[Clang] Failed to compile using default three-way comparison in wrapper class when templated type doesn't define comparison

[scott-zhong]

The following failed to compile on Clang but works with GCC.

#include <memory>

struct UDT
{
};

template <class T>
struct Base {
public:
  T* cbegin() const { return t_; }
  T* cend() const { return t_; }
  friend inline auto operator<=>(const Base& lhs,
                                 const Base& rhs) {
    return std::lexicographical_compare_three_way(
        lhs.cbegin(), lhs.cend(), rhs.cbegin(), rhs.cend(),
        [](auto&& lhs, auto&& rhs) {
          return std::forward_as_tuple(lhs) <=> std::forward_as_tuple(rhs);
        });
  }
  T* t_;
};

template <class T>
class Wrapper {
public:
  friend inline auto operator<=>(const Wrapper& lhs,
                                 const Wrapper& rhs) = default;

private:
  Base<T> impl_;

};

int main()
{
  Wrapper<UDT> c;
}

The compile and output:

$ clang++ --version && clang++ -std=c++20 -c t.cpp
clang version 16.0.6 (Red Hat 16.0.6-2.module+el8.9.0+19521+190d7aba)
Target: x86_64-redhat-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
t.cpp:17:45: error: invalid operands to binary expression ('tuple<UDT &>' and 'tuple<UDT &>')
          return std::forward_as_tuple(lhs) <=> std::forward_as_tuple(rhs);
                 ~~~~~~~~~~~~~~~~~~~~~~~~~~ ^   ~~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/stl_algobase.h:1826:17: note: in instantiation of function template specialization 'operator<=>(const Base<UDT> &, const Base<UDT> &)::(anonymous class)::operator()<UDT &, UDT &>' requested here
    -> decltype(__comp(*__first1, *__first2))
                ^
t.cpp:14:12: note: while substituting deduced template arguments into function template 'lexicographical_compare_three_way' [with _InputIter1 = UDT *, _InputIter2 = UDT *, _Comp = (lambda at t.cpp:16:9)]
    return std::lexicographical_compare_three_way(
           ^
t.cpp:26:22: note: in instantiation of member function 'operator<=>' requested here
  friend inline auto operator<=>(const Wrapper& lhs,
                     ^
t.cpp:36:16: note: in instantiation of template class 'Wrapper<UDT>' requested here
  Wrapper<UDT> c;
               ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/system_error:316:3: note: candidate function not viable: no known conversion from 'tuple<UDT &>' to 'const error_code' for 1st argument
  operator<=>(const error_code& __lhs, const error_code& __rhs) noexcept
  ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/system_error:498:3: note: candidate function not viable: no known conversion from 'tuple<UDT &>' to 'const error_condition' for 1st argument
  operator<=>(const error_condition& __lhs,
  ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/stl_pair.h:819:5: note: candidate template ignored: could not match 'pair' against 'tuple'
    operator<=>(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
    ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/stl_iterator.h:583:5: note: candidate template ignored: could not match 'reverse_iterator' against 'tuple'
    operator<=>(const reverse_iterator<_IteratorL>& __x,
    ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/stl_iterator.h:583:5: note: candidate template ignored: could not match 'reverse_iterator' against 'tuple'
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/stl_iterator.h:601:5: note: candidate template ignored: could not match 'reverse_iterator' against 'tuple'
    operator<=>(const reverse_iterator<_Iterator>& __x,
    ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/stl_iterator.h:1690:5: note: candidate template ignored: could not match 'move_iterator' against 'tuple'
    operator<=>(const move_iterator<_IteratorL>& __x,
    ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/stl_iterator.h:1690:5: note: candidate template ignored: could not match 'move_iterator' against 'tuple'
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/stl_iterator.h:1756:5: note: candidate template ignored: could not match 'move_iterator' against 'tuple'
    operator<=>(const move_iterator<_Iterator>& __x,
    ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/tuple:1938:5: note: candidate template ignored: substitution failure [with _Tps = <UDT &>, _Ups = <UDT &>]: no matching function for call to object of type 'const struct _Synth3way'
    operator<=>(const tuple<_Tps...>& __t, const tuple<_Ups...>& __u)
    ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/tuple:1938:5: note: candidate template ignored: substitution failure [with _Tps = <UDT &>, _Ups = <UDT &>]: no matching function for call to object of type 'const struct _Synth3way'
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/string_view:624:5: note: candidate template ignored: could not match 'basic_string_view' against 'tuple'
