[llvm-bugs] [Bug 44721] New: Defaulted comparison operator with built-in type as member in same namespace as comparison operator template rejected

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=44721

            Bug ID: 44721
           Summary: Defaulted comparison operator with built-in type as
                    member in same namespace as comparison operator
                    template rejected
           Product: clang
           Version: 10.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++2a
          Assignee: unassignedclangbugs at nondot.org
          Reporter: david at doublewise.net
                CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
                    llvm-bugs at lists.llvm.org, richard-llvm at metafoo.co.uk

The following well-formed code:

```
namespace n {

template<typename T>
bool operator==(T const &, T const &) {
    return true;
}

struct s {
        friend bool operator==(s const &, s const &) = default;
        int x;
};

}
```

is rejected by clang with the following message:

```
main.cpp:4:6: error: overloaded 'operator==' must have at least one parameter
of class or enumeration type
bool operator==(T const &, T const &) {
     ^
main.cpp:9:14: note: in instantiation of function template specialization
'n::operator==<int>' requested here
        friend bool operator==(s const &, s const &) = default;
                    ^
1 error generated.
```

A similar error occurs for `operator<=>`. This occurs only when the comparison
operator is defaulted. Manually writing out the equivalent definition is fine.


(In the real code, the generic `operator==` is constrained with a concept that
would not match any built-in type)

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