[llvm-bugs] [Bug 44723] New: Defaulted comparisons deleted for class containing class containing class template with user-provided operator<=> with deduced return type

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=44723

            Bug ID: 44723
           Summary: Defaulted comparisons deleted for class containing
                    class containing class template with user-provided
                    operator<=> with deduced return type
           Product: clang
           Version: trunk
          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 valid code:


```
#include <compare>

template<int>
struct a {
        friend constexpr auto operator<=>(a const & lhs, a const & rhs) {
                return std::strong_ordering::equal;
        }
};

struct b {
        friend auto operator<=>(b const &, b const &) = default;
        a<0> m_value;
};

struct c {
        friend auto operator<=>(c const &, c const &) = default;
        a<0> m_value;
};
auto x = c() <=> c();
```

is rejected with the message:

```
main.cpp:11:14: warning: explicitly defaulted three-way comparison operator is
implicitly deleted [-Wdefaulted-function-deleted]
        friend auto operator<=>(b const &, b const &) = default;
                    ^
main.cpp:12:7: note: return type of defaulted 'operator<=>' cannot be deduced
because return type 'auto' of three-way comparison for member 'm_value' is not
a standard comparison category type
        a<0> m_value;
             ^
main.cpp:5:24: note: selected 'operator<=>' for member 'm_value' declared here
        friend constexpr auto operator<=>(a const & lhs, a const & rhs) {
                              ^
main.cpp:16:14: warning: explicitly defaulted three-way comparison operator is
implicitly deleted [-Wdefaulted-function-deleted]
        friend auto operator<=>(c const &, c const &) = default;
                    ^
main.cpp:17:7: note: return type of defaulted 'operator<=>' cannot be deduced
because return type 'auto' of three-way comparison for member 'm_value' is not
a standard comparison category type
        a<0> m_value;
             ^
main.cpp:5:24: note: selected 'operator<=>' for member 'm_value' declared here
        friend constexpr auto operator<=>(a const & lhs, a const & rhs) {
                              ^
main.cpp:19:14: error: overload resolution selected deleted operator '<=>'
auto x = c() <=> c();
         ~~~ ^   ~~~
main.cpp:16:14: note: candidate function has been implicitly deleted
        friend auto operator<=>(c const &, c const &) = default;
                    ^
2 warnings and 1 error generated.
```

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