[libcxx][nfc] makes comparison operators for `std::*_ordering` types hidden friends

[cjdb]

Summary

The standard leaves it up to the implementation to decide whether or not
these operators are hidden friends. There are several (well-documented)
reasons to prefer hidden friends, as well as an argument for improved
readability.

Depends on D100342.

[cjdb]

rebases to activate CI

[zoecarver]

Can you update the title/commit to say this is a NFC.

Then, get another approval and ship it!!

[cjdb]

updates commit message

[• Quuxplusone]

LGTM, but (re @zoecarver's comment)
Please do not mark this as "NFC", since in fact it has a significant effect on semantics: it makes the operators hidden friends!
Hidden friends are hidden (from non-ADL lookup); non-hidden friends are not hidden from non-ADL name lookup.
@zoecarver, an example program (which we are deliberately breaking and that's fine) would be, like, https://godbolt.org/z/75jM676hx

template<class T>
void f(bool (*fp)(T, std::partial_ordering)) {}

int main() {
    f(std::operator<);
}

libcxx/include/compare

 public:
   // valid values
   static const partial_ordering less;
   static const partial_ordering equivalent;
   static const partial_ordering greater;
   static const partial_ordering unordered;
 
   // comparisons
-  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator==(partial_ordering __v, _CmpUnspecifiedParam) noexcept;
-  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator< (partial_ordering __v, _CmpUnspecifiedParam) noexcept;
-  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator<=(partial_ordering __v, _CmpUnspecifiedParam) noexcept;
-  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator> (partial_ordering __v, _CmpUnspecifiedParam) noexcept;
-  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator>=(partial_ordering __v, _CmpUnspecifiedParam) noexcept;
-  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator< (_CmpUnspecifiedParam, partial_ordering __v) noexcept;
-  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator<=(_CmpUnspecifiedParam, partial_ordering __v) noexcept;
-  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator> (_CmpUnspecifiedParam, partial_ordering __v) noexcept;
-  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator>=(_CmpUnspecifiedParam, partial_ordering __v) noexcept;
-
   _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator==(partial_ordering, partial_ordering) noexcept = default;
 
-  _LIBCPP_INLINE_VISIBILITY friend constexpr partial_ordering operator<=>(partial_ordering __v, _CmpUnspecifiedParam) noexcept;
-  _LIBCPP_INLINE_VISIBILITY friend constexpr partial_ordering operator<=>(_CmpUnspecifiedParam, partial_ordering __v) noexcept;
-
-private:
-  _ValueT __value_;
-};
-
-_LIBCPP_INLINE_VAR constexpr partial_ordering partial_ordering::less(_OrdResult::__less);
-_LIBCPP_INLINE_VAR constexpr partial_ordering partial_ordering::equivalent(_EqResult::__equiv);
-_LIBCPP_INLINE_VAR constexpr partial_ordering partial_ordering::greater(_OrdResult::__greater);
-_LIBCPP_INLINE_VAR constexpr partial_ordering partial_ordering::unordered(_NCmpResult ::__unordered);
-
-_LIBCPP_INLINE_VISIBILITY
-constexpr bool operator==(partial_ordering __v, _CmpUnspecifiedParam) noexcept {
+  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator==(partial_ordering __v, _CmpUnspecifiedParam) noexcept {
     return __v.__is_ordered() && __v.__value_ == 0;
   }
-_LIBCPP_INLINE_VISIBILITY
-constexpr bool operator< (partial_ordering __v, _CmpUnspecifiedParam) noexcept {
+
+  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator< (partial_ordering __v, _CmpUnspecifiedParam) noexcept {
     return __v.__is_ordered() && __v.__value_ < 0;
   }
-_LIBCPP_INLINE_VISIBILITY
-constexpr bool operator<=(partial_ordering __v, _CmpUnspecifiedParam) noexcept {
+
+  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator<=(partial_ordering __v, _CmpUnspecifiedParam) noexcept  {
     return __v.__is_ordered() && __v.__value_ <= 0;
   }
-_LIBCPP_INLINE_VISIBILITY
-constexpr bool operator> (partial_ordering __v, _CmpUnspecifiedParam) noexcept {
+
+  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator> (partial_ordering __v, _CmpUnspecifiedParam) noexcept  {
     return __v.__is_ordered() && __v.__value_ > 0;
   }
-_LIBCPP_INLINE_VISIBILITY
-constexpr bool operator>=(partial_ordering __v, _CmpUnspecifiedParam) noexcept {
+
+  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator>=(partial_ordering __v, _CmpUnspecifiedParam) noexcept  {
     return __v.__is_ordered() && __v.__value_ >= 0;
   }
 
