If Statement with Declaration

[Nick Treleaven via Digitalmars-d]

On Friday, 21 July 2017 at 21:50:02 UTC, Johan Engelen wrote:
> We do have a construct like that already:
> ```
> static if (is(E V == enum)) { // this inserts "V"
>    V v;
> }
> ```

Yes, but this is a bit different as the goal is to avoid 
repeating V (as it may be complex). Repeating a local variable 
name (using e.g. `with...if`) isn't a problem as the variable 
should have a short identifier. With the above `is` expression 
it's unavoidable to have a test and a declaration combined. That 
said, the syntax could be much clearer:

static if (is(E == enum; alias V)) { // this inserts "V"
    V v;
}

This syntax clearly separates the test from the declaration, and 
makes the declaration obvious due to the alias keyword. The test 
comes first as it logically should, as the alias is not being 
used until the following statement. It extends to the other `is` 
expression forms:

is(T; alias A)
is(T : U; alias A)
is(T : U!V, U, V; alias A)
is(T == U!V, U, V; alias A)

(http://dlang.org/spec/expression.html#IsExpression)

Whenever I want to use the identifier variant I always end up 
checking the spec. When I see it in code I usually have to stop 
what I was thinking about to parse the `is` expression.