    operator<=>(basic_string_view<_CharT, _Traits> __x,
    ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/string_view:632:5: note: candidate template ignored: could not match 'basic_string_view' against 'tuple'
    operator<=>(basic_string_view<_CharT, _Traits> __x,
    ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/string_view:632:5: note: candidate template ignored: could not match 'basic_string_view' against 'tuple'
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/basic_string.h:3733:5: note: candidate template ignored: could not match 'basic_string' against 'tuple'
    operator<=>(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
    ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/basic_string.h:3748:5: note: candidate template ignored: could not match 'basic_string' against 'tuple'
    operator<=>(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
    ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/basic_string.h:3748:5: note: candidate template ignored: could not match 'basic_string' against 'tuple'
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/unique_ptr.h:988:5: note: candidate template ignored: could not match 'unique_ptr' against 'tuple'
    operator<=>(const unique_ptr<_Tp, _Dp>& __x,
    ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/unique_ptr.h:988:5: note: candidate template ignored: could not match 'unique_ptr' against 'tuple'
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/unique_ptr.h:997:5: note: candidate template ignored: could not match 'unique_ptr' against 'tuple'
    operator<=>(const unique_ptr<_Tp, _Dp>& __x, nullptr_t)
    ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/unique_ptr.h:997:5: note: candidate template ignored: could not match 'unique_ptr' against 'tuple'
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/shared_ptr_base.h:1805:5: note: candidate template ignored: could not match '__shared_ptr' against 'tuple'
    operator<=>(const __shared_ptr<_Tp, _Lp>& __a,
    ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/shared_ptr_base.h:1805:5: note: candidate template ignored: could not match '__shared_ptr' against 'tuple'
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/shared_ptr_base.h:1811:5: note: candidate template ignored: could not match '__shared_ptr' against 'tuple'
    operator<=>(const __shared_ptr<_Tp, _Lp>& __a, nullptr_t) noexcept
    ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/shared_ptr_base.h:1811:5: note: candidate template ignored: could not match '__shared_ptr' against 'tuple'
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/shared_ptr.h:567:5: note: candidate template ignored: could not match 'shared_ptr' against 'tuple'
    operator<=>(const shared_ptr<_Tp>& __a,
    ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/shared_ptr.h:567:5: note: candidate template ignored: could not match 'shared_ptr' against 'tuple'
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/shared_ptr.h:573:5: note: candidate template ignored: could not match 'shared_ptr' against 'tuple'
    operator<=>(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
    ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/shared_ptr.h:573:5: note: candidate template ignored: could not match 'shared_ptr' against 'tuple'
t.cpp:14:12: error: no matching function for call to 'lexicographical_compare_three_way'
    return std::lexicographical_compare_three_way(
           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
t.cpp:26:22: note: in instantiation of member function 'operator<=>' requested here
  friend inline auto operator<=>(const Wrapper& lhs,
                     ^
t.cpp:36:16: note: in instantiation of template class 'Wrapper<UDT>' requested here
  Wrapper<UDT> c;
               ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/stl_algobase.h:1821:5: note: candidate template ignored: substitution failure [with _InputIter1 = UDT *, _InputIter2 = UDT *, _Comp = (lambda at t.cpp:16:9)]
    lexicographical_compare_three_way(_InputIter1 __first1,
    ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/stl_algobase.h:1869:5: note: candidate function template not viable: requires 4 arguments, but 5 were provided
    lexicographical_compare_three_way(_InputIter1 __first1,
    ^
t.cpp:26:22: error: return type of defaulted 'operator<=>' cannot be deduced because three-way comparison for member 'impl_' has a deduced return type and is not yet defined
  friend inline auto operator<=>(const Wrapper& lhs,
                     ^
t.cpp:36:16: note: in instantiation of template class 'Wrapper<UDT>' requested here
  Wrapper<UDT> c;
               ^
t.cpp:30:11: note: member 'impl_' declared here
  Base<T> impl_;
          ^
t.cpp:12:22: note: selected 'operator<=>' for member 'impl_' declared here
  friend inline auto operator<=>(const Base& lhs,
                     ^
3 errors generated.

