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

[Richard Smith 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}}
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zygoloid 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.

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