bug#36564: 27.0.50; Wrong number of errors in compilation mode-line

[Juri Linkov]

Often compilation-mode displays wrong number of errors
in the mode-line even when compilation is finished.

compilation-mode is based on font-lock, so when the
*compilation* buffer is not displayed during compilation,
some parts of this buffer that contain error messages
are not fontified, and thus these errors are not counted.

This patch ensures the correct number of errors
is displayed on the mode-line:

diff --git a/lisp/progmodes/compile.el b/lisp/progmodes/compile.el
index 1a0d9bdbb7..a28e5f6068 100644
--- a/lisp/progmodes/compile.el
+++ b/lisp/progmodes/compile.el
@@ -2179,6 +2182,8 @@ compilation-handle-exit
     ;; Prevent that message from being recognized as a compilation error.
     (add-text-properties omax (point)
                         (append '(compilation-handle-exit t) nil))
+    ;; Update the number of errors in compilation-mode-line-errors
+    (font-lock-ensure)
     (setq mode-line-process
           (list
            (let ((out-string (format ":%s [%s]" process-status (cdr status)))