I added a patch to https://bugs.llvm.org/show_bug.cgi?id=41578 that fixes it differently.

In D55840#1395019, @ldionne wrote:

TBH, I don't think that's a question the committee even thought about when introducing feature-test macros?

See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89345#c3 for what I've done in libstdc++

As it looks like libc++ isn't going to define the type for earlier dialects, libstdc++ doesn't either (which seems right anyway because the names aren't reserved prior to C++20).

In D56997#1367316, @andrewluo wrote:

You are right, it is not guaranteed per the standard, although it has worked before (at least on MSVC).

I'd be happy to restrict this to > C++17 only (which is automatically the case when using G++ because there's no -fdestroying-delete to enable it, you only get it with -std=c++2a, -std=gnu++2a etc.)

The simplest solution would be to change libstdc++'s <new> to:

@chandlerc thanks for catching this.

The __throw_xxx functions are not part of the public libstdc++ API and whatever Firefox is trying to do with them is not supported by libstdc++ and is undefined behaviour. Make it stop.