[PATCH] libstdc++: Define std::lexicographical_compare_three_way for C++20

[Jonathan Wakely]

On 03/01/20 15:03 +0000, Jonathan Wakely wrote:
>On 03/01/20 14:54 +0000, Jonathan Wakely wrote:
>>On 29/12/19 12:07 +0100, Stephan Bergmann wrote:
>>>FYI, the above fails with -std=c++2a and recent Clang trunk after <https://github.com/llvm/llvm-project/commit/bc633a42dd409dbeb456263e3388b8caa4680aa0> 
>>>"Mark the major papers for C++20 consistent comparisons as 'done', 
>>>and start publishing the corresponding feature-test macro":  Clang 
>>>now defines __cpp_impl_three_way_comparison (so 
>>>libstdc++-v3/include/std/version defines 
>>>__cpp_lib_three_way_comparison) but still doesn't define 
>>>__cpp_lib_concepts, so libstdc++-v3/libsupc++/compare doesn't 
>>>define compare_three_way.
>>>
>>>I locally managed that for now with extending the surrounding
>>>
>>>#if __cpp_lib_three_way_comparison
>>>
>>>with
>>>
>>>&& __cpp_lib_concepts
>>>
>>>>+    }
>>>>+#endif // three_way_comparison
>>
>>Should be fixed at r279861 on trunk (also attached). This leaves the
>>five-argument overload defined for Clang, just not the four-argument
>>one that uses std::compare_three_way.
>>
>>We could define that overload unconditionally and do something like
>>this, but I prefer to just omit it rather than define it badly:
>>
>>     return std::lexicographical_compare_three_way(__first1, __last1,
>>						    __first2, __last2,
>>#if __cpp_lib_concepts
>>						    compare_three_way{}
>>#else
>>						    [](auto&& __l, auto&& __r)
>>						    { return __l <=> __r; }
>>#endif // concepts
>>						    );
>
>After writing that, I realised that a better approach might be:
>
>--- a/libstdc++-v3/libsupc++/compare
>+++ b/libstdc++-v3/libsupc++/compare
>@@ -40,7 +40,9 @@
>
> namespace std
> {
>-#define __cpp_lib_three_way_comparison 201711L
>+#if __cpp_lib_concepts
>+# define __cpp_lib_three_way_comparison 201711L
>+#endif
>
>   // [cmp.categories], comparison category types
>
>
>i.e. don't pretend the library supports three-way comparison if that
>support is incomplete. That will still fix the Clang problem because
>if that macro isn't defined then we don't define either overload of
>std::lexicographical_compare_three_way.
>
>That seems cleaner to me. Anybody disagree?


I'm committing this patch to do that.

Tested powerpc64le-linux, committed to trunk.


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