[Sema] Account for ExistentialType in lookupExistentialConformance.

Author: hborla

lib/AST/Module.cpp

@@ -978,13 +978,17 @@ ModuleDecl::lookupExistentialConformance(Type type, ProtocolDecl *protocol) {
   // existential to an archetype parameter, so for now we restrict this to
   // @objc protocols and marker protocols.
   if (!layout.isObjC() && !protocol->isMarkerProtocol()) {
+    auto constraint = type;
+    if (auto existential = constraint->getAs<ExistentialType>())
+      constraint = existential->getConstraintType();
+
     // There's a specific exception for protocols with self-conforming
     // witness tables, but the existential has to be *exactly* that type.
     // TODO: synthesize witness tables on-demand for protocol compositions
     // that can satisfy the requirement.
     if (protocol->requiresSelfConformanceWitnessTable() &&
-        type->is<ProtocolType>() &&
-        type->castTo<ProtocolType>()->getDecl() == protocol)
+        constraint->is<ProtocolType>() &&
+        constraint->castTo<ProtocolType>()->getDecl() == protocol)
       return ProtocolConformanceRef(ctx.getSelfConformance(protocol));
 
     return ProtocolConformanceRef::forInvalid();