Re: [PATCH] c++: fix parsing with auto(x) [PR112410]

[Jason Merrill <jason@redhat.com>]

On 11/9/23 14:58, Marek Polacek wrote:
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> 
> -- >8 --
> Here we are wrongly parsing
> 
>    int y(auto(42));
> 
> which uses the C++23 cast-to-prvalue feature, and initializes y to 42.
> However, we were treating the auto as an implicit template parameter.
> 
> Fixing the auto{42} case is easy, but when auto is followed by a (,
> I found the fix to be much more involved.  For instance, we cannot
> use cp_parser_expression, because that can give hard errors.  It's
> also necessary to disambiguate 'auto(i)' as 'auto i', not a cast.
> auto(), auto(int), auto(f)(int), auto(*), auto(i[]), auto(...), etc.
> are all function declarations.  We have to look at more than one
> token to decide.

Yeah, this is a most vexing parse problem.  The code is synthesizing 
template parameters before we've resolved whether the auto is a 
decl-specifier or not.

> In this fix, I'm (ab)using cp_parser_declarator, with member_p=false
> so that it doesn't commit.  But it handles even more complicated
> cases as
> 
>    int fn (auto (*const **&f)(int) -> char);

But it doesn't seem to handle the extremely vexing

struct A {
   A(int,int);
};

int main()
{
   int a;
   A b(auto(a), 42);
}

I think we need to stop synthesizing immediately when we see RID_AUTO, 
and instead go back after we successfully parse a declaration and 
synthesize for any autos we saw along the way.  :/

Jason