[Sema] NFC: fix assert-only crasher with overloaded generics

Author: davezarzycki

lib/Sema/CSDiagnostics.cpp

@@ -136,7 +136,9 @@ ValueDecl *RequirementFailure::getDeclRef() const {
   auto *anchor = getRawAnchor();
   auto *locator = cs.getConstraintLocator(anchor);
   if (auto *AE = dyn_cast<CallExpr>(anchor)) {
-    assert(isa<TypeExpr>(AE->getFn()));
+    // NOTE: In valid code, the function can only be a TypeExpr
+    assert(isa<TypeExpr>(AE->getFn()) ||
+           isa<OverloadedDeclRefExpr>(AE->getFn()));
     ConstraintLocatorBuilder ctor(locator);
     locator = cs.getConstraintLocator(
         ctor.withPathElement(PathEltKind::ApplyFunction)

validation-test/Sema/type_checker_crashers_fixed/overloaded_generic.swift

@@ -0,0 +1,13 @@
+// RUN: not %target-swift-frontend %s -typecheck
+
+protocol A {}
+protocol B {}
+
+struct X : A {}
+struct Y : B {}
+
+struct G<T:A> { var v : T }
+struct G<T:B> { var v : T }
+
+let a = G(v: X())
+let b = G(v: Y())