[Bug c++/121443] New: GCC rejects valid lambda with local struct constructor in default argument

[jirehguo at tju dot edu.cn via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121443

            Bug ID: 121443
           Summary: GCC rejects valid lambda with local struct constructor
                    in default argument
           Product: gcc
           Version: 16.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jirehguo at tju dot edu.cn
  Target Milestone: ---

GCC rejects following code, which is accepted by Clang and MSVC.
Reproducer: https://godbolt.org/z/EWjjvdfrh
The same lambda, when used within a function body (e.g., main), is accepted
without error.

Flags: -std=c++23

Code:
```
int g(int i = [] {
  struct Node {
    int value;
    Node(int val) : value(val) {}
  };
  Node node1(45);
  return node1.value;
}()) {
  return i;
}

int main() {
  return g();
}

```

Output:
```
<source>: In lambda function:
<source>:4:10: error: expected unqualified-id before 'int'
    4 |     Node(int val) : value(val) {}
      |          ^~~
<source>:4:10: error: expected ')' before 'int'
    4 |     Node(int val) : value(val) {}
      |         ~^~~
      |          )
Compiler returned: 1
```