Extend the documentation on type constraints to include the new default
functionality, including a detailed example of a nested structure with
multiple levels of defaults.
When a data resource is used for the purposes of verifying a condition
about an object managed elsewhere (e.g. if the managed resource doesn't
directly export all of the information required for the condition) it's
important that we defer the data resource read to the apply step if the
corresponding managed resource has any changes pending.
Typically we'd expect that to come "for free" but unfortunately we have
a pragmatic special case in our handling of data resources where we
normally defer to the apply step only if a _direct_ dependency of the data
resource has a change pending, and allow a plan-time read if there's
a pending change in an indirect dependency. This allowed us to preserve
some compatibility with the questionable historical behavior of always
reading data resources proactively unless the configuration contains
unknown values, since the arguably-more-correct behavior would've been a
regression for anyone who had been depending on that before.
Since preconditions and postconditions didn't exist until now, we are not
constrained in the same way by backward compatibility, and so we can adopt
the more correct behavior in the case where a data resource has conditions
specified. This does unfortunately make the handling of data resources
with conditions subtly inconsistent with those that don't, but this is
a better situation than the alternative where it would be easy to get into
a trapped situation where the remote system is invalid and it's impossible
to plan the change that would make it valid again because the conditions
evaluate too soon, prior to the fix being applied.
Including a note about logical operators not short-circuiting will make the documentation clearer and more useful. https://github.com/hashicorp/terraform/issues/24128 includes examples of people being caught out by this lack of clarity.