Same failure occurs with Clang 19.1.7. The testcase isn't calling any functions that would call the comparison operator, and it shouldn't have failed to compile.

Interestingly, changing the default three-way to the following compiles without any errors.

  friend inline auto operator<=>(const Wrapper& lhs,
                                 const Wrapper& rhs) {
    return lhs.impl_ <=> rhs.impl_;
  }

[EugeneZelenko]

Could you please try 21 or main branch? https://godbolt.org should be helpful.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (scott-zhong)

The following failed to compile on Clang but works with GCC.

#include <memory>

struct UDT
{
};

template <class T>
struct Base {
public:
  T* cbegin() const { return t_; }
  T* cend() const { return t_; }
  friend inline auto operator<=>(const Base& lhs,
                                 const Base& rhs) {
    return std::lexicographical_compare_three_way(
        lhs.cbegin(), lhs.cend(), rhs.cbegin(), rhs.cend(),
        [](auto&& lhs, auto&& rhs) {
          return std::forward_as_tuple(lhs) <=> std::forward_as_tuple(rhs);
        });
  }
  T* t_;
};

template <class T>
class Wrapper {
public:
  friend inline auto operator<=>(const Wrapper& lhs,
                                 const Wrapper& rhs) = default;

private:
  Base<T> impl_;

};

int main()
{
  Wrapper<UDT> c;
}

The compile and output:

