTestsCasts {
 
     def isClassDetermined(X: Type, P: AppliedType)(implicit ctx: Context) = {
       val AppliedType(tycon, _) = P
-      val typeLambda = tycon.ensureLambdaSub.asInstanceOf[TypeLambda]
+
+      def underlyingLambda(tp: Type): TypeLambda = tp.ensureLambdaSub match {
+        case tp: TypeLambda => tp
+        case tp: TypeProxy => underlyingLambda(tp.superType)
+      }
+      val typeLambda = underlyingLambda(tycon)
       val tvars = constrained(typeLambda, untpd.EmptyTree, alwaysAddTypeVars = true)._2.map(_.tpe)
       val P1 = tycon.appliedTo(tvars)
 

tests/pos/i6443.scala

@@ -0,0 +1,9 @@
+object Test {
+  trait Expr { type T }
+  type ExprExact[A] = Expr { type T = A }
+  type IndirectExprExact[A] = Expr { type S = A; type T = S }
+
+  def foo[A](e: ExprExact[A]): Unit = e match {
+    case _: IndirectExprExact[Int] =>
+  }
+}