[Type checker] Make DependentGenericTypeResolver::areSameType() more reasonable.

If the structure of the types matches, consider the types to be the same.
This implementation is from ProtocolRequirementTypeResolver::areSameType().

Author: DougGregor

lib/Sema/TypeCheckGeneric.cpp

@@ -43,11 +43,20 @@ Type DependentGenericTypeResolver::resolveDependentMemberType(
 }
 
 bool DependentGenericTypeResolver::areSameType(Type type1, Type type2) {
-  if (!type1->hasTypeParameter() && !type2->hasTypeParameter())
-    return type1->isEqual(type2);
+  if (type1->isEqual(type2))
+    return true;
+
+  // If both refer to associated types with the same name, they'll implicitly
+  // be considered equivalent.
+  auto depMem1 = type1->getAs<DependentMemberType>();
+  if (!depMem1) return false;
 
-  // Conservative answer: they could be the same.
-  return true;
+  auto depMem2 = type2->getAs<DependentMemberType>();
+  if (!depMem2) return false;
+
+  if (depMem1->getName() != depMem2->getName()) return false;
+
+  return areSameType(depMem1->getBase(), depMem2->getBase());
 }
 
 Type GenericTypeToArchetypeResolver::mapTypeIntoContext(Type type) {