[libcxx-commits] [libcxx] [libc++] Fix instantiation of incomplete type when evaluating tuple's operator<=> (PR #204679)

[A. Jiang via libcxx-commits]

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@@ -1008,11 +1002,20 @@ public:
     return std::__tuple_compare_equal<sizeof...(_Tp)>(__x, __y);
   }
 
+  template <class _Tuple>
+  struct __common_comparison_category_with;
+
+  template <class... _Up>
+    requires requires { typename common_comparison_category_t<__synth_three_way_result<_Tp, _Up>...>; }
+  struct __common_comparison_category_with<tuple<_Up...>> {
+    using type _LIBCPP_NODEBUG = common_comparison_category_t<__synth_three_way_result<_Tp, _Up>...>;
+  };
+
   template <__tuple_like_no_tuple _UTuple>
     requires(sizeof...(_Tp) == tuple_size_v<_UTuple>)
-  _LIBCPP_HIDE_FROM_ABI friend constexpr __tuple_common_comparison_category<tuple, _UTuple>
+  _LIBCPP_HIDE_FROM_ABI friend constexpr typename __common_comparison_category_with<__to_tuple_t<_UTuple>>::type
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frederick-vs-ja wrote:

I guess we need to generalize the change to `operator==`'s constraints to make GCC happy.

https://github.com/llvm/llvm-project/pull/204679