[CSSimplify] Remove all InstanceType at the end of a locator to diagnose mismatched existential conversion

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -8755,8 +8755,17 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyConformsToConstraint(
   if (shouldAttemptFixes() && result.isFailure()) {
     auto *loc = getConstraintLocator(locator);
 
-    if (loc->isLastElement<LocatorPathElt::InstanceType>())
-      loc = getConstraintLocator(loc->getAnchor(), loc->getPath().drop_back());
+    ArrayRef<LocatorPathElt> path = loc->getPath();
+    while (!path.empty()) {
+      if (!path.back().is<LocatorPathElt::InstanceType>())
+        break;
+
+      path = path.drop_back();
+    }
+
+    if (path.size() != loc->getPath().size()) {
+      loc = getConstraintLocator(loc->getAnchor(), path);
+    }
 
     ConstraintFix *fix = nullptr;
     if (loc->isLastElement<LocatorPathElt::ApplyArgToParam>()) {

test/Constraints/existential_metatypes.swift

@@ -125,7 +125,8 @@ func parameterizedExistentials() {
 func testNestedMetatype() {
   struct S: P {}
 
-  func bar<T>(_ x: T) -> T.Type { type(of: x) }
+  func bar<T>(_ x: T) -> T.Type { }
+  func metaBar<T>(_ x: T) -> T.Type.Type { }
   func foo1(_ x: P.Type) {}
   func foo2(_ x: P.Type.Type) { }
 
@@ -134,4 +135,5 @@ func testNestedMetatype() {
   // Make sure we don't crash.
   foo2(bar(S.self))
   foo2(bar(0)) // expected-error {{cannot convert value of type 'Int' to expected argument type 'any P.Type'}}
+  foo2(metaBar(0)) // expected-error {{argument type 'Int' does not conform to expected type 'P'}}
 }