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From: "ppalka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/111284] [11/12/13/14 Regression] Some passing-by-value
 parameters are mishandled since GCC 9, affecting libstdc++'s constexpr
 std::string
Date: Mon, 11 Mar 2024 15:12:04 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D111284

--- Comment #9 from Patrick Palka <ppalka at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #8)
> Created attachment 57648 [details]
> gcc14-pr111284.patch
>=20
> So, I've tried to fix this by constexpr evaluating the arguments passed to
> PARM_DECLs with TREE_ADDRESSABLE types in the caller as lvalues rather th=
an
> rvaluea and later, if we try to evaluate the PARM_DECL in the callee as l=
val,
> lookup the value and use that, if it is rval constexpr evaluate again as
> rvalue.
> There is a complication for qualified type, say if the argument is const =
in
> the callee and caller is passing reference to non-const, adjust_temp_type
> can't handle that when it isn't a rvalue.
Interesting, hopefully this fixes the std::string testcases in PR111258 and
related PRs?

And perhaps the following augmented testcase from this PR with a constexpr =
dtor
that checks valid():

void non_constant();

struct self_locator {
    self_locator() =3D default;
    constexpr self_locator(const self_locator&) noexcept : this_{this} {}
    constexpr self_locator& operator=3D(const self_locator&) noexcept { ret=
urn
*this; }

    constexpr bool valid() const noexcept { return this_ =3D=3D this; }
    constexpr ~self_locator() { if (!valid()) non_constant(); }

    self_locator *this_ =3D this;
};

constexpr bool demonstrator(self_locator x) noexcept
{
    return x.valid();
}

static_assert(demonstrator(self_locator{}), "");
static_assert([](self_locator x){ return x.valid(); }(self_locator{}), "");=