[clang] [C++] Fix parsing of _Alignas in local declarations (PR #81915)

[Aaron Ballman via cfe-commits]

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@@ -0,0 +1,25 @@
+// RUN: %clang_cc1 %s -fsyntax-only -verify=expected,cpp
+// RUN: %clang_cc1 -x c %s -fsyntax-only -verify=expected,c
+
+// Ensure that we correctly parse _Alignas as an extension in C++.
+_Alignas(64) int i1;
+_Alignas(long long) int i2;
+int volatile _Alignas(64) i3; // Test strange ordering
+
+void foo(void) {
+  // We previously rejected these valid declarations.
+  _Alignas(8) char i4;
+  _Alignas(char) char i5;
+
+  (void)(int _Alignas(64))0; // expected-warning {{'_Alignas' attribute ignored when parsing type}}
+  // FIXME: C and C++ should both diagnose the same way, as being ignored.
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AaronBallman wrote:

Note: I plan to fix this in a follow-up

https://github.com/llvm/llvm-project/pull/81915