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From: "jzwinck at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/110752] New: decltype in lambda loses const qualifier
 unless lambda is mutable
Date: Thu, 20 Jul 2023 13:57:13 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 13.1.0
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X-Bugzilla-Who: jzwinck at gmail dot com
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110752

            Bug ID: 110752
           Summary: decltype in lambda loses const qualifier unless lambda
                    is mutable
           Product: gcc
           Version: 13.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jzwinck at gmail dot com
  Target Milestone: ---

I believe this code should compile (and with MSVC it does), but with GCC it
does not:

    template<typename Arg>
    auto & C(Arg && arg) { return arg; }

    void DecltypeWithinLambda()
    {
        int i =3D 0;
        [[maybe_unused]] auto l =3D [i]() // mutable
        {
            // does not preserve constness of 'i' within non-mutable lambda
            // Seems to be evaluating it as if referencing the outer 'i'
            // Though, decltype((i)) does do the right thing.
            // alternative solution to mutable is to 'rename' the captured
variable list in the capture spec
            using T =3D decltype(C(i));
            [[maybe_unused]] T t(C(i)); // actual use of C() only matches
decltype if mutable
        };
    }

GCC says the last line is invalid:

    error: binding reference of type 'T' {aka 'int&'} to 'const int' discar=
ds
qualifiers
    | [[maybe_unused]] T t(C(i)); // actual use of C() only matches decltyp=
e if
mutable
    |                      ~^~~

If the lambda is marked mutable, it compiles as expected.  So does this, wh=
ich
seems like it should be equivalent to the above:

    struct F
    {
        void DecltypeWithinConstMethod() const
        {
            using T =3D decltype(C(i));
            [[maybe_unused]] T t(C(i)); // actual use of C() matches declty=
pe
        }
        int i;
    };

Demo: https://gcc.godbolt.org/z/PzEodY1bW

A somewhat similar bug, but not the same:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D63192=