Sema: Carefully tweak shadowing rules

TypeChecker::lookupMemberType() performs a lookup on the base type,
and disambiguates multiple results by filtering out result decls
that have canonically equal types under the base type substitution.

I'd like to refactor conformance checking to call lookupQualified()
directly and bypass lookupMemberType(), but this requires adding a
new shadowing rule to primitive name lookup to simulate the effect
of the type-based disambiguation.

We already had a shadowing rule which states that concrete type
members shadow protocol members of the same name, but this was
bypassed for member _types_. This change generalizes this rule to
apply to member types also.

Hopefully this doesn't break source compatibility in practice.

Author: slavapestov

lib/AST/NameLookup.cpp

@@ -312,6 +312,19 @@ static void recordShadowedDeclsAfterTypeMatch(
           }
         }
 
+        // If one declaration is in a protocol or extension thereof and the
+        // other is not, prefer the one that is not.
+        if ((bool)firstDecl->getDeclContext()->getSelfProtocolDecl() !=
+              (bool)secondDecl->getDeclContext()->getSelfProtocolDecl()) {
+          if (firstDecl->getDeclContext()->getSelfProtocolDecl()) {
+            shadowed.insert(firstDecl);
+            break;
+          } else {
+            shadowed.insert(secondDecl);
+            continue;
+          }
+        }
+
         continue;
       }
 

test/NameLookup/member_type_shadowing.swift

@@ -29,3 +29,17 @@ class B: A {
     _ = B.Reusable.option2 // expected-error {{type 'B.Reusable' has no member 'option2'; did you mean 'option1'?}}
   }
 }
+
+protocol Q {
+  typealias A = Int
+}
+
+struct S : Q {
+  typealias A = String
+}
+
+func usesA(_: S.A) {} // should resolve to the typealias inside S
+
+func callsA() {
+  usesA("hello")
+}