[Bug c++/95657] New: Bogus error message in "auto" with -std=c++11

[haoxintu at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95657

            Bug ID: 95657
           Summary: Bogus error message in "auto" with -std=c++11
           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: ---

This code bug.cc

decltype (auto) foo( decltype (auto) a ){   }

in GCC-trunk

$g++ -c -std=c++11 bug.cc
bug.cc:1:11: error: expected primary-expression before ‘auto’
    1 | decltype (auto) foo( decltype  (auto) a ){   }
      |           ^~~~
bug.cc:1:33: error: use of ‘auto’ in parameter declaration only available with
‘-std=c++14’ or ‘-std=gnu++14’
    1 | decltype (auto) foo( decltype  (auto) a ){   }
      |                                 ^~~~
bug.cc:1:33: error: use of ‘auto’ in parameter declaration only available with
‘-std=c++14’ or ‘-std=gnu++14’
bug.cc:1:33: error: use of ‘auto’ in parameter declaration only available with
‘-std=c++14’ or ‘-std=gnu++14’
bug.cc:1:33: error: use of ‘auto’ in parameter declaration only available with
‘-std=c++14’ or ‘-std=gnu++14’
bug.cc:1:33: error: use of ‘auto’ in parameter declaration only available with
‘-std=c++14’ or ‘-std=gnu++14’
bug.cc:1:33: error: use of ‘auto’ in parameter declaration only available with
‘-std=c++14’ or ‘-std=gnu++14’
bug.cc:1:33: error: expected primary-expression before ‘auto’
bug.cc:1:22: error: expected primary-expression before ‘decltype’
    1 | decltype (auto) foo( decltype  (auto) a ){   }
      |                      ^~~~~~~~~~~~~~~~

I am sure there were some duplicate error messages about "error: use of ‘auto’
in parameter declaration only available with ‘-std=c++14’ or ‘-std=gnu++14’".

While in clang

$clang++ -c -std=c++11 bug.cc
bug.cc:1:11: warning: 'decltype(auto)' type specifier is a C++14 extension
[-Wc++14-extensions]
decltype (auto) foo( decltype  (auto) a ){   }
          ^
bug.cc:1:33: warning: 'decltype(auto)' type specifier is a C++14 extension
[-Wc++14-extensions]
decltype (auto) foo( decltype  (auto) a ){   }
                                ^
bug.cc:1:22: error: 'decltype(auto)' not allowed in function prototype
decltype (auto) foo( decltype  (auto) a ){   }
                     ^~~~~~~~
bug.cc:1:1: error: deduced return types are a C++14 extension
decltype (auto) foo( decltype  (auto) a ){   }
^
2 warnings and 2 errors generated.