<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/101299>101299</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [Clang][C] What is the type of a bit-field? (For generic selection / constexpr initialization)
        </td>
    </tr>

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

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

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

<pre>
    Seen at https://github.com/llvm/llvm-project/pull/78112#pullrequestreview-2205422888, this compiles surprisingly in C23:

```c++
struct C {
  signed long long i : 8;
};
constexpr struct C c = { 255 };
```

My reading of N3096:

§6.7.1p6:

> If an object or subobject declared with storage-class specifier `constexpr` has [...] integer [..] type, any explicit initializer value for it shall be [.,.] an integer constant expression

§6.6p4:

> Each constant expression shall evaluate to a constant that is in the range of representable values for its type.

§6.7.2.1p12:

> A bit-field is interpreted as having a signed or unsigned integer type consisting of the specified number of bits.

... So the member `i` has a signed type with 8 bits, which 255 is not representable in, and Clang should reject the initializer.

Two other places I can think of where the type of something is observed is `typeof` (which explicitly bans bit-fields), and `_Generic`. Clang currently allows:

```c++
struct C {
  signed long long i : 8;
} c;
static_assert(_Generic(c.i, long long: 1));
```
 which according to §6.7.2.1p12, it shouldn't? The bit-field type with width 8 should not be compatible with a 64 bit wide type. Unless the lvalue conversion done converts the bit-field type to long long, but that doesn't seem to be allowed by §6.3.2.1
 
 CC @AaronBallman @Fznamznon 
</pre>
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