When is

    __traits(compiles, { ... } )

preferred over

    is(typeof( { ... } ))

and vice versa when writing stuff like

    enum isCopyable(S) = is(typeof( { S foo = S.init; S copy = foo; } ));

?

Further, are there cases where the two idioms aren't exchangable?

On Wednesday, 20 September 2017 at 12:01:21 UTC, Nordlöw wrote:
> When is
>
>     __traits(compiles, { ... } )
>
> preferred over
>
>     is(typeof( { ... } ))
>
> and vice versa when writing stuff like
>
>     enum isCopyable(S) = is(typeof( { S foo = S.init; S copy = foo; } ));
>
> ?
>
> Further, are there cases where the two idioms aren't exchangable?

Yes there are.
Prefer __traits(compiles) it includes stricter visibility checks then is() does.
Also compiles does not require the expression to resolve to a type.

On Wednesday, 20 September 2017 at 12:20:18 UTC, Stefan Koch wrote:
> Yes there are.
> Prefer __traits(compiles) it includes stricter visibility checks then is() does.

1. Can you give example of such visibility checks?

2. If so, should `isCopyable` (along with more defs in std.traits) be converted to instead use `__traits(compiles, )` ?

3. And how do the two idioms differ in terms of compilation-speed?