title: Feature test for float/double variants of std::from_chars?
body: <p>I'd like to write a C++17 shim that substitutes custom implementation of <a href="https://en.cppreference.com/w/cpp/utility/from_chars" rel="nofollow noreferrer"><code>std::from_chars</code></a> in case the float/double variants are not available (e.g. GCC &lt; 11 has only integers). However, the feature test macro <a href="https://en.cppreference.com/w/cpp/utility/feature_test" rel="nofollow noreferrer"><code>__cpp_lib_to_chars</code></a> only differentiates general support of <code>from_chars</code> and is satisfied even when only the integer variant is implemented. How should I figure out whether the float/double variants are supported? Do I need to do it via compiled feature tests?</p>