<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/58682>58682</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [clang] template decltype(auto) substitution failure for non-constinit static lval ref argument
        </td>
    </tr>

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

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

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

<pre>
    Rejects valid template specialization with a substitution failure.

E.g. given a class template with a C++17 `decltype(auto)` placeholder parameter
and function template specialized to deduce the argument
then when the argument is an lvalue reference to a non-constinit static object,
clang fails to match it as valid NTTP and rejects with a substitution error
https://godbolt.org/z/aYEaG4jx3
```c++
  template <decltype(auto)v> struct DA {};

  template <decltype(auto)v>
  constexpr decltype(v) get(DA<v>) { return v; }

  int g;
  static_assert( &get(DA<(g)>{}) == &g ); // substitution failure
                  ^~~
// note: candidate template ignored: substitution failure:
// non-type template argument is not a constant expression
```
(the parens `(g)` are needed to convert the id-expression to lvalue)
Same with a class specialization https://godbolt.org/z/Pzo9TMrGd
```c++
  template <typename> struct Get;
  template <decltype(auto)v> struct Get<DA<v>> {};

  Get<DA<(g)>> G;
               ^
//  error: implicit instantiation of undefined template 'Get<DA<g>>'
```
GCC and MSVC accept with no error, over many releases.

A workaround is to employ a hidden friend, as then no 'forwarding' is needed, with direct in-class access to the template argument.
It's an intrusive and non-generic workaround though.
https://godbolt.org/z/E1WfTK6Gh
</pre>
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