[Bug c++/95659] New: Inconsistent error message in "default argument missing for parameter"

[haoxintu at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95659

            Bug ID: 95659
           Summary: Inconsistent error message in "default argument
                    missing for parameter"
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: haoxintu at gmail dot com
  Target Milestone: ---

$cat bug.cc
int a;
long  foo1 ( void  = a, double  param2 ,decltype (auto) param3 = a){ }
long  foo2 ( void  = a, double  param2){ }

In gcc-trunk
$g++-trunk -w -c -std=c++14 bug.cc 
bug.cc:2:41: error: cannot declare a parameter with ‘decltype(auto)’
    2 | long  foo1 ( void  = a, double  param2 ,decltype (auto) param3 = a){ }
      |                                         ^~~~~~~~~~~~~~~
bug.cc:2:14: error: invalid use of type ‘void’ in parameter declaration
    2 | long  foo1 ( void  = a, double  param2 ,decltype (auto) param3 = a){ }
      |              ^~~~~~~~~
bug.cc:3:14: error: invalid use of type ‘void’ in parameter declaration
    3 | long  foo2 ( void  = a, double  param2){ }
      |              ^~~~~~~~~
bug.cc:3:33: error: default argument missing for parameter 2 of ‘long int
foo2(<type error>, double)’
    3 | long  foo2 ( void  = a, double  param2){ }
      |                         ~~~~~~~~^~~~~~
bug.cc:3:14: note: ...following parameter 1 which has a default argument
    3 | long  foo2 ( void  = a, double  param2){ }
      |              ^~~~~~~~~

GCC-trunk misses the "default argument missing for parameter" error message in
function "foo1()" but gives the message in function "foo2()".

In other GCC versions, they have consitent results, for example in GCC-10
$g++-10 -w -c -std=c++14 bug.cc 
bug.cc:2:41: error: cannot declare a parameter with ‘decltype(auto)’
    2 | long  foo1 ( void  = a, double  param2 ,decltype (auto) param3 = a){ }
      |                                         ^~~~~~~~~~~~~~~
bug.cc:2:14: error: invalid use of type ‘void’ in parameter declaration
    2 | long  foo1 ( void  = a, double  param2 ,decltype (auto) param3 = a){ }
      |              ^~~~~~~~~
bug.cc:2:33: error: default argument missing for parameter 2 of ‘long int
foo1(<type error>, double)’
    2 | long  foo1 ( void  = a, double  param2 ,decltype (auto) param3 = a){ }
      |                         ~~~~~~~~^~~~~~
bug.cc:2:14: note: ...following parameter 1 which has a default argument
    2 | long  foo1 ( void  = a, double  param2 ,decltype (auto) param3 = a){ }
      |              ^~~~~~~~~
bug.cc:3:14: error: invalid use of type ‘void’ in parameter declaration
    3 | long  foo2 ( void  = a, double  param2){ }
      |              ^~~~~~~~~
bug.cc:3:33: error: default argument missing for parameter 2 of ‘long int
foo2(<type error>, double)’
    3 | long  foo2 ( void  = a, double  param2){ }
      |                         ~~~~~~~~^~~~~~
bug.cc:3:14: note: ...following parameter 1 which has a default argument
    3 | long  foo2 ( void  = a, double  param2){ }
      |              ^~~~~~~~~