Re: [PATCH] c++: Handle auto(x) in parameter-declaration-clause [PR103401]

[Marek Polacek via Gcc-patches]

On Wed, Dec 01, 2021 at 11:24:58PM -0500, Jason Merrill wrote:
> On 12/1/21 10:16, Marek Polacek wrote:
> > In C++23, auto(x) is valid, so decltype(auto(x)) should also be valid,
> > so
> > 
> >    void f(decltype(auto(0)));
> > 
> > should be just as
> > 
> >    void f(int);
> > 
> > but currently, everytime we see 'auto' in a parameter-declaration-clause,
> > we try to synthesize_implicit_template_parm for it, creating a new template
> > parameter list.  The code above actually has us calling s_i_t_p twice;
> > once from cp_parser_decltype_expr -> cp_parser_postfix_expression which
> > fails and then again from cp_parser_decltype_expr -> cp_parser_expression.
> > So it looks like we have f<auto, auto> and we accept ill-formed code.
> > 
> > So we need to be more careful about synthesizing the implicit template
> > parameter.  cp_parser_postfix_expression looked like a sensible place.
> 
> Does this cover other uses of auto in decltype, such as
> 
> void f(decltype(new auto{0}));

Yes: the clearing of auto_is_implicit_function_template_parm_p will happen here
too.

However, I'm noticing this:

  void f1(decltype(new auto{0}));
  void f2(decltype(new int{0}));

  void
  g ()
  {
    int i;
    void f3(decltype(new auto{0}));
    void f4(decltype(new int{0}));
    f1 (&i); // error: no matching function for call to f1(int*)
             // couldn't deduce template parameter auto:1
    f2 (&i);
    f3 (&i);
    f4 (&i);
  }
 
I think the error we issue is bogus.  (My patch doesn't change this.  clang++
accepts.)  Should I file a PR (and investigate)?

> ?  Should we adjust this flag in cp_parser_decltype along with all the other
> flags?

I think that's possible, but wouldn't cover auto in default arguments, or array
bounds.
 
Thanks,

Marek