Modern C++ has a decent amount of stuff in the standard library but there are a few things I was naively surprised it lacks:

• base64
• any cryptography stuff at all really
• HTTP!

Random discovery of the day is that GNU’s linker has different rules to those of MSVC or LLVM - the latter two are happy to resolve linkages at the final executable or library, while ld seems to require that every intermediate object binary be linked up?