I recently was adding a compilation error to
compilation-error-regexp-alist, and needed to use a custom function to
determine the line number. By inspection I was able to see that all the
primary fields (file, line, line-end, col, col-end) can take either a
subexpression number (for the regexp) or a function. However `line` is
handled differently then all the rest. If you define a function for line,
it ignores all your other fields and calls a legacy function handler
in compilation-parse-errors. It looks like this is to support "old
compile.el" from 2003. I see a lot of code marked obsolete since 24.1.

I was wondering what is your policy on removing obsolete content. I think
it would be a much better for current users if "line" was handled like all
the other fields instead of reverting the entire error to legacy mode in an
undocumented way.