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From: "rs2740 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/101263] non-propagating-cache::emplace-deref missing
 constexpr
Date: Thu, 14 Oct 2021 15:43:15 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101263

TC <rs2740 at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rs2740 at gmail dot com

--- Comment #7 from TC <rs2740 at gmail dot com> ---
(In reply to Barry Revzin from comment #6)
> The "real" answer is allowing constexpr placement new, but that obviously
> doesn't help you right now.
>=20
> But I think the helpful answer is that you can add a constructor to your
> storage like storage(init_from_invoke_t, Args&&... args) that initializes
> the underlying value from invoke((Args&&)args...), and then
> construct_at(&storage, init_from_invoke, [&]() -> decltype(auto) { return
> *i; }).
>=20
> Something like that?

Yes. Something at that level of generality will be needed for the new
optional::transform, so it seems the better approach.

In my proof-of-concept implementation (which didn't have that concern), I u=
sed
something tailored to this specific case, along the lines of=20

struct __deref_tag {};

template<class _Tp>
struct __cache_wrapper {
    template<class _Iter>
    constexpr __cache_wrapper(__deref_tag, const _Iter& __i)
        : __t(*__i) {}
    _Tp __t;
};

and then stored a __non_propagating_cache<__cache_wrapper<range_value_t<...=
>>>
__cache, so that emplace-deref(i) is __cache.emplace(__deref_tag{}, i);=