================
@@ -0,0 +1,22 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++11 %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++14 %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++17 %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++20 %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++23 %s
+
+// Test that 'auto' cannot be combined with a type specifier in C++.
+void f() {
+ auto int x = 1; // expected-error {{'auto' cannot be combined with a
type specifier}}
----------------
erichkeane wrote:
> > Can someone clarify why we would need new annotation or tentative parsing
> > here? It seems to me that we can handle `auto` incrementally while building
> > the list of decl specifiers without any novel lookahead, and I thought
> > that's what we already did. I would expect that we're just converting an
> > extension warning to an error here, and then doing some simplification if
> > we can.
>
> The current logic does: `if (isKnownToBeTypeSpecifier(GetLookAheadToken(1)))
> {` and this fails with:
>
> ```
> template <typename Ty>
> void g() {
> auto Ty x;
> }
> ```
>
> because the token we get from lookahead is `identifier` for `Ty`. We do not
> know that `Ty` is a type unless we do the annotation dance. Are you
> suggesting the better approach is to wait until `DeclSpec::Finish()` and
> determine it's invalid then?
>
> > We know Ty is a type,
>
> We only know that after we do the annotation, unless I'm missing something.
We SHOULD actually know it is a type... Kinda just based on location, but it is
a `typename` in the template, so we should know the type. Also, any type
spelled there is supposed to use the `typename` keyword if it isn't obvious (so
`auto typename Ty::type x`, though we don't actually enforce that (its a
Warning-as-default-error). Not sure what we could/should do in the latter
position, but my kneejerk is to just reject it always.
https://github.com/llvm/llvm-project/pull/166004
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