<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/63192>63192</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            __is_trivially_equality_comparable gives wrong answer when inheritance is involved
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          AMP999
      </td>
    </tr>
</table>

<pre>
    https://godbolt.org/z/ozWvKhddf
``` cpp
    struct Alloc {
      int a;
      bool operator==(const Alloc&) const { return true; }
    };
    struct S : Alloc {
      int b;
      S(int a, int b) : Alloc(a), b(b) {}
      bool operator==(const S&) const = default;
    };
    static_assert(__is_trivially_equality_comparable(S)); // oops!
​
    int main() {
      S a[] = { {1,1}, {1,2}, {1,3} };
      S b[] = { {2,1}, {2,2}, {2,3} };
      assert(a[0] == b[0]);
      assert(a[1] == b[1]);
      assert(a[2] == b[2]);
      assert(!std::ranges::equal(a, b)); // oops!
    }
```
​
The symptom is that `std::equal` and `std::ranges::equal` produce wrong answers for arrays of `S`.
The root cause is that Clang's (relatively new) builtin `__is_trivially_equality_comparable` incorrectly produces `true` for `S` when it should produce `false`. Even though all of `S`'s data members are trivial, the defaulted `S::operator==` isn't trivial because its base class `Alloc::operator==` isn't trivial. I think `QualType::isTriviallyEqualityComparableType` needs to be updated to check base classes as well as members.

Thanks to @Quuxplusone for finding this bug

</pre>
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