$ clang++ --version && clang++ -std=c++20 -c t.cpp
clang version 16.0.6 (Red Hat 16.0.6-2.module+el8.9.0+19521+190d7aba)
Target: x86_64-redhat-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
t.cpp:17:45: error: invalid operands to binary expression ('tuple<UDT &>' and 'tuple<UDT &>')
          return std::forward_as_tuple(lhs) <=> std::forward_as_tuple(rhs);
                 ~~~~~~~~~~~~~~~~~~~~~~~~~~ ^   ~~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/stl_algobase.h:1826:17: note: in instantiation of function template specialization 'operator<=>(const Base<UDT> &, const Base<UDT> &)::(anonymous class)::operator()<UDT &, UDT &>' requested here
    -> decltype(__comp(*__first1, *__first2))
                ^
t.cpp:14:12: note: while substituting deduced template arguments into function template 'lexicographical_compare_three_way' [with _InputIter1 = UDT *, _InputIter2 = UDT *, _Comp = (lambda at t.cpp:16:9)]
    return std::lexicographical_compare_three_way(
           ^
t.cpp:26:22: note: in instantiation of member function 'operator<=>' requested here
  friend inline auto operator<=>(const Wrapper& lhs,
                     ^
t.cpp:36:16: note: in instantiation of template class 'Wrapper<UDT>' requested here
  Wrapper<UDT> c;
               ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/system_error:316:3: note: candidate function not viable: no known conversion from 'tuple<UDT &>' to 'const error_code' for 1st argument
  operator<=>(const error_code& __lhs, const error_code& __rhs) noexcept
  ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/system_error:498:3: note: candidate function not viable: no known conversion from 'tuple<UDT &>' to 'const error_condition' for 1st argument
  operator<=>(const error_condition& __lhs,
  ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/stl_pair.h:819:5: note: candidate template ignored: could not match 'pair' against 'tuple'
    operator<=>(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
    ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/stl_iterator.h:583:5: note: candidate template ignored: could not match 'reverse_iterator' against 'tuple'
    operator<=>(const reverse_iterator<_IteratorL>& __x,
    ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/stl_iterator.h:583:5: note: candidate template ignored: could not match 'reverse_iterator' against 'tuple'
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/stl_iterator.h:601:5: note: candidate template ignored: could not match 'reverse_iterator' against 'tuple'
    operator<=>(const reverse_iterator<_Iterator>& __x,
    ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/stl_iterator.h:1690:5: note: candidate template ignored: could not match 'move_iterator' against 'tuple'
    operator<=>(const move_iterator<_IteratorL>& __x,
    ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/stl_iterator.h:1690:5: note: candidate template ignored: could not match 'move_iterator' against 'tuple'
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/stl_iterator.h:1756:5: note: candidate template ignored: could not match 'move_iterator' against 'tuple'
    operator<=>(const move_iterator<_Iterator>& __x,
    ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/tuple:1938:5: note: candidate template ignored: substitution failure [with _Tps = <UDT &>, _Ups = <UDT &>]: no matching function for call to object of type 'const struct _Synth3way'
    operator<=>(const tuple<_Tps...>& __t, const tuple<_Ups...>& __u)
    ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/tuple:1938:5: note: candidate template ignored: substitution failure [with _Tps = <UDT &>, _Ups = <UDT &>]: no matching function for call to object of type 'const struct _Synth3way'
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/string_view:624:5: note: candidate template ignored: could not match 'basic_string_view' against 'tuple'
    operator<=>(basic_string_view<_CharT, _Traits> __x,
    ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/string_view:632:5: note: candidate template ignored: could not match 'basic_string_view' against 'tuple'
    operator<=>(basic_string_view<_CharT, _Traits> __x,
    ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/string_view:632:5: note: candidate template ignored: could not match 'basic_string_view' against 'tuple'
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/basic_string.h:3733:5: note: candidate template ignored: could not match 'basic_string' against 'tuple'
    operator<=>(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
    ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/basic_string.h:3748:5: note: candidate template ignored: could not match 'basic_string' against 'tuple'
    operator<=>(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
    ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/basic_string.h:3748:5: note: candidate template ignored: could not match 'basic_string' against 'tuple'
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/unique_ptr.h:988:5: note: candidate template ignored: could not match 'unique_ptr' against 'tuple'
    operator<=>(const unique_ptr<_Tp, _Dp>& __x,
    ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/unique_ptr.h:988:5: note: candidate template ignored: could not match 'unique_ptr' against 'tuple'
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/unique_ptr.h:997:5: note: candidate template ignored: could not match 'unique_ptr' against 'tuple'
    operator<=>(const unique_ptr<_Tp, _Dp>& __x, nullptr_t)
    ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/unique_ptr.h:997:5: note: candidate template ignored: could not match 'unique_ptr' against 'tuple'
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/shared_ptr_base.h:1805:5: note: candidate template ignored: could not match '__shared_ptr' against 'tuple'
    operator<=>(const __shared_ptr<_Tp, _Lp>& __a,
    ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/shared_ptr_base.h:1805:5: note: candidate template ignored: could not match '__shared_ptr' against 'tuple'
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/shared_ptr_base.h:1811:5: note: candidate template ignored: could not match '__shared_ptr' against 'tuple'
    operator<=>(const __shared_ptr<_Tp, _Lp>& __a, nullptr_t) noexcept
    ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/shared_ptr_base.h:1811:5: note: candidate template ignored: could not match '__shared_ptr' against 'tuple'
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/shared_ptr.h:567:5: note: candidate template ignored: could not match 'shared_ptr' against 'tuple'
    operator<=>(const shared_ptr<_Tp>& __a,
    ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/shared_ptr.h:567:5: note: candidate template ignored: could not match 'shared_ptr' against 'tuple'
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/shared_ptr.h:573:5: note: candidate template ignored: could not match 'shared_ptr' against 'tuple'
    operator<=>(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
    ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/shared_ptr.h:573:5: note: candidate template ignored: could not match 'shared_ptr' against 'tuple'
t.cpp:14:12: error: no matching function for call to 'lexicographical_compare_three_way'
    return std::lexicographical_compare_three_way(
           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
t.cpp:26:22: note: in instantiation of member function 'operator<=>' requested here
  friend inline auto operator<=>(const Wrapper& lhs,
                     ^
t.cpp:36:16: note: in instantiation of template class 'Wrapper<UDT>' requested here
  Wrapper<UDT> c;
               ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/stl_algobase.h:1821:5: note: candidate template ignored: substitution failure [with _InputIter1 = UDT *, _InputIter2 = UDT *, _Comp = (lambda at t.cpp:16:9)]
    lexicographical_compare_three_way(_InputIter1 __first1,
    ^
/opt/rh/gcc-toolset-13/root/usr/lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/bits/stl_algobase.h:1869:5: note: candidate function template not viable: requires 4 arguments, but 5 were provided
    lexicographical_compare_three_way(_InputIter1 __first1,
    ^
t.cpp:26:22: error: return type of defaulted 'operator<=>' cannot be deduced because three-way comparison for member 'impl_' has a deduced return type and is not yet defined
  friend inline auto operator<=>(const Wrapper& lhs,
                     ^
t.cpp:36:16: note: in instantiation of template class 'Wrapper<UDT>' requested here
  Wrapper<UDT> c;
               ^
t.cpp:30:11: note: member 'impl_' declared here
  Base<T> impl_;
          ^
t.cpp:12:22: note: selected 'operator<=>' for member 'impl_' declared here
  friend inline auto operator<=>(const Base& lhs,
                     ^
3 errors generated.