-_LIBCPP_INLINE_VISIBILITY
-constexpr bool operator< (_CmpUnspecifiedParam, partial_ordering __v) noexcept {
+  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator< (_CmpUnspecifiedParam, partial_ordering __v) noexcept  {
     return __v.__is_ordered() && 0 < __v.__value_;
   }
-_LIBCPP_INLINE_VISIBILITY
-constexpr bool operator<=(_CmpUnspecifiedParam, partial_ordering __v) noexcept {
+
+  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator<=(_CmpUnspecifiedParam, partial_ordering __v) noexcept  {
     return __v.__is_ordered() && 0 <= __v.__value_;
   }
-_LIBCPP_INLINE_VISIBILITY
-constexpr bool operator> (_CmpUnspecifiedParam, partial_ordering __v) noexcept {
+
+  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator> (_CmpUnspecifiedParam, partial_ordering __v) noexcept  {
     return __v.__is_ordered() && 0 > __v.__value_;
   }
-_LIBCPP_INLINE_VISIBILITY
-constexpr bool operator>=(_CmpUnspecifiedParam, partial_ordering __v) noexcept {
+
+  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator>=(_CmpUnspecifiedParam, partial_ordering __v) noexcept  {
     return __v.__is_ordered() && 0 >= __v.__value_;
   }
 
-_LIBCPP_INLINE_VISIBILITY
-constexpr partial_ordering operator<=>(partial_ordering __v, _CmpUnspecifiedParam) noexcept {
+  _LIBCPP_INLINE_VISIBILITY friend constexpr partial_ordering operator<=>(partial_ordering __v, _CmpUnspecifiedParam) noexcept  {
     return __v;
   }
-_LIBCPP_INLINE_VISIBILITY
-constexpr partial_ordering operator<=>(_CmpUnspecifiedParam, partial_ordering __v) noexcept {
+
+  _LIBCPP_INLINE_VISIBILITY friend constexpr partial_ordering operator<=>(_CmpUnspecifiedParam, partial_ordering __v) noexcept  {
     return __v < 0 ? partial_ordering::greater : (__v > 0 ? partial_ordering::less : __v);
   }
+private:
+  _ValueT __value_;
+};
+
+_LIBCPP_INLINE_VAR constexpr partial_ordering partial_ordering::less(_OrdResult::__less);
+_LIBCPP_INLINE_VAR constexpr partial_ordering partial_ordering::equivalent(_EqResult::__equiv);
+_LIBCPP_INLINE_VAR constexpr partial_ordering partial_ordering::greater(_OrdResult::__greater);
+_LIBCPP_INLINE_VAR constexpr partial_ordering partial_ordering::unordered(_NCmpResult ::__unordered);
 
 class weak_ordering {
   using _ValueT = signed char;
 
   _LIBCPP_INLINE_VISIBILITY
   explicit constexpr weak_ordering(_EqResult __v) noexcept : __value_(_ValueT(__v)) {}
   _LIBCPP_INLINE_VISIBILITY
   explicit constexpr weak_ordering(_OrdResult __v) noexcept : __value_(_ValueT(__v)) {}
 
 public:
   static const weak_ordering less;
   static const weak_ordering equivalent;
   static const weak_ordering greater;
 
   _LIBCPP_INLINE_VISIBILITY
   constexpr operator partial_ordering() const noexcept {
     return __value_ == 0 ? partial_ordering::equivalent
         : (__value_ < 0 ? partial_ordering::less : partial_ordering::greater);
   }
 
