Remove check for impossible condition in std::variant::index()

The __index_type is only ever unsigned char or unsigned short, so not
the same type as size_t.

	* include/std/variant (variant::index()): Remove impossible case.

From-SVN: r276100

libstdc++-v3/ChangeLog

 2019-09-24  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/std/variant (variant::index()): Remove impossible case.
+
 	PR libstdc++/91871
 	* testsuite/util/testsuite_hooks.h
 	(conversion::iterator_to_const_iterator()): Do not return an invalid

libstdc++-v3/include/std/variant

@@ -1520,9 +1520,7 @@ namespace __variant
       constexpr size_t index() const noexcept
       {
 	using __index_type = typename _Base::__index_type;
-	if constexpr (is_same_v<__index_type, size_t>)
-	  return this->_M_index;
-	else if constexpr (__detail::__variant::__never_valueless<_Types...>())
+	if constexpr (__detail::__variant::__never_valueless<_Types...>())
 	  return this->_M_index;
 	else if constexpr (sizeof...(_Types) <= __index_type(-1) / 2)
 	  return make_signed_t<__index_type>(this->_M_index);