[Bug c++/97573] Implement C++20 [depr.arith.conv.enum]

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97573

--- 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:455ade18462e5076065b1970e21c622239797392

commit r11-4502-g455ade18462e5076065b1970e21c622239797392
Author: Marek Polacek <polacek@redhat.com>
Date:   Tue Oct 27 11:16:50 2020 -0400

    c++: Deprecate arithmetic convs on different enums [PR97573]

    I noticed that C++20 P1120R0 deprecated certain arithmetic conversions
    as outlined in [depr.arith.conv.enum], but we don't warn about them.  In
    particular, "If one operand is of enumeration type and the other operand
    is of a different enumeration type or a floating-point type, this
    behavior is deprecated."  These will likely become ill-formed in C++23,
    so we should warn by default in C++20.  To this effect, this patch adds
    two new warnings (like clang++): -Wdeprecated-enum-enum-conversion and
    -Wdeprecated-enum-float-conversion.  They are enabled by default in
    C++20.  In older dialects, to enable these warnings you can now use
    -Wenum-conversion which I made available in C++ too.  Note that unlike
    C, in C++ it is not enabled by -Wextra, because that breaks bootstrap.

    We already warn about comparisons of two different enumeration types via
    -Wenum-compare, the rest is handled in this patch: we're performing the
    usual arithmetic conversions in these contexts:
      - an arithmetic operation,
      - a bitwise operation,
      - a comparison,
      - a conditional operator,
      - a compound assign operator.

    Using the spaceship operator as enum <=> real_type is ill-formed but we
    don't reject it yet.  We should also address [depr.array.comp] too, but
    it's not handled in this patch.

    gcc/c-family/ChangeLog:

            PR c++/97573
            * c-opts.c (c_common_post_options): In C++20, turn on
            -Wdeprecated-enum-enum-conversion and
            -Wdeprecated-enum-float-conversion.
            * c.opt (Wdeprecated-enum-enum-conversion,
            Wdeprecated-enum-float-conversion): New options.
            (Wenum-conversion): Allow for C++ too.

    gcc/cp/ChangeLog:

            PR c++/97573
            * call.c (build_conditional_expr_1): Warn about the deprecated
            enum/real type conversion in C++20.  Also warn about a
non-enumerated
            and enumerated type in ?: when -Wenum-conversion is on.
            * typeck.c (do_warn_enum_conversions): New function.
            (cp_build_binary_op): Call it.

    gcc/ChangeLog:

            PR c++/97573
            * doc/invoke.texi: Document -Wdeprecated-enum-enum-conversion
            and -Wdeprecated-enum-float-conversion.  -Wenum-conversion is
            no longer C/ObjC only.

    gcc/testsuite/ChangeLog:

            PR c++/97573
            * g++.dg/cpp0x/linkage2.C: Add dg-warning.
            * g++.dg/parse/attr3.C: Likewise.
            * g++.dg/cpp2a/enum-conv1.C: New test.
            * g++.dg/cpp2a/enum-conv2.C: New test.
            * g++.dg/cpp2a/enum-conv3.C: New test.