[clang] [clang] Fix crashes when passing VLA to va_arg (PR #119563)

[Eli Friedman via cfe-commits]

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@@ -20,4 +20,7 @@ void vla(int n, ...)
   __builtin_va_list ap;
   void *p;
   p = __builtin_va_arg(ap, typeof (int (*)[++n])); // CHECK: add nsw i32 {{.*}}, 1
+  // Don't crash on some undefined behaviors.
+  p = __builtin_va_arg(ap, typeof (int [++n])); // expected-warning{{second argument to 'va_arg' is of variable length array type 'typeof(int[++n])'}}
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efriedma-quic wrote:

Maybe put the warning checks into some test in clang/test/Sema.

https://github.com/llvm/llvm-project/pull/119563