<html>

    <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 --- - Nested template class partial specialization cannot be used"

   href="https://llvm.org/bugs/show_bug.cgi?id=29158">29158</a>

          </td>

        </tr>

        <tr>

          <th>Summary</th>

          <td>Nested template class partial specialization cannot be used

          </td>

        </tr>

        <tr>

          <th>Product</th>

          <td>clang

          </td>

        </tr>

        <tr>

          <th>Version</th>

          <td>3.7

          </td>

        </tr>

        <tr>

          <th>Hardware</th>

          <td>All

          </td>

        </tr>

        <tr>

          <th>OS</th>

          <td>All

          </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++11

          </td>

        </tr>

        <tr>

          <th>Assignee</th>

          <td>unassignedclangbugs@nondot.org

          </td>

        </tr>

        <tr>

          <th>Reporter</th>

          <td>bondvladserg@mail.ru

          </td>

        </tr>

        <tr>

          <th>CC</th>

          <td>dgregor@apple.com, llvm-bugs@lists.llvm.org

          </td>

        </tr>

        <tr>

          <th>Classification</th>

          <td>Unclassified

          </td>

        </tr></table>

      <p>

        <div>

        <pre>Define a class with members in following sequence:

1) forward declaration of nested class.

2) usage of this nested class specialization before definition/specialization.

3) definition + specialization

Result: specialization never used in this case

I'm not sure if it is clang bug, but it works on GCC and VS compilers.

Code sample:

#include <iostream>

template<typename T>

struct Foo

{

    // Dummy for PARTIAL specialization

    // because full specialization is not allowed

    template<typename T2, typename Dummy>

    struct Bar;

    using Type = typename Bar<T, void>::Type;

    template<typename T2, typename Dummy>

    struct Bar

    {

        using Type = T2;

    };

    template<typename Dummy>

    struct Bar<double, Dummy>

    {

        using Type = char;

    };

};

int main()

{

    static_assert(sizeof(Foo<int>::Type) == sizeof(int),

"sizeof(Foo<int>::Type) != sizeof(int)");

    static_assert(sizeof(Foo<double>::Type) == sizeof(char),

"sizeof(Foo<double>::Type) != sizeof(char)");

}