I generally agree with what has been said against this.

Let’s add something about standard features that have not (yet) been implemented by some library. My specific use case is std::filesystem on older macOS. The general idea should be to roll your own implementation if it does not exist yet in the STL library you are using. Fortunately, for filesystem someone has already done the work to write a compatible library (so I don’t have to do it myself). What I do is to use mylib::filesystem/myapp::filesystem everywhere from my own namespace. If std::filesystem is available I use ‘using’ to make my definition to just use the standard library. Otherwise it would declare my own class (or use ‘using’ with the type from the other library).

Certainly, your example with free standing functions is slightly more complicated. At first, I’d implement those functions in my own namespace and use those everywhere. With some macro I can check if the feature is already available. In that case my implementation would be an inline function call to the std implementation. Inline function definitions are usually inlined and thus not a performance bottleneck in an optimized build. With enable_if this might also be possible to do without a macro: choose the correct implementation based on if the function is declared.