IDE: invertible protocols have no members

We can avoid lookup entirely on invertible protocols because they never
can have any members. This saves us from having to update tests and
perhaps some extra work in code completion.

Author: kavon

lib/Sema/LookupVisibleDecls.cpp

@@ -271,6 +271,11 @@ static void lookupTypeMembers(Type BaseType, NominalTypeDecl *LookupType,
   assert(!BaseType->hasTypeParameter());
   assert(LookupType && "should have a nominal type");
 
+  // Skip lookup on invertible protocols. They have no members.
+  if (auto *proto = dyn_cast<ProtocolDecl>(LookupType))
+    if (proto->getInvertibleProtocolKind())
+      return;
+
   Consumer.onLookupNominalTypeMembers(LookupType, Reason);
 
   SmallVector<ValueDecl*, 2> FoundDecls;
@@ -444,6 +449,10 @@ static void lookupDeclsFromProtocolsBeingConformedTo(
 
   for (auto Conformance : CurrNominal->getAllConformances()) {
     auto Proto = Conformance->getProtocol();
+    // Skip conformances to invertible protocols. They have no members.
+    if (Proto->getInvertibleProtocolKind())
+      continue;
+
     if (!Proto->isAccessibleFrom(FromContext))
       continue;