Wow I really thought this would be a compile error. The implicit cast here really is a footgun. Looks like '-Wrange-loop-construct' (included in -Wall) does catch it:

> warning: loop variable 'v' of type 'const std::pair<std::__cxx11::basic_string<char>, int>&' binds to a temporary constructed from type 'std::pair<const std::__cxx11::basic_string<char>, int>' [-Wrange-loop-construct] 11 | for (const std::pair<std::string, int>& v: m) {

As they say, the power of names...