Re: order-only prerequisites themselves get rebuilt when not needed

On Mon, 2025-03-17 at 21:16 -0400, Dmitry Goncharov wrote:
> > To build target A:
> > - First try to build all NON-ORDER-ONLY prerequisites
> > - Compare the modification time of all NON-ORDER-ONLY prerequisites
> > - If any NON-ORDER-ONLY prerequisite is newer than A:
> >    - Try to build all ORDER-ONLY prerequisites
> >   - Run the recipe to build A
> 
> The patch in attachment is slightly different. The patch causes make
> to never update existing order-only prerequisites.

If make never updates them then what's the purpose of having them?  Oh,
I see from the patch: you don't mean "never" you mean, only if the
prerequisite does not exist will it be updated.

I agree this preserves probably the most common reason for using OO
prereqs (to create directories) but I don't think I like this change. 
For one thing it feels too different from normal make behavior, in that
we do "consider" a prerequisite but we do so only to check for
existence, not for out-of-date-ness, which feels odd to me.

Secondly, I believe the behavior provided by your patch is available
already; you can use $(wildcard ...) to check if a file exists.  Isn't
something like this (the gross bits can be hidden in a macro I suppose)
the equivalent of the change in the patch?

    xyz: foo | $(if $(wildcard bar),,bar) ;

    foo bar: ;

If bar exists then we expand to nothing, else we expand to bar.