Modular standard library is part of C++23, unlike the modules themselves which appeared in C++20. In Visual C++ you'd need to install modules for the standard library and use /experimental:module and /std:c++latest[0]. Also half a year ago I had to use import std.core; instead of import std;

[0] https://learn.microsoft.com/en-us/cpp/cpp/modules-cpp?view=m...