<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/141797>141797</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            `[[gnu::weak]]` disables constant evaluation of constexpr variables in C++14
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            good first issue,
            c++11,
            c++14,
            clang:frontend,
            diverges-from:gcc
      </td>
    </tr>

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

    <tr>
      <th>Reporter</th>
      <td>
          cor3ntin
      </td>
    </tr>
</table>

<pre>
    GCC and Clang disagree on their handling of `[[gnu::weak]]` before C++17 https://gcc.godbolt.org/z/s7q67vqjz

```cpp
struct S { static constexpr int var = 0; };
[[gnu::weak]] constexpr int S::var;
static constexpr int usage = S::var;
```

While it would be unsafe to constant-fold this code, it should be fine to constant-evaluate it, for consistency with GCC

In `static bool evaluateVarDeclInit` (ExprConstant.cpp), and in `static bool CheckWeak` (`Interp.cpp`), we should allow constant evaluation -without diagnostics, but not constant folding)  - by looking at `Info.EvalMode`
</pre>
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