[clang] [clang] Reject 'auto' storage class with type specifier in C++ (PR #166004)

[Aaron Ballman via cfe-commits]

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@@ -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}}
----------------
AaronBallman 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.

https://github.com/llvm/llvm-project/pull/166004