[Scope map] Hack around AST issue involving ill-formed associated types.

DeclContext::getSelfTypeInContext() returns null types in some cases
of broken protocols. Hack around this in the ASTScope-based
unqualified name lookup code for now, with a narrower hack than what
the existing unqualified name lookup is doing (which is even *less*
principled).

Author: DougGregor

lib/AST/NameLookup.cpp

@@ -601,6 +601,13 @@ UnqualifiedLookup::UnqualifiedLookup(DeclName Name, DeclContext *DC,
         // Dig out the type we're looking into.
         // FIXME: We shouldn't need to compute a type to perform this lookup.
         Type lookupType = dc->getSelfTypeInContext();
+
+        // FIXME: Hack to deal with missing 'Self' archetypes.
+        if (!lookupType) {
+          if (auto proto = dc->getAsProtocolOrProtocolExtensionContext())
+            lookupType = proto->getDeclaredType();
+        }
+
         if (!lookupType || lookupType->is<ErrorType>()) continue;
 
         // If we're performing a static lookup, use the metatype.

test/NameBinding/scope_map_lookup.swift

@@ -22,6 +22,11 @@ protocol P1 {
   associatedtype A = Self
 }
 
+// Protocols involving associated types.
+protocol AProtocol {
+  associatedtype e : e  // expected-error {{inheritance from non-protocol, non-class type 'Self.e'}}
+}
+
 // Extensions.
 protocol P2 {
 }