[ConstraintSystem] Don't propagate holes through supertype inference

Disallow holes to be inferred as supertype bindings  while inferring
bindings from other type variables (based on transitivity of subtype
conversions), otherwise it would be possible to record a hole without
recording associated fix.

Resolves: rdar://problem/64368545

Author: xedin

lib/Sema/CSBindings.cpp

@@ -69,6 +69,9 @@ void ConstraintSystem::inferTransitiveSupertypeBindings(
 
       auto type = binding.BindingType;
 
+      if (type->isHole())
+        continue;
+
       if (!existingTypes.insert(type->getCanonicalType()).second)
         continue;
 

test/Constraints/overload.swift

@@ -239,3 +239,10 @@ func autoclosure1<T>(_: [T], _: X) { }
 func test_autoclosure1(ia: [Int]) {
   autoclosure1(ia, X()) // okay: resolves to the second function
 }
+
+// rdar://problem/64368545 - failed to produce diagnostic (hole propagated to func result without recording a fix)
+func test_no_hole_propagation() {
+  func test(withArguments arguments: [String]) -> String {
+    return arguments.reduce(0, +) // expected-error {{cannot convert value of type 'Int' to expected argument type 'String'}}
+  }
+}