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
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