AST: Narrower workaround for "concrete conformance under abstract conformance" bug

Instead of just falling back to module lookup any time conformance
substitution fails, only do it in the case we know doesn't work.

This should shake out some more bugs, hopefully without causing
too much pain.

Author: slavapestov

lib/AST/ProtocolConformance.cpp

@@ -162,9 +162,7 @@ ProtocolConformanceRef::subst(Type origType,
     return ProtocolConformanceRef(lookupResults.front());
   }
 
-  // FIXME: Rip this out once ConformanceAccessPaths are plumbed through
-  auto *M = proto->getParentModule();
-  return *M->lookupConformance(substType, proto, nullptr);
+  llvm_unreachable("Invalid conformance substitution");
 }
 
 Type

lib/AST/SubstitutionMap.cpp

@@ -142,8 +142,21 @@ SubstitutionMap::lookupConformance(CanType type, ProtocolDecl *proto) const {
     // If we've hit an abstract conformance, everything from here on out is
     // abstract.
     // FIXME: This may not always be true, but it holds for now.
-    if (conformance->isAbstract())
+    if (conformance->isAbstract()) {
+      // FIXME: Rip this out once we can get a concrete conformance from
+      // an archetype.
+      auto *M = proto->getParentModule();
+      auto substType = type.subst(*this);
+      if (substType &&
+          !substType->is<ArchetypeType>() &&
+          !substType->isTypeParameter() &&
+          !substType->isExistentialType()) {
+        auto lazyResolver = M->getASTContext().getLazyResolver();
+        return *M->lookupConformance(substType, proto, lazyResolver);
+      }
+
       return ProtocolConformanceRef(proto);
+    }
 
     // For the second step, we're looking into the requirement signature for
     // this protocol.

lib/AST/Type.cpp

@@ -2946,17 +2946,7 @@ Optional<ProtocolConformanceRef>
 LookUpConformanceInSubstitutionMap::operator()(CanType dependentType,
                                        Type conformingReplacementType,
                                        ProtocolType *conformedProtocol) const {
-  auto result = Subs.lookupConformance(dependentType, conformedProtocol->getDecl());
-  if ((result && result->isConcrete()) ||
-      conformingReplacementType->hasError() ||
-      conformingReplacementType->isTypeParameter())
-    return result;
-
-  // FIXME: Rip this out once ConformanceAccessPaths are plumbed through
-  auto *M = conformedProtocol->getDecl()->getParentModule();
-  return M->lookupConformance(conformingReplacementType,
-                              conformedProtocol->getDecl(),
-                              nullptr);
+  return Subs.lookupConformance(dependentType, conformedProtocol->getDecl());
 }
 
 Optional<ProtocolConformanceRef>