Re: [PATCH] c++: Implement __is_{nothrow_,}convertible [PR106784]

[Marek Polacek <polacek@redhat.com>]

On Fri, Sep 23, 2022 at 05:34:21PM +0100, Jonathan Wakely wrote:
> On Fri, 23 Sept 2022 at 15:43, Jonathan Wakely wrote:
> >
> > On Fri, 23 Sept 2022 at 15:34, Marek Polacek wrote:
> > >
> > > On Thu, Sep 22, 2022 at 06:14:44PM -0400, Jason Merrill wrote:
> > > > On 9/22/22 09:39, Marek Polacek wrote:
> > > > > To improve compile times, the C++ library could use compiler built-ins
> > > > > rather than implementing std::is_convertible (and _nothrow) as class
> > > > > templates.  This patch adds the built-ins.  We already have
> > > > > __is_constructible and __is_assignable, and the nothrow forms of those.
> > > > >
> > > > > Microsoft (and clang, for compatibility) also provide an alias called
> > > > > __is_convertible_to.  I did not add it, but it would be trivial to do
> > > > > so.
> > > > >
> > > > > I noticed that our __is_assignable doesn't implement the "Access checks
> > > > > are performed as if from a context unrelated to either type" requirement,
> > > > > therefore std::is_assignable / __is_assignable give two different results
> > > > > here:
> > > > >
> > > > >    class S {
> > > > >      operator int();
> > > > >      friend void g(); // #1
> > > > >    };
> > > > >
> > > > >    void
> > > > >    g ()
> > > > >    {
> > > > >      // #1 doesn't matter
> > > > >      static_assert(std::is_assignable<int&, S>::value, "");
> > > > >      static_assert(__is_assignable(int&, S), "");
> > > > >    }
> > > > >
> > > > > This is not a problem if __is_assignable is not meant to be used by
> > > > > the users.
> > > >
> > > > That's fine, it's not.
> > >
> > > Okay then.  libstdc++ needs to make sure then that it's handled right.
> >
> > That's fine, the type traits in libstdc++ are always "a context
> > unrelated to either type", unless users do something idiotic like
> > declare std::is_assignable as a friend.
> >
> > https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1339r1.pdf
> > wants to explicitly say that's idiotic.
> 
> And I just checked that a variable template like std::is_assignable_v
> also counts as "a context unrelated to either type", even when
> instantiated inside a member function of the type:
> 
> #include <type_traits>
> 
> template<typename T, typename U>
> constexpr bool is_assignable_v = __is_assignable(T, U);
> 
> class S {
>   operator int();
>   friend void g(); // #1
> };
> 
> void
> g ()
> {
>   // #1 doesn't matter
>   static_assert(std::is_assignable<int&, S>::value, "");
>   static_assert(std::is_assignable_v<int&, S>, "");
>   static_assert(__is_assignable(int&, S), "");
> }
> 
> The first two assertions are consistent, and fail, which is what we
> want.  The direct use of the built-in succeeds, but we don't care.

Great, thanks. 

Marek