[libc++] Fix chrono::duration constructor constraint

[ldionne]

Summary

As per [time.duration.cons]/1 the constructor constraint should be on
const Rep2&. As it is now the code will fail to compile in certain
cases, ex:

https://godbolt.org/z/c7fPrcTYM

struct S{
operator int() const&& noexcept = delete;
operator int() const& noexcept;
};

const S &fun();

auto k = std::chrono::microseconds{fun()};

[ldionne]

Thanks for your patch!

Can you please add a test for this? The correct file seems like it should be libcxx/test/std/utilities/time/time.duration/time.duration.cons/rep.pass.cpp.

Also, if you need someone to commit this for you, please provide Author Name <email@domain> for commit attribution.

[tiagoma]

In D118902#3293588, @ldionne wrote:

Thanks for your patch!

Can you please add a test for this? The correct file seems like it should be libcxx/test/std/utilities/time/time.duration/time.duration.cons/rep.pass.cpp.

Also, if you need someone to commit this for you, please provide Author Name <email@domain> for commit attribution.

"Tiago Macarios <[e-mail]>"

[ldionne]

Commandeer per offline discussion.

[ldionne]

Fix the test.

libcxx/include/__chrono/duration.h

#else
         duration() {}
 #endif
 
     template <class _Rep2>
         _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
         explicit duration(const _Rep2& __r,
             typename enable_if
             <
-               is_convertible<_Rep2, rep>::value &&
+               is_convertible<const _Rep2&, rep>::value &&
                (treat_as_floating_point<rep>::value ||
                !treat_as_floating_point<_Rep2>::value)
             >::type* = nullptr)
                 : __rep_(__r) {}
 
     // conversions
     template <class _Rep2, class _Period2>
         _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR