How to compile against GitHub fork of Phobos?

[Q. Schroll via Digitalmars-d-learn]

On Tuesday, 7 March 2017 at 01:45:48 UTC, Adam D. Ruppe wrote:
> You pass your modified file to the compiler:
>
> dmd yourfile.d format.d
>
> The format.d there can be a copy of the one from phobos (or a 
> fork or whatever) and since you passed it explicitly on the 
> command line, it takes precedence over the one in the library.
>
> You still import it as std.format. You can do that with as many 
> modules as you like.

I just tried it out.

    dmd myfile.d phobos/std/format.d

worked fine, but interestingly,

    rdmd myfile.d phobos/std/format.d

did not. The latter definitely takes format from the library. I 
have no idea why.