[Diagnostics] Do more checking before recording force downcast fix

Solver should do more checking upfront before recording
`force downcast` fix, to make sure that it's indeed always
applicable when recorded, otherwise it would be possible
to misdiagnose or omit diagnostics in certain situations.

Resolves: rdar://problem/65254452

Author: xedin

lib/Sema/CSDiagnostics.cpp

@@ -988,8 +988,6 @@ bool MissingExplicitConversionFailure::diagnoseAsError() {
     return false;
 
   bool useAs = TypeChecker::isExplicitlyConvertibleTo(fromType, toType, DC);
-  if (!useAs && !TypeChecker::checkedCastMaySucceed(fromType, toType, DC))
-    return false;
 
   auto *expr = findParentExpr(anchor);
   if (!expr)

lib/Sema/CSSimplify.cpp

@@ -2968,6 +2968,9 @@ static bool
 repairViaBridgingCast(ConstraintSystem &cs, Type fromType, Type toType,
                       SmallVectorImpl<RestrictionOrFix> &conversionsOrFixes,
                       ConstraintLocatorBuilder locator) {
+  if (fromType->hasTypeVariable() || toType->hasTypeVariable())
+    return false;
+
   auto objectType1 = fromType->getOptionalObjectType();
   auto objectType2 = toType->getOptionalObjectType();
 
@@ -2986,6 +2989,9 @@ repairViaBridgingCast(ConstraintSystem &cs, Type fromType, Type toType,
   if (!canBridgeThroughCast(cs, fromType, toType))
     return false;
 
+  if (!TypeChecker::checkedCastMaySucceed(fromType, toType, cs.DC))
+    return false;
+
   conversionsOrFixes.push_back(ForceDowncast::create(
       cs, fromType, toType, cs.getConstraintLocator(locator)));
   return true;

test/Constraints/rdar65254452.swift

@@ -0,0 +1,17 @@
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -verify %s
+// REQUIRES: objc_interop
+
+import Foundation
+
+class Obj: NSObject {
+}
+
+class Container {
+  var objects: [Obj]
+  init(objects: [Obj]) {}
+}
+
+func test(other: Container) {
+  _ = Container(objects: other)
+  // expected-error@-1 {{cannot convert value of type 'Container' to expected argument type '[Obj]'}}
+}