   // comparisons
-  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator==(weak_ordering __v, _CmpUnspecifiedParam) noexcept;
-  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator< (weak_ordering __v, _CmpUnspecifiedParam) noexcept;
-  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator<=(weak_ordering __v, _CmpUnspecifiedParam) noexcept;
-  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator> (weak_ordering __v, _CmpUnspecifiedParam) noexcept;
-  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator>=(weak_ordering __v, _CmpUnspecifiedParam) noexcept;
-  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator< (_CmpUnspecifiedParam, weak_ordering __v) noexcept;
-  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator<=(_CmpUnspecifiedParam, weak_ordering __v) noexcept;
-  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator> (_CmpUnspecifiedParam, weak_ordering __v) noexcept;
-  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator>=(_CmpUnspecifiedParam, weak_ordering __v) noexcept;
-
   _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator==(weak_ordering, weak_ordering) noexcept = default;
 
-  _LIBCPP_INLINE_VISIBILITY friend constexpr weak_ordering operator<=>(weak_ordering __v, _CmpUnspecifiedParam) noexcept;
-  _LIBCPP_INLINE_VISIBILITY friend constexpr weak_ordering operator<=>(_CmpUnspecifiedParam, weak_ordering __v) noexcept;
-
-private:
-  _ValueT __value_;
-};
-
-_LIBCPP_INLINE_VAR constexpr weak_ordering weak_ordering::less(_OrdResult::__less);
-_LIBCPP_INLINE_VAR constexpr weak_ordering weak_ordering::equivalent(_EqResult::__equiv);
-_LIBCPP_INLINE_VAR constexpr weak_ordering weak_ordering::greater(_OrdResult::__greater);
-
-_LIBCPP_INLINE_VISIBILITY
-constexpr bool operator==(weak_ordering __v, _CmpUnspecifiedParam) noexcept {
+  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator==(weak_ordering __v, _CmpUnspecifiedParam) noexcept {
     return __v.__value_ == 0;
   }
-_LIBCPP_INLINE_VISIBILITY
-constexpr bool operator< (weak_ordering __v, _CmpUnspecifiedParam) noexcept {
+
+  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator< (weak_ordering __v, _CmpUnspecifiedParam) noexcept {
     return __v.__value_ < 0;
   }
-_LIBCPP_INLINE_VISIBILITY
-constexpr bool operator<=(weak_ordering __v, _CmpUnspecifiedParam) noexcept {
+
+  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator<=(weak_ordering __v, _CmpUnspecifiedParam) noexcept {
     return __v.__value_ <= 0;
   }
-_LIBCPP_INLINE_VISIBILITY
-constexpr bool operator> (weak_ordering __v, _CmpUnspecifiedParam) noexcept {
+
+  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator> (weak_ordering __v, _CmpUnspecifiedParam) noexcept {
     return __v.__value_ > 0;
   }
-_LIBCPP_INLINE_VISIBILITY
-constexpr bool operator>=(weak_ordering __v, _CmpUnspecifiedParam) noexcept {
+
+  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator>=(weak_ordering __v, _CmpUnspecifiedParam) noexcept {
     return __v.__value_ >= 0;
   }
-_LIBCPP_INLINE_VISIBILITY
-constexpr bool operator< (_CmpUnspecifiedParam, weak_ordering __v) noexcept {
+
+  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator< (_CmpUnspecifiedParam, weak_ordering __v) noexcept {
     return 0 < __v.__value_;
   }
-_LIBCPP_INLINE_VISIBILITY
-constexpr bool operator<=(_CmpUnspecifiedParam, weak_ordering __v) noexcept {
+
+  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator<=(_CmpUnspecifiedParam, weak_ordering __v) noexcept {
     return 0 <= __v.__value_;
   }
-_LIBCPP_INLINE_VISIBILITY
-constexpr bool operator> (_CmpUnspecifiedParam, weak_ordering __v) noexcept {
+
+  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator> (_CmpUnspecifiedParam, weak_ordering __v) noexcept {
     return 0 > __v.__value_;
   }
-_LIBCPP_INLINE_VISIBILITY
-constexpr bool operator>=(_CmpUnspecifiedParam, weak_ordering __v) noexcept {
+
+  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator>=(_CmpUnspecifiedParam, weak_ordering __v) noexcept {
     return 0 >= __v.__value_;
   }
 
