<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/81577>81577</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Unexpected behavior in deduction of nested templates with template template parameters (inconsistent with GCC, MSVC)
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

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

    <tr>
      <th>Reporter</th>
      <td>
          13steinj
      </td>
    </tr>
</table>

<pre>
    I don't have enough practice with standard-ese to express myself any better than that, unfortunately. Define `EVEN_WORSE` or `ALTERNATIVE_EVEN_WORSE` for a different, but seemingly related, inconsistency. Define `FIXED` to fix the original issue, and "fix" the "even worse" inconsistencies (successful compile, unexpectedly).

I'm happy to provide the motivating example, it's just larger. A whole load of transitional layers from one set of types to another.

<details>
<summary><a href="https://gcc.godbolt.org/z/Whqfnjv49">Reduced Example</href></summary>

```cpp
template<typename>
struct Wrapper;

template<template<typename> typename, typename T>
using ComposedWrapper = Wrapper<T>;

template<typename...>
struct mp_list;

template<typename>
struct converter_impl;

template<typename T>
using converter = converter_impl<T>::type;

template<
#if !defined(EVEN_WORSE)
 template<typename> typename Wrapper,
#endif
    typename T
>
struct converter_impl<mp_list<Wrapper<T>>> {
    using type = void;
};

template<typename T>
struct converter_impl<ComposedWrapper<Wrapper, T>> {
    using type = converter<T>;
};

#if defined(__clang__) && defined(FIXED)

template<typename T>
struct converter_impl<ComposedWrapper<Wrapper, mp_list<T>>> {
    using type = int;
};

#endif

template<typename>
concept valid = true;

static_assert(valid<converter<ComposedWrapper<Wrapper,
#if !defined(ALTERNATIVE_EVEN_WORSE)
    mp_list<
#endif
 mp_list<int>
#if !defined(ALTERNATIVE_EVEN_WORSE)
 >
#endif
>>>);
```

</details>
</pre>
<img width="1px" height="1px" alt="" src="http://email.email.llvm.org/o/eJy0VlGP4jYQ_jXmZXQoTDgCDzxwAaqV2qt0t73rGzLxhHjl2Kk9YZf--soJCznudvdaqRIKju35Zr4ZTz7LEPTBEi3F-w_i_XokW66cX07SwKTtw2jv1Gl5B8pZgRlDJY8EZF17qKDxsmBdEDxqriCwtEp69Y4CATugp8ZTCFCfApkSpD3BnpjJA1fSxgcLzKG1pfPcWslkTmNYU6ktgZglmy-bj7uvv3_6vBGzBJyPc6tf7zefPq7u775sdt-ul86DBKXLkjzZDnnfMgSiWtuDOYEnI5lUXNC2cDbowGSLb1xu7_7crCMaOyj1E3BF4Lw-aCsN6BBaiubSKhCIpX4SiN0egUhHsvDofKA4OfSgKYDAeWiLgkIoWwOFqxttqGdPTw0VTMqcBC7GIlmLZNU_7wRmNVSyaU4xoMa7o1bUOawd66NkbQ9AT7JuejDNArMAD21gMNIfyI9hBY-VMwTGSQWuBPbSBs3aRUpGnsgHKL2rwVmCQNztOTUUoktpHVfkv4lKpLkiltoEkW4uU6Gta-lPcSrNJVSeSpGuBWLF3ASRrgRuBW4PRTE-OLV3hsfOHwRu_xa4_Vr9VdqH43QhEEW6-USqLUjB5swszQVue8RN_zLwNoxslvS_omn6Gaa6iVUXaR45WVnTxSawbwuGr142DXmRfhhCDQ1_iAGXMeaXMdxf0NsQa5O7unGB1NkJiHR9dZh3u1_2ewYdj8e3MdfNzujAb9veGhbOHskz-Z2uG_O2_XeELgAdlVu4M6OVSFcR4WX88zSmugSBE9U1oBI4H_Q0Lvpd8EYBLgnFKyxZpcuzOcCwPufz-npa8kuC89tqxR-I7MMVvE9MdNHl5Oi0uhLP1v8qyS_Fc3OOBnFhDvc_EdUF8btj912EfVWuJdntCiPtYbcTuACBM4GzwWr_xXyu1f_G8VqQnyyDtvw6x8ERebt9CmcLahiO0mjV4bNvb893YMm62MkQyLPAebdZpPkw96-RfLEpXhC9S4MADPLzwx64Lnd52fxnV0PbYQafaxJ3XrLy_Dm-UQ-B26uAjNQyVYt0IUe0nGRJNk_myXQ2qpZTOVMFTSakKJtjoWSR4nSxz9SCsvkEJyO9xASnCU7SJEsynI33qpSJ3E-mOJ1miSrFNKFaajM25lhHtRl1Cr6cT95n2cjIPZnQXXoQLT0-yzvGO5BfRpt3-_YQxDSJqQtXFNZsaPnHRbhhT5U8audBW1BRu6K6Rh21FOL688kK_T3p-fU6aKSXNXGUYoHzwd2Be4tf8jw2wW-fv-QCF6PWm-WNrGqu2v24cLXAbQz0_Peu8e6BCha47egFgduO_j8BAAD__8f4Gjg">