AST: Perform a top-level lookup in directReferencesForUnqualifiedTypeLookup() if necessary

UnqualifiedLookupRequest takes a DeclContext and a SourceLoc. If the
SourceLoc is valid, it locates the innermost ASTScope containing this
location, and starts the lookup from that scope.

Also, ASTScope currently walks up the scope tree to find the
innermost scope that corresponds to the given DeclContext, in case
the DeclContext is a parent of the 'natural' DeclContext for this
source location.

We want to remove this additional behavior and make ASTScope only
depend on a source file and source location.

This requires changing directReferencesForUnqualifiedTypeLookup()
to handle the top-level lookup case explicitly. See the comment for
more details.

Author: slavapestov

lib/AST/NameLookup.cpp

@@ -1946,7 +1946,33 @@ static DirectlyReferencedTypeDecls
 directReferencesForUnqualifiedTypeLookup(DeclNameRef name,
                                          SourceLoc loc, DeclContext *dc,
                                          LookupOuterResults lookupOuter) {
+  // In a protocol or protocol extension, the 'where' clause can refer to
+  // associated types without 'Self' qualification:
+  //
+  // protocol MyProto where AssocType : Q { ... }
+  //
+  // extension MyProto where AssocType == Int { ... }
+  //
+  // For this reason, ASTScope maps source locations inside the 'where'
+  // clause to a scope that performs the lookup into the protocol or
+  // protocol extension.
+  //
+  // However, protocol and protocol extensions can also put bounds on 'Self',
+  // for example:
+  //
+  // protocol MyProto where Self : MyClass { ... }
+  //
+  // We must start searching for 'MyClass' at the top level, otherwise
+  // we end up with a cycle, because qualified lookup wants to resolve
+  // 'Self' bounds to build the set of declarations to search inside of.
+  //
+  // To make this work, we handle the top-level lookup case explicitly
+  // here, bypassing unqualified lookup and ASTScope altogether.
+  if (dc->isModuleScopeContext())
+    loc = SourceLoc();
+
   DirectlyReferencedTypeDecls results;
+
   UnqualifiedLookupOptions options =
       UnqualifiedLookupFlags::TypeLookup |
       UnqualifiedLookupFlags::AllowProtocolMembers;
@@ -1958,8 +1984,8 @@ directReferencesForUnqualifiedTypeLookup(DeclNameRef name,
   auto lookup = evaluateOrDefault(ctx.evaluator,
                                   UnqualifiedLookupRequest{descriptor}, {});
   for (const auto &result : lookup.allResults()) {
-    if (auto typeDecl = dyn_cast<TypeDecl>(result.getValueDecl()))
-      results.push_back(typeDecl);
+    auto typeDecl = cast<TypeDecl>(result.getValueDecl());
+    results.push_back(typeDecl);
   }
 
   return results;