Bug in dmd?

[Andrey Zherikov]

I have [pretty simple code in my 
library](https://github.com/andrey-zherikov/argparse/blob/bug/source/argparse/help.d#L27-L47):
```d
alias CC = SumType!(AA,BB);
struct AA {}
struct BB
{
     CC[] c;
}

private void ppp(T, Output)(auto ref Output output, in 
CommandArguments!T cmd, in Config config)
{
     auto cc = CC(AA());  // (1)
}
private void printHelp(T, Output)(auto ref Output output, in 
CommandArguments!T cmd, in Config config)
{
     auto cc = CC(AA());  // (2)
}

void printHelp(T, Output)(auto ref Output output, in Config 
config)
{
     ppp(output, CommandArguments!T(config), config);
     printHelp(output, CommandArguments!T(config), config);
}
```

[Line (2) 
produces](https://github.com/andrey-zherikov/argparse/runs/6880350900?check_suite_focus=true#step:5:12) `undefined reference to '_D3std7sumtype__T7SumTypeTS8argparse4help2AATSQtQm2BBZQBl6__dtorMFNaNbNiNfZv'` (it demangles to `pure nothrow @nogc @safe void std.sumtype.SumType!(argparse.help.AA, argparse.help.BB).SumType.__dtor()`)

If I comment line (2) then everything is good (no link errors). 
Note that there is the same code at (1) which doesn't produce any 
error. Any idea what's going on?