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List:       gcc-bugs
Subject:    [Bug c++/103876] New: Parameter pack not expanded in lambda within static_assert in a fold-expressio
From:       johelegp at gmail dot com via Gcc-bugs <gcc-bugs () gcc ! gnu ! org>
Date:       2021-12-31 14:11:45
Message-ID: bug-103876-4 () http ! gcc ! gnu ! org/bugzilla/
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103876

            Bug ID: 103876
           Summary: Parameter pack not expanded in lambda within
                    static_assert in a fold-expression of a lambda
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: johelegp at gmail dot com
                CC: johelegp at gmail dot com
  Target Milestone: ---

See https://godbolt.org/z/5W1fYn7E4.

```C++
void f() {
  [](auto... op) {
    (..., void([] {
      static_assert([]<class T> {
        return requires(T v) { op(v); };
      }.template operator()<int>());
    }));
  }([](...) {});
}
```

```
In instantiation of 'f()::<lambda(auto:1 ...)> [with auto:1 =
{f()::<lambda(...)>}]':
<source>:8:4:   required from here
<source>:5:16: error: parameter packs not expanded with '...':
    5 |         return requires(T v) { op(v); };
      |                ^~~~~~~~~~~~~~~~~~~~~~~~
<source>:5:16: note:         'op'
<source>:6:33: error: could not convert '<lambda closure
object>f()::<lambda(auto:1 ...)>::<lambda()>::<lambda()>().f()::<lambda(auto:1
...)>::<lambda()>::<lambda()>()' from 'void' to 'bool'
    4 |       static_assert([]<class T> {
      |                     ~~~~~~~~~~~~~
    5 |         return requires(T v) { op(v); };
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    6 |       }.template operator()<int>());
      |       ~~~~~~~~~~~~~~~~~~~~~~~~~~^~
      |                                 |
      |                                 void"
```=
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