You must have missed the prior discussion: https://github.com/D-Programming-Language/phobos/pull/1911#discussion-diff-9568563

Highlights include the replies:
>  > Why is this block indented?
>
> Because I thought it looked nicer.
>
> > In general, do we want to document contracts?
>
> I don't find trivia to be helpful - in this case, any additional documentation would be trivia.

And:

> Indenting it is also consistent with how constraints are used in templates, and is analogous in that they apply to the function parameters.

For what it's worth, there is not *much* consistency in how constraints are used, and there definitely isn't a consensus about it (sometimes indented, sometimes not). What's for sure is that in regards to an "in" block, this is a new style.

But I kind of like it. It *does* help separate what's body and what's not in a quick glimpse. IMO, as long as the entire module adheres to a consistent style, I'm fine with it. It's just style.