<table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Issue</th>
<td>
<a href=https://github.com/llvm/llvm-project/issues/89293>89293</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>
`__is_trivially_equality_comparable(T)` false positive when the defaulted operator is ineligible
</td>
</tr>
<tr>
<th>Labels</th>
<td>
new issue
</td>
</tr>
<tr>
<th>Assignees</th>
<td>
</td>
</tr>
<tr>
<th>Reporter</th>
<td>
AMP999
</td>
</tr>
</table>
<pre>
Clang's `__is_trivially_equality_comparable(T)` builtin assumes that if a type has _any_ defaulted trivial comparison operator, then _all_ its comparison operators will be tantamount-to-trivial. That's a sane assumption... but only if the defaulted operator is actually eligible for the given `T`! If the defaulted operator is `requires`-clause'd away, then we shouldn't consider it relevant to the question of triviality at all.
https://godbolt.org/z/3hs4EKPKT
```
template<bool B>
struct S {
int i;
bool operator==(const S&) const requires B = default;
bool operator==(const S& rhs) const { return (i % 3) == (rhs.i % 3); }
};
static_assert(__is_trivially_equality_comparable(S<true>)); // OK
static_assert(not __is_trivially_equality_comparable(S<false>)); // Oops, this line assert-fails
```
This compiler misbehavior causes libc++ to miscompile `std::equal`:
https://godbolt.org/z/YPfEG6cv9
```
S<false> a[] = {1,2,3};
S<false> b[] = {4,5,6};
return std::equal(a, a+3, b, b+3);
// should be true, not false
```
</pre>
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