Same failure occurs with Clang 19.1.7. The testcase isn't calling any functions that would call the comparison operator, and it shouldn't have failed to compile.

Interestingly, changing the default three-way to the following compiles without any errors.

  friend inline auto operator<=>(const Wrapper& lhs,
                                 const Wrapper& rhs) {
    return lhs.impl_ <=> rhs.impl_;
  }

[shafik]

Note edg/MSVC also reject: https://godbolt.org/z/3r4KK9Tef

[scott-zhong]

@EugeneZelenko 21 and main branch failed as well.

[keinflue]

I think Clang is correct here. It needs to instantiate the operator<=> for Base<T> in order to determine whether the defaulted operator<=> for Wrapper<T> is to be defined as deleted according to [class.spaceship]/2.1, i.e. it needs to deduce the return type of operator<=> for Base<T> to check whether it is a comparison category type.

[keinflue]

As for whether the compiler should delay checking whether the function is going to be defined as deleted to a later point (i.e. what GCC does), see CWG 2997.

[scott-zhong]

@keinflue thank you for the explanation. Another question: if I changed Base's three-way to default, then I don't see Clang generating any error, why is that?

  friend inline auto operator<=>(const Base& lhs,
                                 const Base& rhs) = default;

[keinflue]

@keinflue thank you for the explanation. Another question: if I changed Base's three-way to default, then I don't see Clang generating any error, why is that?

  friend inline auto operator<=>(const Base& lhs,
                                 const Base& rhs) = default;

I explained in previous comments why Clang instantiates Base's operator<=>. That instantiation then fails because std::forward_as_tuple(lhs) <=> std::forward_as_tuple(rhs) is not valid given that lhs and rhs are of type T (i.e. UDT) which can't be compared.

If you default this operator there is never an attempt to compare Ts, just T*s, which can be compared. Even if the defaulted operator would attempt to compare Ts, it would simply be defined as deleted instead of attempting to instantiate that invalid comparison.