<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/121208>121208</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [clang] Incomplete treatement for non-deduced context with nested template
        </td>
    </tr>

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

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

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

<pre>
    Currently, Clang doesn't accept the following code snippet, while other implementations do ([demo](https://godbolt.org/z/Yccjc95Eb)).

```C++
template <template <typename> class>
struct B;

struct C {
  template <typename>
  struct Nested;
};

template <typename T>
void f(T*, B<T::template Nested>*);

void g(C *cp, B<C::template Nested>* bcnp) {
  f(cp, 0);
}
```

When `0` is replaced with `{}` (but not `nullptr` or any real null pointer constant), Clang also accept it ([demo](https://godbolt.org/z/19P6MK15P)).

The following similar example is rejected by all implemenations ([demo](https://godbolt.org/z/5oej5ohGT)), and shows that Clang also considers that `T` is not deducible from `B<T::template Nested>*` (and thus `B<T::template Nested>*` forms non-deduced context for `T`). Although the current standard wording is possibly defective on this (see cplusplus/CWG#660).

```C++
template <template <typename> class>
struct B;

struct C {
  template <typename>
  struct Nested;
};

template <typename T>
void f(T*, B<T::template Nested>*);

void g(C *cp, B<C::template Nested>* bcnp) {
  f({}, bcnp);
}
```

Given that `T` is (or should be  considered) in the non-deduced context in `B<T::template Nested>*`, Clang should not try to match `B<T::template Nested>*` against `int` (or `std::nullptr_t`), and should just take `0` (or `nullptr`) as null pointer constant instead.
</pre>
<img width="1" height="1" alt="" src="http://email.email.llvm.org/o/eJzsVk1v4zYQ_TX0ZRBDpiJbOuhgy_GiKFrswcCip4IiRxazFCmQI2fdX19Qku2kTbdZoMcCCmJw9ObzzaNECPpkEUuW7Vi2X4iBWufLxqNCr-XXh3N4eBaL2qlLWQ3eoyVzYbyCygh7AuUwWMY3BEJK7AmoRWicMe5F2xNIpxCC1X2PFEEvrTYIjlr0oLveYIeWBGlnAygHjOcs2ynsHMv2jOctUR9YumX8wPjh5FTtDC2dPzF--IPxw29SPssie6oZLxgvlizZxmedTE_F-C4-yZaw640gBJZWb35ferSiQ5Y-gTQiBJY-sWQbyA-SYMfS3eRyPqiAbeIJwD84GW3zy79iIFSzi83-5us9KBwn8NlpBQ3j-ZHxbWzYjqXVMXYgveOujp_Gd4qb4xF8YjyvgPGt7K_46jt4qKXtGS9uhcXgEzS5-97sX7d1ivalRQtsncQT0AE89kZIVPCiqY2G6HGzj1bG83ogsI7iuR2M6clHg_Mg7AU8CgPxGHqnLaEH6WwgYWmc65VrwgR3pZmmHybLqvi8_uXnVfb5DVmOb_gadKeN8IDfRGTnVNczSkIF9QWEMTfazqz90Swyh8-Zaz8dpyxidcIqCK17CUCtoNfFxjZohX62sHVynLsde6lQDVLXBqHxrovWf2XLNIwYkNohfBTSON_FkPZhDIkqJkb4jaLlmlVsKWwNtW44taMMyEkuIE5SCa_gxXkVu6wD9C4EXZsLKGxQkj4jOAvU6rGjARFkb4YQ_xg_VF8-MZ6upyD_L_l_suTzevLqav3Orn_SZ7R_4yDjufORuYNRUCPc6IoqBtN2ZMF7tNH2g9S7L_8cJvKe_AXIQSdIth-lsDgJbcOYvrY078FE3kBqAs_C9DtNbH61mjHy8xAISHzFm-jdPNwVLdYtwvtaBjEBFGq5UGWqirQQCyxXmzRLk3VSpIu2TAtsEBuVrmVW5Ooxrzc8W0ve5IWQSb5a6JIn_HHF-WaV8nyVLrN0o1SdpI9rlWeYF-wxwU5oszTm3EXVWegQBixXfMWTfGFEjSaMVz3nMvaVpdvGx6lYxTiPHwC-jNiHejgF9pgYHSjcvZEmM34qTOBsDz9Z6aIiEgJ5FDTe6KMuvDf48W6w42xu27UYvCn_Ipua2qFeStcxfojB538PvXdRjRk_jHVFbZhLO5f8zwAAAP__Xre0Gg">