[libcxx-commits] [PATCH] D131395: [libc++] Implement `lexicographical_compare_three_way`

[Adrian Vogelsgesang via Phabricator via libcxx-commits]

avogelsgesang added inline comments.


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Comment at: libcxx/test/std/algorithms/alg.sorting/alg.three.way/lexicographical_compare_three_way.pass.cpp:62
+}
+
+constexpr bool test() {
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philnik wrote:
> avogelsgesang wrote:
> > philnik wrote:
> > > avogelsgesang wrote:
> > > > mumbleskates wrote:
> > > > > avogelsgesang wrote:
> > > > > > huixie90 wrote:
> > > > > > > The spec has explicitly specifies the return type ` -> decltype(__comp(*__first1, *__first2))` and this has a SFINAE effect.
> > > > > > > It would be good to test the SFINAE effect as well (if __comp is not callbale then the function should be SFINAEed out)
> > > > > > Sorry, but I don't quite understand this point. How should I check for SFINAE here?
> > > > > in C++20 (which you have the luxury of relying upon since this function is new in C++20) you can just make a concept and then `static_check` it.
> > > > still can't follow. Did you mean `static_assert`? Or is `static_check` something different?
> > > > How would I use a concept to `static_assert` that `std::lexicographical_compare_three_way` is excluded from a function overload set due to SFINAE?
> > > You can do something like
> > > ```
> > > template <class T>
> > > concept HasLexicographicalCompare = requires (T whatever) { std::lexicographical_compare_three_way(whatever); };
> > > 
> > > static_assert(hasLexicographicalCompare<CorrectType>)
> > > static_assert(!HasLexicographicalCompare<IncorrectType>);
> > > ```
> > Thanks for that example! I was able to use this for `lexicographical_compare_three_way_comp.pass.cpp`.
> > 
> > However, I am still struggling to understand what you want to me to change in this file, i.e. `lexicographical_compare_three_way.pass.cpp` where we don't pass a comparator.
> > 
> > What I could come up with so far is:
> > 
> > ```
> > template <class T>
> > concept has_lexicographical_compare = requires (T t) { std::lexicographical_compare_three_way(std::declval<T*>(), std::declval<T*>(), std::declval<T*>(), std::declval<T*>()); };
> > 
> > // Test that `std::lexicographical_compare_three_way` accepts valid types
> > static_assert(has_lexicographical_compare<int>);
> > static_assert(has_lexicographical_compare<WeakInt>);
> > static_assert(has_lexicographical_compare<PartialInt>);
> > // Test that `std::lexicographical_compare_three_way` rejects invalid types
> > static_assert(!has_lexicographical_compare<UnexpectedlyComparableInt>);
> > static_assert(!has_lexicographical_compare<UncomparableInt>);
> > ```
> > 
> > However that led to the error
> > 
> > ```
> > In file included from /home/tsi/avogelsgesang/Documents/llvm-project/libcxx/test/std/algorithms/alg.sorting/alg.three.way/lexicographical_compare_three_way.pass.cpp:18:
> > In file included from /home/tsi/avogelsgesang/Documents/llvm-project/build/include/c++/v1/algorithm:1771:
> > /home/tsi/avogelsgesang/Documents/llvm-project/build/include/c++/v1/__algorithm/lexicographical_compare_three_way.h:71:10: error: no matching function for call to 'lexicographical_compare_three_way'  return std::lexicographical_compare_three_way(__first1, __last1, __first2, __last2, compare_three_way());                                                                                                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > ```
> Yes, that's a bug in your code :P. You have to add `-> decltype(std::lexicographical_compare_three_way(__first1, __last1, __first2, __last2, compare_three_way()))` to the overload. Also, the `T t` isn't just there for good looks ;-). In your case you can rewrite it to
> ```
> template <class T, class U>
> concept has_lexicographical_compare = requires (T first1, T last1, U first2, U last2) { std::lexicographical_compare_three_way(first1, last1, first2, last2); };
> ```
> and then also check with different iterator types.
> You have to add `-> decltype(std::lexicographical_compare_three_way(__first1, __last1, __first2, __last2, compare_three_way()))` to the overload

Afaict, this explicit return type is not present in the standard (https://eel.is/c++draft/alg.sorting#alg.three.way). Or am I looking in the wrong place?


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