-_LIBCPP_INLINE_VISIBILITY
-constexpr weak_ordering operator<=>(weak_ordering __v, _CmpUnspecifiedParam) noexcept {
+  _LIBCPP_INLINE_VISIBILITY friend constexpr weak_ordering operator<=>(weak_ordering __v, _CmpUnspecifiedParam) noexcept {
     return __v;
   }
-_LIBCPP_INLINE_VISIBILITY
-constexpr weak_ordering operator<=>(_CmpUnspecifiedParam, weak_ordering __v) noexcept {
+
+  _LIBCPP_INLINE_VISIBILITY friend constexpr weak_ordering operator<=>(_CmpUnspecifiedParam, weak_ordering __v) noexcept {
     return __v < 0 ? weak_ordering::greater : (__v > 0 ? weak_ordering::less : __v);
   }
+
+private:
+  _ValueT __value_;
+};
+
+_LIBCPP_INLINE_VAR constexpr weak_ordering weak_ordering::less(_OrdResult::__less);
+_LIBCPP_INLINE_VAR constexpr weak_ordering weak_ordering::equivalent(_EqResult::__equiv);
+_LIBCPP_INLINE_VAR constexpr weak_ordering weak_ordering::greater(_OrdResult::__greater);
 class strong_ordering {
   using _ValueT = signed char;
 
   _LIBCPP_INLINE_VISIBILITY
   explicit constexpr strong_ordering(_EqResult __v) noexcept : __value_(_ValueT(__v)) {}
   _LIBCPP_INLINE_VISIBILITY
   explicit constexpr strong_ordering(_OrdResult __v) noexcept : __value_(_ValueT(__v)) {}
 
public:
 
   _LIBCPP_INLINE_VISIBILITY
   constexpr operator weak_ordering() const noexcept {
     return __value_ == 0 ? weak_ordering::equivalent
         : (__value_ < 0 ? weak_ordering::less : weak_ordering::greater);
   }
 
   // comparisons
-  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator==(strong_ordering __v, _CmpUnspecifiedParam) noexcept;
-  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator< (strong_ordering __v, _CmpUnspecifiedParam) noexcept;
-  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator<=(strong_ordering __v, _CmpUnspecifiedParam) noexcept;
-  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator> (strong_ordering __v, _CmpUnspecifiedParam) noexcept;
-  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator>=(strong_ordering __v, _CmpUnspecifiedParam) noexcept;
-  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator< (_CmpUnspecifiedParam, strong_ordering __v) noexcept;
-  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator<=(_CmpUnspecifiedParam, strong_ordering __v) noexcept;
-  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator> (_CmpUnspecifiedParam, strong_ordering __v) noexcept;
-  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator>=(_CmpUnspecifiedParam, strong_ordering __v) noexcept;
-
   _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator==(strong_ordering, strong_ordering) noexcept = default;
 
