--- a/mercurial/commands.py	Wed Jul 11 21:15:52 2018 +0900
+++ b/mercurial/commands.py	Wed Jul 11 21:23:18 2018 +0900
@@ -2533,6 +2533,9 @@
     """
     opts = pycompat.byteskwargs(opts)
     diff = opts.get('all') or opts.get('diff')
+    if diff and opts.get('all_files'):
+        raise error.Abort(_('--diff and --all-files are mutually exclusive'))
+
     reflags = re.M
     if opts.get('ignore_case'):
         reflags |= re.I

--- a/tests/test-grep.t	Wed Jul 11 21:15:52 2018 +0900
+++ b/tests/test-grep.t	Wed Jul 11 21:23:18 2018 +0900
@@ -412,6 +412,12 @@
   $ hg grep -r "." "unmod" --all-files
   um:1:unmod
 
+--diff --all-files makes no sense since --diff is the option to grep history
+
+  $ hg grep --diff --all-files um
+  abort: --diff and --all-files are mutually exclusive
+  [255]
+
   $ cd ..
 
 Fix_Wdir(): test that passing wdir() t -r flag does greps on the