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I searched for uses of when/2 in Prolog code on my machine. These predicates would benefit from delay/1 support:

• error:must_be/2
• =:=/2
• callable/2

I can probably generalize from those few examples to include other operators (like =:=) and other type checks (like callable).

Jan pointed out

delaying over a cut changes the semantics.
So, if you use this you either should not use cuts or be very
careful with them.

The first improvement is to document this circumstance so that developers are aware of the problem.

It'd also be nice to generate warnings about delay/1 that occur before !/0 in a clause. This doesn't cover all problem scenarios but it should help notice some common ones. If I go this route, there needs to be a way to say, "I know what I'm doing, leave me alone". Ideally, there should be a style_check/1 mode that enables this warning.

The library should provide support for maplist/3, maplist/4, etc. It should also work even if the maplist calls have been expanded via apply_macros.pl.

We can probably just look for a predicate whose name starts with __aux_maplist to detect that it's a maplist expansion.

I want users to be able to add delay/1 support to any predicates they care about. Making that happen requires a decent rewrite. As follows:

• strip all macros from this code. it must work with mode/1 as it's written
• replace g and n with ground and nonvar, respectively
• make sure that each delay/1 clause cuts before calling when/2. otherwise, when/2 can fail because all arguments are ground and the underlying goal fails. that would backtrack to the final delay/1 clause and throw an exception. Be sure to write a failing test for this first, then make changes to make it pass. without a test, I'm sure I'll encounter this same corner case again.
• have delay/1 consult mode/1 to build a when/2 goal then assert it as an additional clause on delay/1 (the assertion is an optimization)
• change delay/1 into meta_predicate delay(0) and change mode/1 so that each built-in predicate is prefaced with system:. Look at mode/1 with the module-qualified head. If there's no mode for that, call predicate_property(Head, imported_from(Module)) to determine if this predicate is originally from another module. Perform the mode/1 look up using that module. If there's no mode/1 declaration in either case, throw an error with instructions about how to add support for other predicates.
• make sure delay_followup/1 always has a nonvar(List) guard in the list(_) case
• :- multifile mode/1.
• at this point, one can just add delay:mode(foo:bar(ground,_)) facts to support additional predicates.
• clean up and document before releasing a new version and posting to the mailing list

SWI-Prolog's SICSTUS compatibility layer includes the block/1 directive which fills a niche similar to delay/1. Link to it from the documentation and perhaps comment on the differences between each approach. For example, does block/1 work with built-in predicates? Does it work with foreign predicates? Does it work with dynamic predicates? I'm pretty sure that delay/1 works with all of them. I think that delay/1 has a more flexible mode language than block/1