Don’t repeat lookup work that may not round-trip

lib/AST/NameLookup.cpp

@@ -3238,6 +3238,11 @@ static llvm::TinyPtrVector<TypeDecl *> directReferencesForQualifiedTypeLookup(
 static DirectlyReferencedTypeDecls directReferencesForDeclRefTypeRepr(
     Evaluator &evaluator, ASTContext &ctx, DeclRefTypeRepr *repr,
     DeclContext *dc, DirectlyReferencedTypeLookupOptions options) {
+  // If we've already bound this TypeRepr, don't repeat the work.
+  if (repr->isBound()) {
+    return DirectlyReferencedTypeDecls({ repr->getBoundDecl() }, {});
+  }
+
   if (auto *qualIdentTR = dyn_cast<QualifiedIdentTypeRepr>(repr)) {
     auto result = directReferencesForTypeRepr(
         evaluator, ctx, qualIdentTR->getBase(), dc, options);

test/NameLookup/module_selector.swift

@@ -440,3 +440,11 @@ func dependentTypeLookup<T, U, V, W, X>(_: T, _: U, _: V, _: W, _: X) where
   X: Identifiable, X: ComparableIdentifiable, X.NonexistentModule::ID == Int
   // expected-error@-1 {{module selector is not allowed on generic member type; associated types with the same name are merged instead of shadowing one another}} {{49-68=}}
 {}
+
+// rdar://164647850 -- conforming to `Swift::Error` doesn't work when there's an `Error` type around
+
+enum Error {}
+enum E: Swift::Error { case err }
+
+func testErrorConformance() -> Result<Void, E> { .success(()) }
+// no-error@-1

validation-test/compiler_crashers/RequirementMachine-areReducedTypeParametersEqual-1801de.swift → validation-test/compiler_crashers_fixed/RequirementMachine-areReducedTypeParametersEqual-1801de.swift

@@ -1,4 +1,4 @@
 // {"kind":"typecheck","signature":"swift::rewriting::RequirementMachine::areReducedTypeParametersEqual(swift::Type, swift::Type) const"}
-// RUN: not --crash %target-swift-frontend -typecheck %s
+// RUN: not %target-swift-frontend -typecheck %s
 protocol a {
   typealias Index extension Collection where Self : a{b : Index} protocol a