get!T does not work when T is a class with non-inout constructor

[tom-tan]

The following code should work without errors but it does not.
Here is the same code in run.dlang.io: https://run.dlang.io/is/7bHmjV

/+ dub.sdl:
dependency "dyaml" version="~>0.8.3"
+/
import dyaml : Loader, Node;
import std : split, to;

class MyClass
{
    int x, y, z;

    this(Node node)
    {
        auto parts = node.as!string().split(":");
        x = parts[0].to!int;
        y = parts[1].to!int;
        z = parts[2].to!int;
    }
}

void main()
{
	auto loader = Loader.fromString(`"1:2:3"`);
	Node node = loader.load();
	auto mc = node.as!MyClass;
}

It fails to compile with the following error message:

onlineapp.d(24,16): Error: template instance `dyaml.node.Node.as!(MyClass)` does not match template declaration `get(T, Flag stringConversion = Yes.stringConversion)()`
  with `T = onlineapp.MyClass`
  must satisfy one of the following constraints:
`       allowed!(Unqual!T)
       hasNodeConstructor!(Unqual!T)`
dmd failed with exit code 1.

It works when inout is added to the constructor.
However, in the most cases, using inout constructor does not work as intended because it creates only const objects unless the constructor is used in inout functions (See https://run.dlang.io/is/9zhOL8).

[tom-tan]

FYI:

D-YAML includes a unittest using an inout constructor (line 729 in node.d).
I guess using a constructor that can create a unique object is more appropriate for the constructor for any qualifiers.

However, it seems to be a difficult task because Node.get cannot be pure (it is used in MyClass.this) due to std.variant.Algebraic.type. Using assumePure and @trusted can make Node.get pure but this solution makes more difficult to solve #33.

Using std.sumtype that will be included in 2.097 instead of Algebraic may solve this problem.