<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/113530>113530</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [Clang] Accessing a reference member of a volatile-qualified class glvalue is misinterpreted as volatile read
        </td>
    </tr>

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

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

    <tr>
      <th>Reporter</th>
      <td>
          frederick-vs-ja
      </td>
    </tr>
</table>

<pre>
    Currently, Clang (and GCC) rejects the following example ([link](https://godbolt.org/z/v9fKKEaGW)), which doesn't seem conforming.

```C++
struct S { const int& ref; };
constexpr int n = 42;

template<class T>
constexpr int readref(T&& t) {
  return static_cast<T&&>(t).ref;
}

static_assert(::readref(static_cast<volatile S&&>(S{n})) == 42, "");
```

Error messages:
```
<source>:9:15: error: static assertion expression is not an integral constant expression
    9 | static_assert(::readref(static_cast<volatile S&&>(S{n})) == 42, "");
 | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:6:30: note: read of volatile temporary is not allowed in a constant expression
    6 | return static_cast<T&&>(t).ref;
      | ^
<source>:9:15: note: in call to 'readref<volatile S>(S{n})'
    9 | static_assert(::readref(static_cast<volatile S&&>(S{n})) == 42, "");
 | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:9:51: note: volatile temporary created here
    9 | static_assert(::readref(static_cast<volatile S&&>(S{n})) == 42, "");
 |                                                   ^
1 error generated.
Compiler returned: 1
```

I think the relevant standard wording is [[expr.const]/5.9](https://eel.is/c++draft/expr.const#5.9):

> an lvalue-to-rvalue conversion unless it is applied to
> - a non-volatile glvalue that refers to an object that is usable in constant expressions, or
> - a non-volatile glvalue of literal type that refers to a non-volatile object whose lifetime began within the evaluation of _E_;

However, there doesn't seem any wording treating evaluating a reference non-static data member of a class object as lvalue-to-rvalue conversion. So perhaps this example should be well-formed.
(A reference member behaves like it were an "auto-dereferencing" pointer in many cases. But IIUC this example is not such a case.)
</pre>
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