Re: [PATCH] c++: unsynthesized defaulted constexpr fn [PR110122]

[Jason Merrill <jason@redhat.com>]

On 6/6/23 14:29, Patrick Palka wrote:
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk?
> 
> -- >8 --
> 
> In the second testcase of PR110122, during regeneration of the generic
> lambda with V=Bar{}, substitution followed by coerce_template_parms for
> A<V>'s template argument naturally yields a copy of V in terms of Bar's
> (implicitly) defaulted copy constructor.
> 
> This however happens inside a template context so although we introduced
> a use of the copy constructor, mark_used didn't actually synthesize it,
> which causes subsequent constant evaluation of the template argument to
> fail with:
> 
>    nontype-class58.C: In instantiation of ‘void f() [with Bar V = Bar{Foo()}]’:
>    nontype-class58.C:22:11:   required from here
>    nontype-class58.C:18:18: error: ‘constexpr Bar::Bar(const Bar&)’ used before its definition
> 
> Conveniently we already make sure to instantiate eligible constexpr
> functions before such (manifestly) constant evaluation, as per P0859R0.
> So this patch fixes this by making sure to synthesize eligible defaulted
> constexpr functions beforehand as well.

We probably also want to do this in cxx_eval_call_expression, under

>   /* We can't defer instantiating the function any longer.  */

Jason