[llvm-bugs] [Bug 51874] New: [C++20] Wrongly worded warning on defaulted-as-deleted operator!=

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=51874

            Bug ID: 51874
           Summary: [C++20] Wrongly worded warning on defaulted-as-deleted
                    operator!=
           Product: clang
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++2a
          Assignee: unassignedclangbugs at nondot.org
          Reporter: arthur.j.odwyer at gmail.com
                CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
                    llvm-bugs at lists.llvm.org, richard-llvm at metafoo.co.uk

// https://godbolt.org/z/GP1d8f5Kx
#include <compare>
struct C {
  friend std::strong_ordering operator<=>(const C&, const C&);
  bool operator!=(const C&) const = default;
};

<source>:5:8: warning: explicitly defaulted equality comparison operator is
implicitly deleted [-Wdefaulted-function-deleted]
  bool operator!=(const C&) const = default;
       ^
<source>:5:8: note: defaulted 'operator!=' is implicitly deleted because there
is no viable three-way comparison function for 'C'


The compiler is right to diagnose this ill-formed program, but wrong to claim
that there is no viable "three-way comparison function".
GCC gives a more accurate diagnostic (but only at the point where the
defaulted-as-deleted function is actually called):

<source>:5:8: note: 'bool C::operator!=(const C&) const' is implicitly deleted
because the default definition would be ill-formed:
    5 |   bool operator!=(const C&) const = default;
      |        ^~~~~~~~
<source>:5:8: error: no match for 'operator==' (operand types are 'const C' and
'const C')

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