Sema: Require CS to certify conversions associated w/ existential member accesses

lib/Sema/CSSimplify.cpp

@@ -5620,6 +5620,23 @@ bool ConstraintSystem::repairFailures(
     }
   }
 
+  // If there is a conversion associated with an existential member access
+  // along the path, the problem is that the constraint system does not support
+  // the (formally sane) upcast required to access the member.
+  if (llvm::find_if(path, [](const LocatorPathElt &elt) -> bool {
+        return elt.is<LocatorPathElt::ExistentialMemberAccessConversion>();
+      }) != path.end()) {
+    if (auto overload = findSelectedOverloadFor(castToExpr(anchor))) {
+      auto &choice = overload->choice;
+      conversionsOrFixes.push_back(AllowMemberRefOnExistential::create(
+          *this, choice.getBaseType(), choice.getDecl(),
+          DeclNameRef(choice.getDecl()->getName()),
+          getConstraintLocator(locator)));
+
+      return true;
+    }
+  }
+
   auto elt = path.back();
   switch (elt.getKind()) {
   case ConstraintLocator::LValueConversion: {
@@ -15153,7 +15170,8 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyFixConstraint(
   case FixKind::GenericArgumentsMismatch:
   case FixKind::AllowConcreteTypeSpecialization:
   case FixKind::IgnoreGenericSpecializationArityMismatch:
-  case FixKind::IgnoreKeyPathSubscriptIndexMismatch: {
+  case FixKind::IgnoreKeyPathSubscriptIndexMismatch:
+  case FixKind::AllowMemberRefOnExistential: {
     return recordFix(fix) ? SolutionKind::Error : SolutionKind::Solved;
   }
   case FixKind::IgnoreThrownErrorMismatch: {
@@ -15376,7 +15394,6 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyFixConstraint(
   case FixKind::RemoveCall:
   case FixKind::RemoveUnwrap:
   case FixKind::DefineMemberBasedOnUse:
-  case FixKind::AllowMemberRefOnExistential:
   case FixKind::AllowTypeOrInstanceMember:
   case FixKind::AllowInvalidPartialApplication:
   case FixKind::AllowInvalidInitRef: