GenEx: add $<LIST:SORT,...,COMPARATOR,body> comparator
Add a COMPARATOR form to $<LIST:SORT> that orders the list by a caller-defined
rule: a <body> evaluated per comparison with the two elements bound to $<_0>
and $<_1>, yielding "1" when the first should sort before the second. This
brings the custom ordering of list(SORT ... COMPARATOR) to generate time, so
elements can be ordered by target properties or any other generator expression.
CASE: and ORDER: still apply, while COMPARE: is rejected because the body
defines the ordering.
Fixes: #27892
23 files changed
