On 8/12/21 12:12 PM, Paul Backus wrote:
The reason for this is a bit subtle. Normally, `inout` can convert to `const`, so you might expect that the `const` copy constructor could be used to construct a copy of an `inout` object. However, copy constructors take their arguments by `ref`, and implicit conversions are not allowed for arguments passed to `ref` parameters. (I cannot find a citation in the spec for this, but you can verify it yourself.)
implicit const conversions are possible via a single ref, but not through a double reference. In this case, it's not the passing of the inout object to the const constructor.
The issue is that you can't convert const (or immutable or mutable) to inout implicitly, and the member variable is inout inside an inout constructor. Therefore, there's no viable copy constructor to call for the member, and the outer copy constructor cannot be generated.
As a side note, inout actually *can* bind to double references of any mutability, unlike const, which is a nice feature that is seldom talked about.
-Steve
