Re: -minstd: Require a minimum std version, without being specific

[Ben Boeckel <ben.boeckel@kitware.com>]

On Wed, Dec 21, 2022 at 19:33:48 +0100, Alejandro Colomar via Gcc wrote:
> I've long had this wish: an option similar to -std, but which would not specify 
> the standard.  Rather, mark a requirement that the standard be at least a version.
> 
> This would be especially useful for libraries, which might for example require 
> C99 or C11 to work.  They would be able to specify -minstd=c11 in their pc(5) 
> file (for use with pkgconf(1)).  That way, a program using such library, would 
> be free to use -std to specify the C version that the project should be compiled 
> with; maybe gnu17, maybe even gnu2x.  But if the program tries to compile under, 
> say gnu89, the compiler would report an error.

(FD: CMake developer, ISO C++ SG15 committee member)

I'd like to see us move away from "flag soup" and instead towards more
structured information here. This request corresponds to CMake's
`C_STANDARD` target property[1] which CMake then puts together to mean
something.

Note that there are real hazards with just putting the flags like this
into `.pc` files directly. If you have a C library and say "-minstd=c99"
and I'm C++, what is supposed to happen with this flag for a C++
compilre? Does it translate to C++11 (which is the first C++ standard to
"fully contain" C99)? What if there is no answer (e.g., `-minstd=c23`)?

FWIW, my idea is to broaden the concept CMake has of "usage
requirements" to not be so CMake-centered and to encompass things like
"you need an rpath to X to use my libraries" or even "here's an entry
for `PYTHONPATH` to use my Python code". ISO C++'s SG15 has started
discussion of such things with an eye towards standardization here:

    https://github.com/isocpp/pkg-fmt

--Ben

[1]https://cmake.org/cmake/help/latest/prop_tgt/C_STANDARD.html