clang 18 gives error "non C++20 importable modules" for "import std"

Question

the following code

import std;
int main(){}

compiled with clang 18:

clang++ -std=c++20 -stdlib=libc++ -fmodules -fbuiltin-module-map

gives the error:

import of module 'std' imported non C++20 importable modules

while it works on clang 17.

anybody have an idea why?