[Bug c++/105130] New: gcc does not warn about unused return value of last expression of statement expr

[Bug c++/105130] New: gcc does not warn about unused return value of last expression of statement expr

[dani at danielbertalan dot dev]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105130

            Bug ID: 105130
           Summary: gcc does not warn about unused return value of last
                    expression of statement expr
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dani at danielbertalan dot dev
  Target Milestone: ---

Created attachment 52739
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=52739&action=edit
Proposed patch

The following code does not cause a warning about the return value of the foo()
function being ignored:

$ cat unused.cc
[[nodiscard]] int foo();

int main() {
    ({ foo(); });
}

$ g++ -Wunused-value -std=c++20 unused.cc
[no warnings]

The issue seems to be that CLEANUP_POINT_EXPR and STATEMENT_LIST -- which are
what statement expressions are turned into -- are not handled in
convert_to_void() (cp/cvt.cc).

The attached patch resolves the issue, but as a newbie to hacking on gcc, I
can't evaluate if it has any unintended consequences.

[Bug c++/105130] gcc does not warn about unused return value of last expression of statement expr

[dani at danielbertalan dot dev]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105130

--- Comment #1 from Daniel Bertalan <dani at danielbertalan dot dev> ---
Created attachment 52740
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=52740&action=edit
Proposed patch #2

Actually, my proposed patch was definitely incorrect. It didn't allow me to
circumvent the warning by explicitly casting to void. This new version fixes
that issue.

[Bug c++/105130] gcc does not warn about unused return value of last expression of statement expr

[egallager at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105130

Eric Gallager <egallager at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |egallager at gcc dot gnu.org

--- Comment #2 from Eric Gallager <egallager at gcc dot gnu.org> ---
Please send your patch to the gcc-patches mailing list for review.

[Bug c++/105130] gcc does not warn about unused return value of last expression of statement expr

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105130

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I would have thought this was by design. a statement expression last statement
is "used" there.

[Bug c++/105130] gcc does not warn about unused return value of last expression of statement expr

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105130

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=39808

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
PR 39808 is the C version.