[Bug c++/109367] New: bogus -Wunused-function warning

[Bug c++/109367] New: bogus -Wunused-function warning

[f.heckenbach@fh-soft.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109367

            Bug ID: 109367
           Summary: bogus -Wunused-function warning
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: f.heckenbach@fh-soft.de
  Target Milestone: ---

% cat test.cpp
using T = decltype ([]{});
struct S { static void f(T); };
void S::f(T) {}
% g++ --std=c++20 -Wall -c test.cpp
test.cpp:3:6: warning: 'static void S::f(T)' defined but not used
[-Wunused-function]
    3 | void S::f(T) {}
      |      ^

This happens with decltype of a lambda or a type containing such (in my actual
use case, a multimap with a comparator defined this way) in the function's
arguments or return type.

The warning is bogus since static member functions can be accessed from
elsewhere.

[Bug c++/109367] bogus -Wunused-function warning

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109367

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Please file a different bug with your full testcase as I think decltype of a
lamba is a type which has local linkage but I could be wrong.

[Bug c++/109367] bogus -Wunused-function warning

[f.heckenbach@fh-soft.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109367

--- Comment #2 from Frank Heckenbach <f.heckenbach@fh-soft.de> ---
My full testcase consists of many includes files, libraries etc.

The type declarations (corresponding to the first two lines of the
stripped-down example) are in a header to be called from other translation
units, and the function implementation (the last line) in a cpp file, as usual.

So this means, decltype can't be used this way at all? If so, not a compiler
bug; but severely disappointed in the standard again, rendering a hopeful
feature mostly useless by too strict limitations. I hope it will be rectified
in C++38 or so ... :/

[Bug c++/109367] bogus -Wunused-function warning with decltype of a lambda as an argument

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109367

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Actually I think the bug is:
using T = decltype ([]{});

is broken with GCC. There are multiple testcases dealing with that even.

[Bug c++/109367] bogus -Wunused-function warning with decltype of a lambda as an argument

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109367

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2024-04-13

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed. GCC thinks the type is a local type and then marks S::f as being
local to that TU.