std.sumtype?

[Oleg B]

On Thursday, 18 March 2021 at 18:09:38 UTC, Paul Backus wrote:
> union U1 { int n; float f; }
> union U2 { float f; int n; }
>
> static assert(is(U1 == U2)); // fails
>
> D is a nominally-typed language, not a structurally-typed one. 
> That means types that are structurally identical (like the 
> unions and SumTypes in the above examples) are still considered 
> separate types if their names are spelled differently.
>
> IMO the behavior of SumType in this regard is perfectly 
> consistent with the rest of the language. If you want to use 
> structural typing instead of nominal typing in your own code, 
> you can define template predicates to check for SumTypes with a 
> particular structure, the same way Phobos defines predicates 
> like `isInputRange` to check for types with a particular 
> structure.

okey, I agree

but read-only tag (kind, type index) is key feature in chouse 
between sumtype and other for me, and I hope it will be added 
before next compiler release...