> As to 'vetting' the use of std library — I do not see how to do it without making a fork (or writing own "stripped down" version from scratch). E.g., you cannot simply decide not to use container allocator parameters, to reduce symbol sizes.

Yes, correct, what I meant is that you can still use many features like type_traits, bit_cast, initializer_list, span, array etc. Of course, std::string and friends are a bit no-no in very memory constrained cases.

> So current C++ standard library must be treated like part of the compiler implementation.

Indeed. Though even in C, compiler will assume C standard library is present unless you explicitly tell it not to (it optimizes calls to memcpy and will emit memset calls when initializing variables to 0)