<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/120211>120211</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            `__builtin_bit_cast` to `bool` diverges from GCC's behavior
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            clang:frontend,
            constexpr
      </td>
    </tr>

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

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

<pre>
    This is the specific case:
```c++
constexpr bool A = __builtin_bit_cast(bool, 'A');
static_assert(A);
```

GCC happily compiles this while clang prints an error:
```
array.cpp:128:16: error: constexpr variable 'a' must be initialized by a constant expression
  128 | constexpr bool a = __builtin_bit_cast(bool, 'A');
      |                ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
array.cpp:128:20: note: value 65 cannot be represented in type 'bool'
  128 | constexpr bool a = __builtin_bit_cast(bool, 'A');
      |                    ^
```

This specific diagnostic is tested exactly _once_, in `SemaCXX/constexpr-builtin-bit-cast.cpp`. It's an edge case which clang checks for all integral types though:
https://github.com/llvm/llvm-project/blob/0693b9e9ccdec5f09a3080b1bec73f5004a8dfa3/clang/lib/AST/ExprConstant.cpp#L7495-L7507

I think the relevant part from https://eel.is/c++draft/bit.cast is
> For the result and each object created within it, if there is no value of the object's type corresponding to the value representation produced, the behavior is undefined[.](https://eel.is/c++draft/bit.cast#2.sentence-5)
If there are multiple such values, which value is produced is unspecified[.](https://eel.is/c++draft/bit.cast#2.sentence-6)

Does the first sentence apply here or the second?

CC @zygoloid 
</pre>
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