I always found it odd that there's no printf extension to just automatically print the correct integer size. GCC is already smart enough to tell me if %d is wrong for a long; why can't I just do %N and have it print out the correct length for the argument?
It's not even necessary for the compiler to do it: writing a type-safe wrapper is pretty easy in C++11 and a decent approximation is doable in pure C99 (ask me if you want a sample). Yet using the ugly PRI macros seems common even in C++ projects. It's understandable why one wouldn't want to use ostream type formatting, but at least write a better printf...
(Type-safe at runtime, that is, via automatically passing a type info argument along with each format argument; C++ is braindead enough that doing the check at compile time is only somewhat possible with C++14 - as far as I can tell, it only works if you define the format outside of a function - and probably fairly slow, since that involves a template instantiation per character in the format. Oh well.)
Just a hypothetical idea: what if the standard allowed a compiler to modify the format string during compilation, replacing e.g. %N with the appropriate conversion specification, subject to proving that a given format string is a plain old literal that is not touched anywhere else?
Although compilers will warn about it, it's still possible to generate a format string dynamically at runtime. This might be done if you want different formats for the same fixed argument types (though there are probably better/safer ways).
It's a good idea, but then you are tied to one compiler.
So, what if Clang implements it, but not GCC? Or what if Clang and GCC implement it, but not the Sun or Intel compilers? Or what about all the GCC copies every board maker forks when they create something custom?
It's tricky making in-compiler behavior non-standard (though, I guess that's what compiler flags are for).
