How to use pragma in a compound literal in GCC 9

How to use pragma in a compound literal in GCC 9

[Daurnimator]

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A project I help maintain has had a report that it fails to compile with GCC 9
https://github.com/wahern/cqueues/issues/212#issuecomment-461693111

I've attached a minimal reproduction of the issue.
Trying to compile it results in:

<source>: In function 'main':
<source>:46:15: error: lvalue required as unary '&' operand
   46 |     void *x = &quietinit((struct bar){ 0, .a = 0 });
      |               ^
Compiler returned: 1

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/*
 * C O M P I L E R  A N N O T A T I O N S
 *
 * GCC with -Wextra, and clang by default, complain about overrides in
 * initializer lists. Overriding previous member initializers is well
 * defined behavior in C. We rely on this behavior to define default,
 * overrideable member values when instantiating configuration objects.
 *
 * quietinit() guards a compound literal expression with pragmas to
 * silence these shrill warnings. This alleviates the burden of requiring
 * third-party projects to adjust their compiler flags.
 *
 * NOTE: If you take the address of the compound literal, take the address
 * of the transformed expression, otherwise the compound literal lifetime is
 * tied to the scope of the GCC statement expression.
 *
 * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */

#if defined __clang__
#define PRAGMA_PUSH _Pragma("clang diagnostic push")
#define PRAGMA_QUIET _Pragma("clang diagnostic ignored \"-Winitializer-overrides\"")
#define PRAGMA_POP _Pragma("clang diagnostic pop")

#define quietinit(...) \
	PRAGMA_PUSH PRAGMA_QUIET __VA_ARGS__ PRAGMA_POP
#elif (__GNUC__ == 4 && __GNUC_MINOR__ >= 6) || __GNUC__ > 4
#define PRAGMA_PUSH _Pragma("GCC diagnostic push")
#define PRAGMA_QUIET _Pragma("GCC diagnostic ignored \"-Woverride-init\"")
#define PRAGMA_POP _Pragma("GCC diagnostic pop")

/* GCC parses the _Pragma operator less elegantly than clang. */
#define quietinit(...) \
	__extension__ ({ PRAGMA_PUSH PRAGMA_QUIET __VA_ARGS__; PRAGMA_POP })
#else
#define PRAGMA_PUSH
#define PRAGMA_QUIET
#define PRAGMA_POP
#define quietinit(...) __VA_ARGS__
#endif

struct bar {
	int a;
};

int main() {
    void *x = &quietinit((struct bar){ 0, .a = 0 });
}

Re: How to use pragma in a compound literal in GCC 9

[Jakub Jelinek]

On Tue, Feb 12, 2019 at 06:19:56PM +1100, Daurnimator wrote:
> A project I help maintain has had a report that it fails to compile with GCC 9
> https://github.com/wahern/cqueues/issues/212#issuecomment-461693111
> 
> I've attached a minimal reproduction of the issue.
> Trying to compile it results in:
> 
> <source>: In function 'main':
> <source>:46:15: error: lvalue required as unary '&' operand
>    46 |     void *x = &quietinit((struct bar){ 0, .a = 0 });
>       |               ^
> Compiler returned: 1

Guess you need to restructure the code somewhat, with the way these macros
are used it is not possible.  GCC doesn't accept _Pragma at arbitrary spots
within a single expression.

You could just add -Wno-override-init to the options and use the macro
without pragma.

Or better, don't do what the macro complains about, if I understand it
right, at least in the package I was looking at, there is just 0, 0, { 0, 0 }
style initialization in some macros and a macro that uses quietinit
sometimes ammends it with , .a = whatever, .b = whateverelse.
Those 0, 0, { 0, 0 } are useless, that is the default behavior, if the
compound literal is just (struct bar){}, it will be initialized to all
zeros, and (struct bar){ .a = 1 } will initialize just the designated
elements.  And you won't get any warnings for that (at least not those in
-Wall/-Wextra).

Or use those quietinit-like macros as toplevel statements only and pass
to the macro also a name of the pointer you assign the address to, so in the
end it will be:
  _Pragma("GCC diagnostic push")
  _Pragma("GCC diagnostic ignored \"-Woverride-init\"")
  void *x = &(struct bar){ 0, .a = 0 };
  _Pragma("GCC diagnostic pop")

Or use the no pragmas quietinit and add another set of macros,
quietinit_start/quietinit_end or whatever, define those macros to empty
on everything but GCC9+, on GCC9+ define those to the starting or ending
pragmas and put those in the source around the whole statements with that,
rather than just in subexpressions.

	Jakub