[Bug c++/78058] New: Complex initialization of nested std::optional does not work

[akrzemi1 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78058

            Bug ID: 78058
           Summary: Complex initialization of nested std::optional does
                    not work
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: akrzemi1 at gmail dot com
  Target Milestone: ---

The following program fails to compile:

```
#include <cassert>
#include <optional>

using std::optional;
using std::nullopt;
using std::in_place;

int main()
{
   optional<optional<optional<int>>> o (in_place, in_place, nullopt);
   assert (o);
   assert (*o);
   assert (!**o);
}
```

Overload resolution cannot find the right constructor. I expect this to
compile, the meaning is described with the assertions.

The middle optional is initialized with two arguments {in_place, nullopt},
which means that the innermost optional is initialized with nullptr.

The same program compiles with the reference implementation:
https://github.com/akrzemi1/Optional