Report use of std::vector<const T> (and similar containers of const elements). These are not allowed in standard C++, and should usually be std::vector<T> instead.”

Per C++ [allocator.requirements.general]: “T is any cv-unqualified object type”, std::allocator<const T> is undefined. Many standard containers use std::allocator by default and therefore their const T instantiations are undefined.

libc++ defines std::allocator<const T> as an extension which will be removed in the future.

libstdc++ and MSVC do not support std::allocator<const T>:

// libstdc++ has a better diagnostic since https://gcc.gnu.org/bugzilla/show_bug.cgi?id=48101
std::deque<const int> deque; // error: static assertion failed: std::deque must have a non-const, non-volatile value_type
std::set<const int> set; // error: static assertion failed: std::set must have a non-const, non-volatile value_type
std::vector<int* const> vector; // error: static assertion failed: std::vector must have a non-const, non-volatile value_type

// error C2338: static_assert failed: 'The C++ Standard forbids containers of const elements because allocator<const T> is ill-formed.'

Code bases only compiled with libc++ may accrue such undefined usage. This check finds such code and prevents backsliding while clean-up is ongoing.