Re: clang and _Noreturn

[Paul Eggert]

Bruno Haible wrote:
> What is the semantic difference between _Noreturn and
> __attribute_noreturn__?

__attribute__ ((__noreturn__)), which the latter expands to, also works with 
function pointers, whereas _Noreturn does not. The distinction can matter when a 
function's address is assigned to a function pointer. Clang checks for 
__attribute__ ((__noreturn__)) compatibility when assigning function pointers; 
GCC does not, which can lead to weird results.  For example:

_Noreturn void nr (void) { for (;;); }
__attribute__ ((__noreturn__)) void anr (void) { for (;;); }

/* Valid.  */
void (*a) (void) = nr;
void (*b) (void) = anr;
__attribute__ ((__noreturn__)) void (*c) (void) = anr;

/* Invalid, as _Noreturn applies only to function definitions.  */
_Noreturn void (*d) (void) = nr;
_Noreturn void (*e) (void) = anr;

/* Allowed by GCC, but weirdly disallowed by clang because f is not declared 
with
   __attribute__ ((__noreturn__)).  */
__attribute__ ((__noreturn__)) void (*f) (void) = nr;

GCC does a better job in this area, and it's not clear that it's worth catering 
to clang's idiosyncracies here.