Hi,

I noticed that compilation-mode parses and successfully detect errors
according to set regexp int the compilation buffer header, iow in text
inserted by the mode itself. This is weird since there isn't any error
there to visit.

Reproduce: emacs -Q -l reproduce.el

Expected output:
No error fontification of header line: Compilation started at Sun Sep 27 11:48:24

Actual output:
Error fontified line: Compilation started at Sun Sep 27 11:48:24

I noticed this in my own package rg.el which is similar to grep.el in
that it reuses compilation mode and has similar regexps for parsing
file name and line number that happens to match the "$CMD started at..."
line. The attached test case somewhat emulates what happens in those
packages. The command output is unrelated though.

Looking at the code in compile.el, my impression is that this would be
taken care of by compilation-filter but for some reason that doesn't happen.

Best regards,
David Landell