[Bug c++/103553] New: Show what decltype deduced in static_assert

[Bug c++/103553] New: Show what decltype deduced in static_assert

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103553

            Bug ID: 103553
           Summary: Show what decltype deduced in static_assert
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mpolacek at gcc dot gnu.org
  Target Milestone: ---

struct false_type { static constexpr bool value = false; };
struct true_type { static constexpr bool value = true; };
template<class T, class U>
struct is_same : false_type {}; 
template<class T>
struct is_same<T, T> : true_type {};

void
g ()
{
  int &&r = 42;
  static_assert (is_same<decltype(r), int>::value, "");
}

yields

$ ./cc1plus -quiet w.C
w.C: In function ‘void g()’:
w.C:12:45: error: static assertion failed
   12 |   static_assert (is_same<decltype(r), int>::value, "");
      |                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~

It would be nice to see why though, show what type the decltype produced.

$ xclang++ -c w.C
w.C:12:3: error: static_assert failed due to requirement 'is_same<int &&,
int>::value' ""
  static_assert (is_same<decltype(r), int>::value, "");
  ^              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.

which is much better.

[Bug c++/103553] Show what decltype deduced in static_assert

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103553

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement

[Bug c++/103553] Show what decltype deduced in static_assert

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103553

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2021-12-07
     Ever confirmed|0                           |1

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed, we do it for the template case but if it was not in a template we
don't, notice:

#include <type_traits>

template<class T>
class f
{
   int && r;
  static_assert (std::is_same_v<decltype(r), int>,"");
};
class f1
{
   int && r;
  static_assert (std::is_same_v<decltype(r), int>,"");
};

void
g ()
{
  int &&r = 42;
  f<decltype(r)> t;
}

[Bug c++/103553] Show what decltype deduced in static_assert

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103553

--- Comment #2 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Yes, I added that in GCC 11.

To make this work, I think we'll have to print parser->scope when there's one. 
In this case it's

full-name "struct is_same<int&&, int>"

so that could be extracted and used in finish_static_assert.