No we should not support enum types derived from strings

[deadalnix]

On Tuesday, 11 May 2021 at 13:50:46 UTC, Andrei Alexandrescu 
wrote:
> No it isn't.
>
> EnumString and String are reference types. A reference to an 
> enum value does not convert to a reference to its 
> representation. Very very very VERY different.
>

Here we hit at the core of the problem. A reference to a type B 
that is a subtype of A is not a subtype of ref A. Or, in simlpler 
terms, B is a subtype of A doesn't imply that ref B is a subtype 
of ref A.

This means that you can pass a B where an A is expected, but not 
a ref B where a ref A is expected.

You'll note that the example I provided with classes for 
understanding will also demonstrate the same behavior:

class A { ... }
class B : A { ... }

void foo(ref A a) { ... }

B b = ...;
foo(b); // This must be an error because, while B is a subtype of 
A, ref B is not a subtype of ref A.

That means that you shouldn't be able to pass SomeEnumString to 
any function, in phobos or elsewhere, that will mutate it, such 
as popFront. But you should be able to do so, transparently, so 
any function that won't. That includes all compile time 
parameters.