Allow a binding operation to bind more than one operand at once, exposed as
$<_0>, $<_1>, .... A single value remains the common case, but the upcoming
SORT COMPARATOR must bind the two elements being compared, so the binding
becomes an indexed frame and $<_1> is added. Referencing an index the active
binding does not provide is reported as an error.
Issue: #27892
Introduce "binding operations": generator expressions that evaluate a <body>
once for each value they supply, with $<_0> expanding to that value. This is
the foundation the $<LIST:TRANSFORM,...,APPLY> action and the predicate
selectors build on, letting a <body> refer to the element being processed.
Using $<_0> outside a binding operation is reported as an error rather than
silently expanding to nothing.
Issue: #27892
Policy CMP0189, introduced by commit b3da9c6d60 (GenEx: Evaluate
LINK_LIBRARIES target properties transitively, 2025-02-24,
v4.1.0-rc1~731^2), takes effect at generation time, and so uses the
policy value as of the end of each directory. However, some projects
may rely on `file(GENERATE)` with the policy's OLD behavior in order
to extract targets' *direct* dependencies from `LINK_LIBRARIES`.
Pending a first-class solution to that problem, make it easier for
projects to port to the policy's NEW behavior in general while
retaining the OLD behavior in an isolated context.
Fixes: #27220
Mickaël Germain
Brad King