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From: Antony Polukhin <antoshkka@gmail.com>
Date: Wed, 12 May 2021 12:58:16 +0300
Message-ID: <CAKqmYPaDPDAU_Pud9jZ6aWDAb8NxiW0tefQjj28AP1tjySvnSw@mail.gmail.com>
Subject: Re: [PATCH] PR libstdc++/89728 diagnose some missuses of
 [locale.convenience] functions
To: Jonathan Wakely <jwakely@redhat.com>
Cc: "libstdc++" <libstdc++@gcc.gnu.org>,
 gcc-patches List <gcc-patches@gcc.gnu.org>
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=D1=81=D1=80, 12 =D0=BC=D0=B0=D1=8F 2021 =D0=B3. =D0=B2 12:18, Jonathan Wak=
ely <jwakely@redhat.com>:
<...>
> Or just leave it undefined, as libc++ seems to do according to your
> comment in PR 89728:
>
> error: implicit instantiation of undefined template 'std::__1::ctype<std:=
:__1::basic_string<char> >'
>
> Was your aim to have a static_assert that gives a more descriptive
> error? We could leave it undefined in C++98 and have the static assert
> for C++11 and up.

Leaving it undefined would be the best. It would allow SFINAE on ctype
and a compile time error is informative enough.

However, there may be users who instantiate ctype<ThierChar> in a
shared library without ctype<ThierChar> template specializations in
the main executable. Making the default ctype undefined would break
their compilation:

#include <locale>
// no ctype<ThierChar> specialization
c =3D std::tolower(ThierChar{42}, locale_from_shared_library()); // OK
right now in libstdc++, fails on libc++


Should we care about those users?

--=20
Best regards,
Antony Polukhin