lib/sizeof.h: typeas(): Add support for 'void' and function types

[alejandro-colomar]

typeas() should be a generic macro that accepts any types that typeof() accepts.

The current implementation, while simple, didn't allow 'void' and function types, since one can't create a compound literal of such a type.

The solution needs to create a pointer type, which accepts a compound literal even for void and function pointee types. This requires using typeof() within the parenthesized type of the compound literal, and thus we need a different way to validate that the input is a type name (not an expression), which is done through _Generic().

Here's an expanded version of it, to help review it:

	#define typeas(T)  typeof                                     \
	(                                                             \
		*(typeof(T) *){                                       \
			_Generic(0, T: NULL, default: NULL)           \
		}                                                     \
	)

For the source code, let's keep it as a one-liner.

BTW, a nice side effect of this implementation is that we use {NULL} instead of {0}, and thus we get rid of -Wzero-as-null-pointer-constant diagnostics when the type T is a pointer type.

This implementation is compatible to GNU C11 --and ISO C23, which standardized typeof()--.

Cc: @uecker, @kees , @chrisbazley

---

Revisions:

v2

• Replace some remaining uses of typeof() by typeas(). After this, only two uses of typeof() remain, and they're both strictly necessary (they can't be replaced by typeas()).

$ git rd 
1:  1c42851aafc3 = 1:  1c42851aafc3 lib/sizeof.h: typeas(): Add support for 'void' and function types
-:  ------------ > 2:  c5a6a7adb832 lib/: Use typeas() instead of typeof() where possible

v2b

• Rebase

$ git rd 
1:  1c42851aafc3 = 1:  8c1ed7af27a5 lib/sizeof.h: typeas(): Add support for 'void' and function types
2:  c5a6a7adb832 = 2:  24c7117e116e lib/: Use typeas() instead of typeof() where possible

v3

• Drop second commit. It didn't work with old compilers.

$ git rd 
1:  8c1ed7af27a5 = 1:  8c1ed7af27a5 lib/sizeof.h: typeas(): Add support for 'void' and function types
2:  24c7117e116e < -:  ------------ lib/: Use typeas() instead of typeof() where possible