[Type checker] Maintain "base" type sugar for typealiases.

Author: DougGregor

lib/Sema/TypeCheckType.cpp

@@ -3154,6 +3154,8 @@ Type TypeResolver::buildProtocolType(
 Type TypeChecker::substMemberTypeWithBase(ModuleDecl *module,
                                           TypeDecl *member,
                                           Type baseTy) {
+  Type sugaredBaseTy = baseTy;
+
   // For type members of a base class, make sure we use the right
   // derived class as the parent type.
   if (auto *ownerClass = member->getDeclContext()
@@ -3207,7 +3209,8 @@ Type TypeChecker::substMemberTypeWithBase(ModuleDecl *module,
   // If we're referring to a typealias within a generic context, build
   // a sugared alias type.
   if (aliasDecl && aliasDecl->getGenericSignature()) {
-    resultType = BoundNameAliasType::get(aliasDecl, baseTy, subs, resultType);
+    resultType = BoundNameAliasType::get(aliasDecl, sugaredBaseTy, subs,
+                                         resultType);
   }
 
   return resultType;

test/decl/typealias/dependent_types.swift

@@ -58,3 +58,15 @@ let _: GenericStruct.MetaAlias = metaFoo()
 // ... but if the typealias has a fully concrete underlying type,
 // we are OK.
 let _: GenericStruct.Concrete = foo()
+
+class SuperG<T, U> {
+  typealias Composed = (T, U)
+}
+
+class SubG<T> : SuperG<T, T> { }
+
+typealias SubGX<T> = SubG<T?>
+
+func checkSugar(gs: SubGX<Int>.Composed) {
+  let i4: Int = gs // expected-error{{cannot convert value of type 'SubGX<Int>.Composed' (aka '(Optional<Int>, Optional<Int>)') to specified type 'Int'}}
+}