[Bug c++/96103] New: Unclear diagnostic for a function return with "decltype(auto)"

[Bug c++/96103] New: Unclear diagnostic for a function return with "decltype(auto)"

[haoxintu at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96103

            Bug ID: 96103
           Summary: Unclear diagnostic for a function return with
                    "decltype(auto)"
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: haoxintu at gmail dot com
  Target Milestone: ---

Hi, all.

This code, "decltype(auto)" in return type is a c++14 extension and I guess GCC
might diagnose this better. 

$cat test.cc
decltype(auto) foo () {}

$g++ -c -std=c++11 test.cc
test.cc:1:10: error: expected primary-expression before ‘auto’
    1 | decltype(auto) foo () {}
      |          ^~~~
test.cc: In function ‘int foo()’:
test.cc:1:24: warning: no return statement in function returning non-void
[-Wreturn-type]
    1 | decltype(auto) foo () {}
      |                        ^

$g++ -c -std=c++14 test.cc
//emits nothing

While in Clang:
$clang++ -c -std=c++11 test.cc 
test.cc:1:10: warning: 'decltype(auto)' type specifier is a C++14 extension
[-Wc++14-extensions]
decltype(auto) foo () {}
         ^
test.cc:1:1: error: deduced return types are a C++14 extension
decltype(auto) foo () {}
^
1 warning and 1 error generated.

I guess users can not fix the code according to the errors emitted until they
realized this is a C++14 extension. 

Should GCC recognizes the C++14 extension first when parsing this code and then
emits the appropriate diagnostic information?

Thanks,
Haoxin

[Bug c++/96103] Unclear diagnostic for a function return with "decltype(auto)"

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96103

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2020-07-07
                 CC|                            |mpolacek at gcc dot gnu.org

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Confirmed, we should tweak cp_parser_decltype to give a better error.

[Bug c++/96103] Unclear diagnostic for a function return with "decltype(auto)"

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96103

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |mpolacek at gcc dot gnu.org

[Bug c++/96103] Unclear diagnostic for a function return with "decltype(auto)"

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96103

--- Comment #2 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:a51de1af063b0a9233762dcd6ecf2ea0bdf4cdff

commit r11-1913-ga51de1af063b0a9233762dcd6ecf2ea0bdf4cdff
Author: Marek Polacek <polacek@redhat.com>
Date:   Tue Jul 7 17:09:42 2020 -0400

    c++: Better diagnostic for decltype(auto) in C++11 [PR96103]

    If you try to use decltype(auto) in C++11, we emit obscure

      error: expected primary-expression before 'auto'

    giving the user no hint as to what's wrong.  This patch improves that
    diagnostic.  Since we've been giving an error, I'm also using error().

    gcc/cp/ChangeLog:

            PR c++/96103
            * parser.c (cp_parser_decltype): Print error about using
decltype(auto)
            in C++11.  Check that the token following "auto" is ")".

    gcc/testsuite/ChangeLog:

            PR c++/96103
            * g++.dg/cpp0x/decltype77.C: New test.

[Bug c++/96103] Unclear diagnostic for a function return with "decltype(auto)"

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96103

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Fixed.

[Bug c++/96103] Unclear diagnostic for a function return with "decltype(auto)"

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96103

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |11.0

[Bug c++/96103] Unclear diagnostic for a function return with "decltype(auto)"

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96103

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 95658 has been marked as a duplicate of this bug. ***