[Bug c/97748] Preincrement of _Complex gives bogus warning = "value computed is not used"

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97748

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-10 branch has been updated by Jakub Jelinek
<jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:9d488c191c62cb114fa40239c495ac773ff76751

commit r10-9011-g9d488c191c62cb114fa40239c495ac773ff76751
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue Nov 10 15:56:20 2020 +0100

    c, c++: Fix up -Wunused-value on COMPLEX_EXPRs [PR97748]

    The -Wunused-value warning in both C and C++ FEs (implemented
    significantly differently between the two) sees the COMPLEX_EXPRs created
    e.g. for complex pre/post increment and many other expressions as useless
    and warns about it.

    For the C warning implementation, on e.g.
    COMPLEX_EXPR < ++REALPART_EXPR <x>, IMAGPART_EXPR <x>>;
    would warn even on the IMAGPART_EXPR <x> there alone etc., so what works
    is check if we'd warn about both operands of COMPLEX_EXPR and if yes,
    warn on the whole COMPLEX_EXPR, otherwise don't warn.

    The C++ warning implementation is significantly different and for that one
    the only warn if both would be warned about doesn't really work,
    we then miss warnings e.g. about
    COMPLEX_EXPR <REALPART_EXPR <SAVE_EXPR <x>> + 1.0e+0, IMAGPART_EXPR
<SAVE_EXPR <x>>> >>>>>
    The patch replaces the warning_at call with call to the c-family
    warn_if_unused_value function.

    On the testcase which after the initial new tests contains pretty much
    everything from gcc.dg/Wunused-value-1.c both approaches seem to work
    nicely.

    2020-11-10  Jakub Jelinek  <jakub@redhat.com>

            PR c/97748
    gcc/c-family/
            * c-common.h (warn_if_unused_value): Add quiet argument defaulted
            to false.
            * c-warn.c (warn_if_unused_value): Likewise.  Pass it down
            recursively and just return true instead of warning if it is true.
            Handle COMPLEX_EXPR.
    gcc/cp/
            * cvt.c (convert_to_void): Check (complain & tf_warning) in the
outer
            if rather than twice times in the inner one.  Use
warn_if_unused_value.
            Formatting fix.
    gcc/testsuite/
            * c-c++-common/Wunused-value-1.c: New test.

    (cherry picked from commit 0000ea4fb4eaacbd2c954d78d7f8e9f03c7be739)