[PATCH] D44865: [libc++] Implement P0608R1 - A sane variant converting constructor

[Eric Fiselier via Phabricator via cfe-commits]

EricWF added inline comments.


================
Comment at: include/variant:1109
 
+#define _LIBCPP_VARIANT_BOOLEAN_CONVERSION(bool_type)                    \
+    template <class... _Types>                                           \
----------------
lichray wrote:
> EricWF wrote:
> > lichray wrote:
> > > EricWF wrote:
> > > > EricWF wrote:
> > > > > I would like to see a version of this patch that doesn't use preprocessor expansion to specialize `__overload` multiple times.
> > > > > 
> > > > > Perhaps adding a `bool = is_same<remove_cv_t<First>, bool>` to `__overload` and then specializing once on that?
> > > > Or perhaps SFINAE-ing the `test` member instead of specializing `__overload`.
> > > > 
> > > > Something like this maybe?
> > > > 
> > > > ```
> > > > 
> > > > template <class _Tp, class... _Types>
> > > > struct __overload< _Tp, _Types...> : __overload<_Types...>
> > > > {
> > > >     using __overload<_Types...>::operator();
> > > > 
> > > >     template <bool _EnableBool>
> > > >     using _DepIsBool = typename __dependent_type<
> > > >         enable_if<is_same_v<remove_cv_t<_Tp>, bool> == _EnableBool>, _EnableBool
> > > >     >::type;
> > > > 
> > > >     template <class _Up, bool _EnableBool = false, class = _DepIsBool<_EnableBool>>
> > > >     auto operator()(_Tp, _Up&& __t) const
> > > >       -> decltype(_Tp{__t}, __identity<_Tp>());
> > > > 
> > > >     template <class _Up, class _Ap = __uncvref_t<_Up>,
> > > >         bool _EnableIfBool = true, class = _DepIsBool<_EnableIfBool>>
> > > >     auto operator()(bool, _Up&&, int _Ap::*) const
> > > >         -> __identity<_Tp>;
> > > > 
> > > >     template <class _Up, class _Ap = __uncvref_t<_Up>,
> > > >         bool _EnableIfBool = true, class = _DepIsBool<_EnableIfBool>>
> > > >     auto operator()(bool, _Up&&) const
> > > >         -> enable_if_t<is_same_v<_Ap, bool>, __identity<_Tp>>;
> > > > };
> > > > 
> > > > ```
> > > If we have concepts we can do so, but `__overload` is already variadic, leaving no space for sfinae...
> > Hmm. That doesn't seem to quite work..
> Instantiating function templates and sfinae are the slowest operations in meta programming, and I don't want to do this for all alternatives just because of a rare bool.
How would you go about testing this?


Repository:
  rCXX libc++

https://reviews.llvm.org/D44865