[CSBindings] Don't attempt to join types that contain holes

Author: xedin

lib/AST/TypeJoinMeet.cpp

@@ -87,6 +87,9 @@ struct TypeJoin : CanTypeVisitor<TypeJoin, CanType> {
     assert(!first->hasTypeVariable() && !second->hasTypeVariable() &&
            "Cannot compute join of types involving type variables");
 
+    assert(!first->hasHole() && !second->hasHole() &&
+           "Cannot compute join of types involving type holes");
+
     assert(first->getWithoutSpecifierType()->isEqual(first) &&
            "Expected simple type!");
     assert(second->getWithoutSpecifierType()->isEqual(second) &&

lib/Sema/CSBindings.cpp

@@ -481,6 +481,7 @@ void ConstraintSystem::PotentialBindings::addPotentialBinding(
   if (binding.Kind == AllowedBindingKind::Supertypes &&
       !binding.BindingType->hasUnresolvedType() &&
       !binding.BindingType->hasTypeVariable() &&
+      !binding.BindingType->hasHole() &&
       !binding.BindingType->hasUnboundGenericType() &&
       !binding.hasDefaultedLiteralProtocol() &&
       !binding.isDefaultableBinding() && allowJoinMeet) {