Yes. They were approved for C++20 with no working reference implementation. This was done over objections from representatives from every compiler and build system. 5 years later they're still not widely (or fully) implemented.
They're impossible to implement in Make, which is without exaggeration the world's most widely-used build tool. Even CMake has had a very difficult time implementing them. They break most methods for incremental builds, and mean that a compiler is needed just to determine staleness. They also make fully parallelized compilation impossible, because dependencies can't be fully resolved by the build system.
They're impossible to implement in Make, which is without exaggeration the world's most widely-used build tool. Even CMake has had a very difficult time implementing them. They break most methods for incremental builds, and mean that a compiler is needed just to determine staleness. They also make fully parallelized compilation impossible, because dependencies can't be fully resolved by the build system.