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    <head>
      <base href="https://llvm.org/bugs/" />
    </head>
    <body><table border="1" cellspacing="0" cellpadding="8">
        <tr>
          <th>Bug ID</th>
          <td><a class="bz_bug_link 
          bz_status_NEW "
   title="NEW --- - Trailing return type accepted, even though it should be rejected"
   href="https://llvm.org/bugs/show_bug.cgi?id=30551">30551</a>
          </td>
        </tr>

        <tr>
          <th>Summary</th>
          <td>Trailing return type accepted, even though it should be rejected
          </td>
        </tr>

        <tr>
          <th>Product</th>
          <td>clang
          </td>
        </tr>

        <tr>
          <th>Version</th>
          <td>3.9
          </td>
        </tr>

        <tr>
          <th>Hardware</th>
          <td>PC
          </td>
        </tr>

        <tr>
          <th>OS</th>
          <td>Linux
          </td>
        </tr>

        <tr>
          <th>Status</th>
          <td>NEW
          </td>
        </tr>

        <tr>
          <th>Severity</th>
          <td>normal
          </td>
        </tr>

        <tr>
          <th>Priority</th>
          <td>P
          </td>
        </tr>

        <tr>
          <th>Component</th>
          <td>C++14
          </td>
        </tr>

        <tr>
          <th>Assignee</th>
          <td>unassignedclangbugs@nondot.org
          </td>
        </tr>

        <tr>
          <th>Reporter</th>
          <td>michele.caini@gmail.com
          </td>
        </tr>

        <tr>
          <th>CC</th>
          <td>llvm-bugs@lists.llvm.org
          </td>
        </tr>

        <tr>
          <th>Classification</th>
          <td>Unclassified
          </td>
        </tr></table>
      <p>
        <div>
        <pre>The code below should be rejected, but clang accepts it:

    #include <type_traits>

    template<typename T>
    constexpr auto check(T &&t) -> decltype(typename std::integral_constant<T,
t>::type{}) { }

    int main() {  
        //check(42); // (1)
    }

I suspect the function definition should be rejected for it is ill-formed, t
cannot be used with std::integral_constant that way.
If you uncomment (1), the compilation fails with the right error:

<span class="quote">> candidate template ignored: substitution failure [with T = int]: non-type template argument is not a constant expression</span ></pre>
        </div>
      </p>
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