[libcxx-commits] [libcxx] [libc++] Make bad_function_call::what() existence a matter of availability instead of ABI (PR #127697)

[Louis Dionne via libcxx-commits]

================
@@ -372,6 +372,10 @@
 #define _LIBCPP_AVAILABILITY_HAS_HASH_MEMORY _LIBCPP_INTRODUCED_IN_LLVM_21
 // No attribute, since we've had hash in the headers before
 
+// This controls whether `bad_function_call::what()` is available in the dylib
----------------
ldionne wrote:

```suggestion
// This controls whether we provide a message for `bad_function_call::what()` that is
// specific to `std::bad_function_call`. See https://wg21.link/LWG2233. This requires
// `std::bad_function_call::what()` to be available in the dylib.
```

https://github.com/llvm/llvm-project/pull/127697