[Bug c++/116052] [15 Regression] ICE in diagnostic_context::diagnostic_impl

[curdeius at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116052

--- Comment #6 from Curdeius Curdeius <curdeius at gmail dot com> ---
A smaller reproduce: https://godbolt.org/z/YGbdbPqh7:
```
#include <stdexec/execution.hpp>

stdexec::sender auto f() {
    return stdexec::just() | stdexec::then([](auto... args) {
           });  // Removing then makes it compile.
}

int main() {
    stdexec::sync_wait(f());  // Removing sync_wait makes it compile.
}
```

I'll try to find an even smaller repro if possible.