As reported by deadalnix on IRC. The culprit is likely std.concurrency using is(T == function) when trying to check for a function pointer.
https://github.com/D-Programming-Language/phobos/pull/362
https://github.com/D-Programming-Language/phobos/commit/e8e92f3f99f348ddc0a5b512544775b2f1fcd602