[Issue 20018] static foreach with static if and is should permit a declaration

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20018

timon.gehr at gmx.ch changed:

           What    |Removed                     |Added
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                 CC|                            |timon.gehr at gmx.ch

--- Comment #2 from timon.gehr at gmx.ch ---
> But static if(is( ... identifier)) is a kinda special case. That does introduce a new symbol... and it is scoped to the body of the static if.

The underlying issue is that this is not true.

---
struct S{}
static if(is(S T)){ }
static assert(is(S==T)); // passes
---

I agree that it would make sense to scope `T` to the body of the `static if`.
The implementation would be pretty simple, it could just reuse the mechanism I
used to scope the loop variable within the body of `static foreach` (but this
is a breaking change).

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