<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/112859>112859</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Different ODR-usage between 'auto' and 'decltype' when declaring a static constexpr member.
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

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

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

<pre>
    Hello,

Let's say I have a template class with a member function which is not ODR-used. Clang will remove it from its AST, to avoid that, I reference it in a constexpr static variable:

```c++
template<typename T>
struct foo_t {
  int a() { return 42; }

  static constexpr auto __a = &foo_t::a;
};

int main() {
  foo_t<int> f;
}
```

Now replace 'auto' with a 'decltype':
```diff
--- with_auto.cpp       2024-10-18 11:02:43.389178174 +0200
+++ with_decltype.cpp   2024-10-18 11:02:51.985095752 +0200
@@ -4,3 +4,3 @@
 
-  static constexpr auto __a = &foo_t::a;
+  static constexpr decltype(&foo_t::a) __a = &foo_t::a;
 };
```
And compare the AST of the `-ClassTemplateSpecializationDecl` with `clang++ -Xclang -ast-dump <file>`:
With "auto", the function has a body:
```
| |-CXXMethodDecl 0x77f9ae0 <line:3:3, col:24> col:7 used a 'int ()' implicit_instantiation implicit-inline instantiated_from 0x77f9378
|   | `-CompoundStmt 0x781a0e8 <col:11, col:24>
|   |   `-ReturnStmt 0x781a0d8 <col:13, col:20>
|   |     `-IntegerLiteral 0x77f95e8 <col:20> 'int' 42
```

But the body is absent when using `decltype`:
```
| |-CXXMethodDecl 0x71bdeb0 <line:3:3, col:24> col:7 referenced a 'int ()' implicit_instantiation implicit-inline instantiated_from 0x71bd6c8
```

Is it an expected behavior?

Best regards and thank you for the great work!

Dorian
</pre>
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