allocator<const T> is ill-formed

[fsb4000]

Summary

I noticed that MS STL and libstdc++ already forbid this, so I decided do the same for libc++.

https://gcc.godbolt.org/z/3WsT4j3c1

[fsb4000]

Oh, I found https://github.com/llvm/llvm-project/commit/a54d028895c91da356a4aaf30e27a5a5b90dd313
ok, I'll close.

[ldionne]

Yes, we are still working with trying to ship that without breaking too many users :)

libcxx/include/__memory/allocator.h

 class _LIBCPP_TEMPLATE_VIS allocator
     : private __non_trivial_if<!is_void<_Tp>::value, allocator<_Tp> >
 {
+    static_assert(!is_const<_Tp>::value, "std::allocator does not support const types");
     static_assert(!is_volatile<_Tp>::value, "std::allocator does not support volatile types");
 public:
     typedef size_t      size_type;