-  _LIBCPP_INLINE_VISIBILITY friend constexpr strong_ordering operator<=>(strong_ordering __v, _CmpUnspecifiedParam) noexcept;
-  _LIBCPP_INLINE_VISIBILITY friend constexpr strong_ordering operator<=>(_CmpUnspecifiedParam, strong_ordering __v) noexcept;
-
-private:
-  _ValueT __value_;
-};
-
-_LIBCPP_INLINE_VAR constexpr strong_ordering strong_ordering::less(_OrdResult::__less);
-_LIBCPP_INLINE_VAR constexpr strong_ordering strong_ordering::equal(_EqResult::__equal);
-_LIBCPP_INLINE_VAR constexpr strong_ordering strong_ordering::equivalent(_EqResult::__equiv);
-_LIBCPP_INLINE_VAR constexpr strong_ordering strong_ordering::greater(_OrdResult::__greater);
-
-_LIBCPP_INLINE_VISIBILITY
-constexpr bool operator==(strong_ordering __v, _CmpUnspecifiedParam) noexcept {
+  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator==(strong_ordering __v, _CmpUnspecifiedParam) noexcept {
     return __v.__value_ == 0;
   }
-_LIBCPP_INLINE_VISIBILITY
-constexpr bool operator< (strong_ordering __v, _CmpUnspecifiedParam) noexcept {
+
+  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator< (strong_ordering __v, _CmpUnspecifiedParam) noexcept {
     return __v.__value_ < 0;
   }
-_LIBCPP_INLINE_VISIBILITY
-constexpr bool operator<=(strong_ordering __v, _CmpUnspecifiedParam) noexcept {
+
+  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator<=(strong_ordering __v, _CmpUnspecifiedParam) noexcept {
     return __v.__value_ <= 0;
   }
-_LIBCPP_INLINE_VISIBILITY
-constexpr bool operator> (strong_ordering __v, _CmpUnspecifiedParam) noexcept {
+
+  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator> (strong_ordering __v, _CmpUnspecifiedParam) noexcept {
     return __v.__value_ > 0;
   }
-_LIBCPP_INLINE_VISIBILITY
-constexpr bool operator>=(strong_ordering __v, _CmpUnspecifiedParam) noexcept {
+
+  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator>=(strong_ordering __v, _CmpUnspecifiedParam) noexcept {
     return __v.__value_ >= 0;
   }
-_LIBCPP_INLINE_VISIBILITY
-constexpr bool operator< (_CmpUnspecifiedParam, strong_ordering __v) noexcept {
+
+  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator< (_CmpUnspecifiedParam, strong_ordering __v) noexcept {
     return 0 < __v.__value_;
   }
-_LIBCPP_INLINE_VISIBILITY
-constexpr bool operator<=(_CmpUnspecifiedParam, strong_ordering __v) noexcept {
+
+  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator<=(_CmpUnspecifiedParam, strong_ordering __v) noexcept {
     return 0 <= __v.__value_;
   }
-_LIBCPP_INLINE_VISIBILITY
-constexpr bool operator> (_CmpUnspecifiedParam, strong_ordering __v) noexcept {
+
+  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator> (_CmpUnspecifiedParam, strong_ordering __v) noexcept {
     return 0 > __v.__value_;
   }
-_LIBCPP_INLINE_VISIBILITY
-constexpr bool operator>=(_CmpUnspecifiedParam, strong_ordering __v) noexcept {
+
+  _LIBCPP_INLINE_VISIBILITY friend constexpr bool operator>=(_CmpUnspecifiedParam, strong_ordering __v) noexcept {
     return 0 >= __v.__value_;
   }
 
-_LIBCPP_INLINE_VISIBILITY
-constexpr strong_ordering operator<=>(strong_ordering __v, _CmpUnspecifiedParam) noexcept {
+  _LIBCPP_INLINE_VISIBILITY friend constexpr strong_ordering operator<=>(strong_ordering __v, _CmpUnspecifiedParam) noexcept {
     return __v;
   }
-_LIBCPP_INLINE_VISIBILITY
-constexpr strong_ordering operator<=>(_CmpUnspecifiedParam, strong_ordering __v) noexcept {
+
+  _LIBCPP_INLINE_VISIBILITY friend constexpr strong_ordering operator<=>(_CmpUnspecifiedParam, strong_ordering __v) noexcept {
     return __v < 0 ? strong_ordering::greater : (__v > 0 ? strong_ordering::less : __v);
   }
 
+private:
+  _ValueT __value_;
+};
+
+_LIBCPP_INLINE_VAR constexpr strong_ordering strong_ordering::less(_OrdResult::__less);
+_LIBCPP_INLINE_VAR constexpr strong_ordering strong_ordering::equal(_EqResult::__equal);
+_LIBCPP_INLINE_VAR constexpr strong_ordering strong_ordering::equivalent(_EqResult::__equiv);
+_LIBCPP_INLINE_VAR constexpr strong_ordering strong_ordering::greater(_OrdResult::__greater);
+
 // named comparison functions
 _LIBCPP_INLINE_VISIBILITY
 constexpr bool is_lt(partial_ordering __cmp) noexcept { return __cmp < 0; }
 
 _LIBCPP_INLINE_VISIBILITY
 constexpr bool is_lteq(partial_ordering __cmp) noexcept { return __cmp <= 0; }
 
 _LIBCPP_INLINE_VISIBILITY