[Bug libstdc++/55713] std::tuple incorrectly is convertible to "ElementType" when it is an empty class

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55713
Bug 55713 depends on bug 63579, which changed state.

Bug 63579 Summary: New attribute for empty member optimization
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63579

           What    |Removed                     |Added
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             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED