<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/131611>131611</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [Clang][Windows] Microsoft mangling cannot handle lambda in decltype in return type
        </td>
    </tr>

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

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

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

<pre>
    Given this code:
```c++
template <class>
struct B {};

struct A {
 template <class T>
    auto f(T) -> B<decltype([] (T) {})> {
        return {};
    }
};

int main() {
    A{}.f(0);
}
```

Clang on Windows error:
```console
<source>(6,5): error: cannot mangle this template specialization type yet
    6 |     auto f(T) -> B<decltype([] (T) {})> {
 |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    7 |         return {};
 |         ~~~~~~~~~~
    8 |     }
      |     ~
```

Similar error happens on simpler code:
```c++
template <class T> // must be a function template
auto f(T) -> decltype([] {}) {
    return {};
}
int main() { f(0)(); }
```

On other platforms with Itanium mangling, or if using MSVC, it can be compiled.

See https://compiler-explorer.com/z/vPz6E5ve7. (Compiler explorer seems to only have version 18 for `clang-cl`)

I can reproduce this using the main branch.
</pre>
<img width="1" height="1" alt="" src="http://email.email.llvm.org/o/eJysVEGPqzYQ_jXDZZQITAjkwIGwm-odVq30Vu3ZwACujI1sk9fdw_vtlcHJ7utupVZ6CCkwmRl_3zcfw60VgyIqITtD9hDxxY3alE98IOX4syEa9WIpanT3Uv4irqTQjcJiqzuCtIK4gmO83S2ws7_jytE0S-4IIa1bya2F9BHiyjqztA7PCPkZ8gdIfe5bvFrjcYUfyvF5a4CIyBensQdWPAM74Q7SRzxDWnfUSvcyE7BiY4K3lHAYO_nUcEK4DLnFqB_g-LB_iav3CIVyOHGhfPfQckutttq9BxSvZ5xD6Tthth615GpArfAPoTr9zSIZo80_NdTKakk-lNZWL6YlT50VR2B1th5Q3Qux5Uppj0wNkra53LWzM7WCS_HKndAKvTb4Qi7gPiLkNf40PW_NIHv8_h-uACK_1_3bMN7__6G8eDt21RvD8xb7_nECX8UkJDebfjjyeSZl_USsmGZJ5n-aenUlArsAu-C0WIcNIcd-Ue0meSiAuPpE5E8Evgn7zl-fiLKR_ehIvHtwDUF6xk99-KtC7UYy6MH12kwWvwk34hfHlVimzU1CDcBq1AZFj4sVasCnr7_XPiac953n2uppFpK6fZCXCEfnZuslXFUJCWZHf81SGzL7Vk_ALq_ALtffXo-P2ZXyvXdWHTLxlomWaLLoNGolX3DkV8IrGeuFTQrstUE_Hf9N7VrpybHTBuPLCs_QbHS3tOGz2Ci4kVbRsDFcteM-6sq0O6UnHlGZ5AfGkvyYFNFYxmnRU8wPTd5lh_xE1GU8S5q-74jzIjtGomQxy-I0yZOUxUmx5x31bU5Jl7CkY-wEh5gmLuReyuu012aIhLULlUmaHJMkkrwhadely9jKAhjz-9eUvmDXLIOFQyyFdfathRNOrpt63SWQPUB2DtvEG-hJtEZb3bv7DG8bYuSqk4SST03HUai7-_xz8Jh_jRYjyx9nOAg3Lk0YnEcSfnaz0X9S64BdVmYW2CWQu5bs7wAAAP__2WjfvQ">