Fastest way to check using if identifier has already been defined, using static if or similar?

[Simen Kjærås]

On Wednesday, 3 June 2020 at 09:39:34 UTC, Basile B. wrote:
> You can use this template:
>
>   enum Exists(alias T) = is(typeof(T));
>
> I don't know if there's a faster way bu this technic is used, 
> notatbly in phobos, to workaroud issues of double declaration 
> in `static foreach`

enum Exists(alias T) = is(typeof(T));
static assert(!Exists!bar); // undefined identifier bar

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   Simen