DeclContext::getAsTypeOrTypeExtensionContext() generic typealiases.

This function was unable to "look through" an unbound generic typealias to
find the underlying nominal type that was getting extended, which can occur
if we end up performing a name lookup into this context before the extension
itself has been fully-checked.

Also, test that extending DictionaryIndex works.

Author: DougGregor

lib/AST/DeclContext.cpp

@@ -47,15 +47,33 @@ ASTContext &DeclContext::getASTContext() const {
 
 GenericTypeDecl *
 DeclContext::getAsTypeOrTypeExtensionContext() const {
-  if (auto decl = const_cast<Decl*>(getAsDeclOrDeclExtensionContext())) {
-    if (auto ED = dyn_cast<ExtensionDecl>(decl)) {
-      if (auto type = ED->getExtendedType())
-        return type->getAnyNominal();
-      return nullptr;
+  auto decl = const_cast<Decl*>(getAsDeclOrDeclExtensionContext());
+  if (!decl) return nullptr;
+
+  auto ext = dyn_cast<ExtensionDecl>(decl);
+  if (!ext) return dyn_cast<GenericTypeDecl>(decl);
+
+  auto type = ext->getExtendedType();
+  if (!type) return nullptr;
+
+  do {
+    // expected case: we reference a nominal type (potentially through sugar)
+    if (auto nominal = type->getAnyNominal())
+      return nominal;
+
+    // early type checking case: we have a typealias reference that is still
+    // unsugared, so explicitly look through the underlying type if there is
+    // one.
+    if (auto typealias =
+          dyn_cast_or_null<TypeAliasDecl>(type->getAnyGeneric())) {
+      type = typealias->getUnderlyingTypeLoc().getType();
+      if (!type) return nullptr;
+
+      continue;
     }
-    return dyn_cast<GenericTypeDecl>(decl);
-  }
-  return nullptr;
+
+    return nullptr;
+  } while (true);
 }
 
 /// If this DeclContext is a NominalType declaration or an

test/Compatibility/CountableRange.swift renamed to test/Compatibility/stdlib_generic_typealiases.swift

@@ -9,3 +9,10 @@ extension CountableRange { // expected-warning{{'CountableRange' is deprecated:
   }
 }
 
+struct RequiresHashable<T: Hashable> { }
+
+extension DictionaryIndex {
+  func testHashable() {
+    _ = RequiresHashable<Key>()
+  }
+}