[libc++] These throwing allocation functions shouldn't return null.

[DianQK]

Summary

According to https://isocpp.org/wiki/faq/freestore-mgmt#new-never-returns-null and https://en.cppreference.com/w/cpp/memory/new/operator_new, a new operator that throws an exception should not return null.

This D20677 breaks that convention with -fno-exceptions.

I noticed the C++ standard doesn't mention such a convention under using -fno-exceptions. I'm new to the C++ standard library, so please let me know if I'm missing something.

But this patch breaks another convention.

5. Called by the non-throwing non-array new-expressions. The standard library implementation calls the version (1) and returns a null pointer on failure instead of propagating the exception.

I am submitting another candidate. (Mark the void* operator new ( std::size_t count ) as noexcept?)

If there are no relevant standards, are these the possible ways?

1. The current patch (Don't call the throwing function). This broken the standard.
2. Mark the void* operator new ( std::size_t count ) as noexcept.
3. Return the -1 in the throwing function? (Too tricky).
4. Remove the throwing function declaration (Breaking many api).
5. Revert D20677. Because the standard requires that exceptions be thrown.

[DianQK]

This comment was removed by DianQK.

[Mordante]

Thanks for the quick fix. I just saw on Discord @ldionne wants to look into this further.

libcxxabi/src/stdlib_new_delete.cpp

43

I really dislike std::__libcpp_unreachable since it just results in undefined behaviour. Either we should do nothing or assert(false);.