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

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

```c++
#include

struct UDT
{
};

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

private:
Base impl_;

};

int main()
{
Wrapper 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' and 'tuple')
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 &, const Base &)::(anonymous class)::operator()' 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' requested here
Wrapper 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' 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' 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 = , _Ups = ]: 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 = , _Ups = ]: 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' requested here
Wrapper 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' requested here
Wrapper c;
^
t.cpp:30:11: note: member 'impl_' declared here
Base 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_;
}
```

Start with the standalone t.cpp testcase from the issue and compile it using clang++ -std=c++20 -c t.cpp, comparing the failure with GCC and the manual comparison definition. Investigate Clang's handling of a defaulted three-way comparison whose member comparison has a deduced return type. Done means the testcase compiles without calling the comparison operator, while preserving correct diagnostics for genuinely invalid comparisons.

Written by the indexing model from the issue text.