[bug #35062] Add lazy evaluation of prerequisites

[Paul D. Smith]

Follow-up Comment #3, bug #35062 (project make):

Well, yes, it would be possible to do something like store the internal
variable database as a kind of "versioned" database, where changes to variable
values do not overwrite previous ones but instead are added on.  Then when a
target is defined it would keep a reference to the state of the variable
database at that time so that all variable references in prerequisite lists
would be expanded in the context of that state, instead of the "current"
(ending) state.  Or something similar.

However functions that referenced "outside influences" like wildcard etc.
could not be duplicated (a wildcard expansion while the makefile is read in
and an expansion well on into the work of building targets could well give
very different results for example).

And my final comment stands: it would be an enormous amount of work.


One possibility would be to use SECONDEXPANSION to avoid the immediate
expansion of prerequisites.  However, today make will secondarily expand all
(explicit) targets, not just targets that are considered during a given run. 
I would be willing to look into an enhancement that deferred secondary
expansion until a target was considered and not secondarily expanding targets
that are not considered.

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