Why doesn't to!MyEnumType(42) work

[Tommi]

According to the docs, std.conv.to uses cast operator under the 
hood, and since the following works:

enum MyEnumType {a}
auto val = cast(MyEnumType) 42;
...it seems natural that to!MyEnumType(42) should work as well. 
But currently it gives error:
template instance std.conv.to!(MyEnumType).to!(int) error 
instantiating