switch case expressions

[Martin Krejcirik via Digitalmars-d]

> https://github.com/D-Programming-Language/dmd/pull/2887
>
> Ugh...
>
> at the VERY LEAST, the error message should be fixed, as "can't 
> be read at compile time" is clearly not the issue.

Looks like this issue surfaces periodically. Somehow I've missed 
it so far.
Anyway, either the spec is wrong or the implementation is wrong.

> at the VERY LEAST, the error message should be fixed, as "can't 
> be read at compile time" is clearly not the issue.

This is auto solved by allowing cast in case expression.