If C23 is why not use constexpr?

https://en.cppreference.com/w/c/language/constexpr

  #define C(x) ((constexpr typeof(x)){ (x) })

Is there a use case for such a macro, or is it just a puzzle for its own sake?

  // Create a map with int keys and values that are vectors of floats:
  map( int, vec( float ) ) our_map;
  init( &our_map );
  
  // Create a vector of floats:
  vec( float ) our_vec;
  init( &our_vec );
  push( &our_vec, 1.23f );
  
  // Insert the vector into the map.
  insert( &our_map, 456, our_vec );
  
  // Generates a warning because get checks its first argument for side
  // effects and the compiler can't tell that the first argument of the
  // outermost get has none:
  printf( "%f", *get( get( &our_map, 456 ), 0 ) );
  
  // The "proper", albeit cumbersome, way to achieve the same thing without a
  // warning:
  vec( float ) *itr = get( &our_map, 456 );
  printf( "%f", *get( itr, 0 ) );

The macro should be called IS_CONST(), not C()

One can use _Pragma inside macros

Apparently the static_assert trick doesn't work with GCC, it just compiles it with a warning if it's not a constant expression:

  warning: expression in static assertion is not an integer constant expression [-Wpedantic]

Instead you can use the sizeof + compound literal with array type, use the comma operator to preserve the type of the expression and cast the result of sizeof to void to suppress the warning:

  #define C(x) ( (void)sizeof( (char [(int)(x) || 1]){0} ), (x) )

The only problem is that it doesn't support floating point expressions

> And I'd rather keep the library warning free instead of telling the users to switch warnings off.

Thank you! Separately, but related: fuck you, Google! (every time I have to deal with protobuf in C++, I curse Google and their "we don't give a shit about signed vs unsigned comparisons").

__builtin_choose_expr can be used instead of a ternary to avoid the type conversion rules that require the typeof cast

It's great of programmers to aim for portability, but frankly it's kind of a stretch that an arbitrary C compiler that is limited in standard support would the same time be sophisticated enough to process these tricks as intended.

In my fork of chibicc (a small C11 compiler) there are plenty of additional logic that were implemented solely to play nice with C tricks from real world projects that could have been easier if they target later standards. The most recent being how curl's build script determines the size of primitive types: they use (sizeof(T) == N) as a case index and expect the compiler to error on case-duplication[1], I had to add a backtracking loop to check exactly that[2]. I'm not complaining as more error checks isn't a bad thing, however, I'll advise programmers willing to invest in obscure tricks to actually test them on obscure compilers (instead of just flipping -std).

[1]: https://github.com/curl/curl/blob/339464432555b9bd71a5e4a4c4...

[2]: https://github.com/fuhsnn/slimcc/blob/54563ecae8480f836a0bb2...

>And I'd rather keep the library warning free instead of telling the users to switch warnings off.

Why not push/pop warnings to ignore in the library?

  _Pragma("GCC diagnostic push")
  _Pragma("GCC diagnostic ignored \"-Wshadow\"")
  int a = 1;
  {
      int a = 2;
  }
  _Pragma("GCC diagnostic pop")

This takes me back - all those compiler hacks just to keep stuff portable kinda drive me nuts tbh. I love seeing people push for warning-free code, though.

> And this cannot be silenced with #pragma since it's a macro, so the warning occurs at the location where the macro is invoked.

I seem to remember there's actually a solution for this .. at least on clang and I think MSVC .. you can programmatically turn warnings on/on with the _Pragma() macro. I don't remember exactly what you put in it, but it's designed specifically for this kind of macro nonsense

gcc will not let you actually define a negatively-sized array. Check it with some simple code -- I did. Even with -Wall -Wextra -O1 -std=c11 -Wpedantic, if I actually try to create foo[-1], on the stack or in BSS, I get the proper error: error: size of array 'foo' is negative