Hi Marcin,

On 2026-02-10T14:12:42+0100, Marcin Jaczewski wrote:
> > > > alx_at_devuan:~/tmp$ cat nodiscard.c++
> > > > [[nodiscard]] int f(void);
> > > >
> > > > int
> > > > main(void)
> > > > {
> > > > typeof(int (void) ) *fp1;
> > > > typeof(int (void)[[nodiscard]]) *fp2;
> > > >
> > > > fp1 = &f; // I would expect a diagnostic here.
> > > > fp1();
> > > >
> > > > fp2 = &f;
> > > > fp2();
> > > > }
> > > > alx_at_devuan:~/tmp$ g++ -S -Wall -Wextra nodiscard.c++
> > > > nodiscard.c++: In function ‘int main()’:
> > > > nodiscard.c++:7:38: warning: ‘nodiscard’ attribute can only be applied to functions or to class or enumeration types [-Wattributes]
> > > > 7 | typeof(int (void)[[nodiscard]]) *fp2;
> > > > | ^

[...]
> > No, that was not the point. The point is that the conversion in
> > fp1 = &f;
> > discards an attribute from the function type, and thus results in calls
> > through that function pointer to not have a diagnostic when the return
> > value is discarded. This is a hole in the rules.
> >
> > And the diagnostic you see in fp2 is in fact another hint that the
> > design of [[nodiscard]] was bogus. That code should be accepted, and it
> > should be the only way of storing a function pointer that holds &f. If
> > that's not accepted, there is no way of using [[nodiscard]] safely.
> >
>
> But what does this have to do with ignorability of attributes?
> It's possible that you could not add the "not-ignorable" attribute there anyway.
> ```
> std::is_same<int, [[foo]] int>
> ```
> how C++ should handle it? even in case where it can't ignore this attribute.
>
> Only solution I see is that `fp1` itself should have `[[nodiscard]]`,

Yes; it should be like fp2, and the attribute should be part of the type
(and thus we should have no diagnostic in line 7).

> and this could be still ignorable.

In part, this is a problem because either

a) ignorability of attributes is the problem
or
b) [[nodiscard]] should not be ignorable, and should be replaced by a
    non-ignorable attribute.

And if b) is chosen, then few very ignorable attributes would remain;
so that ignorability of attributes would render attributes DoA. *The*
truly ignorable attribute is [[maybe_unused]].


Cheers,
Alex

-- 
<https://www.alejandro-colomar.es>