Segfault with std.variant

[Mitchell via Digitalmars-d-learn]

Howdy,

I've just tried out `std.variant` and I've noticed that I can 
induce a segfault by having a variant of variants. Should this 
work?

```d
import std.stdio;
import std.variant;

void main()
{
  Variant variant = Variant([
    "one": Variant(1),
    "two": Variant(2),
    "three": Variant(3)
  ]);

  writefln("Segfault occurs on the next line");
  variant["four"] = Variant(4); // Segfault
  writefln("This line will not be reached");
}
```

For whatever reason, the `variant["four"] = Variant(4)` line will 
fail with a segfault. I'm using LDC2:
```
LDC - the LLVM D compiler (1.31.0):
  based on DMD v2.101.2 and LLVM 14.0.3
  built with LDC - the LLVM D compiler (1.31.0)
  Default target: x86_64-unknown-linux-gnu
  Host CPU: ivybridge
```