Sema: Fix requirement/witness disambiguation for subclass existentials

When performing a lookup into a class C, we might find a member of
C that witnesses a requirement in a protocol Q that C conforms to.

In this case, AST name lookup returns both results.

There are two further levels of fitering which can eliminate the
ambiguity:

- Sema name lookup maps requirements to witnesses if the base type
  of the lookup is a nominal type or a class-constrained archetype.

  Imported conformances don't have this mapping recorded, but there's
  another hack in this code path where if a requirement maps to
  itself inside a conformance, it is dropped from the results
  altogether.

- If the duplicate results were not filtered out in Sema name lookup,
  there was another hack in CSRanking which would a witness higher
  than it's requirement when comparing solutions. This doesn't work
  for imported conformances, but usually name lookup filters out
  the duplicate results sooner, which also eliminates the exponential
  behavior from having multiple constraint system solutions.

However, if we have a subclass existential C & P where C conforms
to Q and we find a member of C that witnesses a requirement of Q,
then (C & P) does not conform to Q.

So if the conformance was imported, *both* of the above checks
would fail to disambiguate the witness and requirement, and the
member access would fail to type check.

To make this work with imported conformances, teach Sema name lookup
to extract the superclass from an existential before performing the
conformance check. If the conformance check fails, it means we
found the protocol member via the 'existential' part of the subclass
existential (eg, in our case, a member of P), and we proceed as
before.

Fixes <rdar://problem/33291112>.

Author: slavapestov

lib/Sema/TypeCheckNameLookup.cpp

@@ -16,6 +16,7 @@
 //
 //===----------------------------------------------------------------------===//
 #include "TypeChecker.h"
+#include "swift/AST/ExistentialLayout.h"
 #include "swift/AST/Initializer.h"
 #include "swift/AST/NameLookup.h"
 #include "swift/AST/ProtocolConformance.h"
@@ -121,27 +122,48 @@ namespace {
       if (!Options.contains(NameLookupFlags::ProtocolMembers) ||
           !isa<ProtocolDecl>(foundDC) ||
           isa<GenericTypeParamDecl>(found) ||
-          (isa<FuncDecl>(found) && cast<FuncDecl>(found)->isOperator()) ||
-          foundInType->isAnyExistentialType()) {
+          (isa<FuncDecl>(found) && cast<FuncDecl>(found)->isOperator())) {
         addResult(found);
         return;
       }
 
       assert(isa<ProtocolDecl>(foundDC));
 
+      auto conformingType = foundInType;
+
+      // When performing a lookup on a subclass existential, we might
+      // find a member of the class that witnesses a requirement on a
+      // protocol that the class conforms to.
+      //
+      // Since subclass existentials don't normally conform to protocols,
+      // pull out the superclass instead, and use that below.
+      if (foundInType->isExistentialType()) {
+        auto layout = foundInType->getExistentialLayout();
+        if (layout.superclass)
+          conformingType = layout.superclass;
+      }
+
       // If we found something within the protocol itself, and our
       // search began somewhere that is not in a protocol or extension
       // thereof, remap this declaration to the witness.
       if (foundInType->is<ArchetypeType>() ||
+          foundInType->isExistentialType() ||
           Options.contains(NameLookupFlags::PerformConformanceCheck)) {
         // Dig out the protocol conformance.
-        auto conformance = TC.conformsToProtocol(foundInType, foundProto, DC,
+        auto conformance = TC.conformsToProtocol(conformingType, foundProto, DC,
                                                  conformanceOptions);
-        if (!conformance)
+        if (!conformance) {
+          // If there's no conformance, we have an existential
+          // and we found a member from one of the protocols, and
+          // not a class constraint if any.
+          assert(foundInType->isExistentialType());
+          addResult(found);
           return;
+        }
 
         if (conformance->isAbstract()) {
-          assert(foundInType->is<ArchetypeType>());
+          assert(foundInType->is<ArchetypeType>() ||
+                 foundInType->isExistentialType());
           addResult(found);
           return;
         }
@@ -158,6 +180,10 @@ namespace {
 
         // FIXME: the "isa<ProtocolDecl>()" check will be wrong for
         // default implementations in protocols.
+        //
+        // If we have an imported conformance or the witness could
+        // not be deserialized, getWitnessDecl() will just return
+        // the requirement, so just drop the lookup result here.
         if (witness && !isa<ProtocolDecl>(witness->getDeclContext()))
           addResult(witness);
 

test/ClangImporter/subclass_existentials.swift

@@ -35,3 +35,7 @@ class OldSwiftLaundryService : NSLaundry {
     return g!
   }
 }
+
+// Make sure the method lookup is not ambiguous
+
+_ = Coat.fashionStatement.wear()

test/Inputs/clang-importer-sdk/usr/include/Foundation.h

@@ -1098,13 +1098,21 @@ typedef enum __attribute__((ns_error_domain(FictionalServerErrorDomain))) Fictio
   FictionalServerErrorMeltedDown = 1
 } FictionalServerErrorCode;
 
+@protocol Wearable
+- (void)wear;
+@end
+
 @protocol Garment
 @end
 
 @protocol Cotton
 @end
 
-@interface Coat
+@interface Coat : NSObject<Wearable>
+
+- (void)wear;
+@property (class) Coat <Wearable> *fashionStatement;
+
 @end
 
 @protocol NSLaundry