For compatibility with the man(1) implementations of the man-1.6

and man-db packages, print the manpath if the -w option is given
without a following name argument.

This quirk has been in man-1.6 since at least man-1.5e (1998)
and in man-db since 2012.

Using this feature in portable software is a dubious idea because the
internal organization of manual page directories varies in about a
dozen respects among operating systems, so even if you get the answer,
there is no portable way to use it for looking up anything inside.
However, Matej Cepl <mcepl at suse dot cz> made me aware that some
software, for example the manual viewing functionality in the newest
editors/neovim code, unwisely relies on this feature anyway.

No objections were raised when this patch was shown on tech@.

usr.bin/mandoc/main.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: main.c,v 1.245 2020/02/06 19:41:34 schwarze Exp $ */
+/*	$OpenBSD: main.c,v 1.246 2020/02/10 13:49:04 schwarze Exp $ */
 /*
  * Copyright (c) 2008-2012 Kristaps Dzonsons <kristaps@bsd.lv>
  * Copyright (c) 2010-2012, 2014-2020 Ingo Schwarze <schwarze@openbsd.org>
@@ -427,8 +427,20 @@ main(int argc, char *argv[])
 	/* man(1): Resolve each name individually. */
 
 	if (search.argmode == ARG_NAME) {
-		if (argc < 1)
-			usage(ARG_NAME);
+		if (argc < 1) {
+			if (outmode != OUTMODE_FLN)
+				usage(ARG_NAME);
+			if (conf.manpath.sz == 0) {
+				warnx("The manpath is empty.");
+				mandoc_msg_setrc(MANDOCLEVEL_BADARG);
+			} else {
+				for (i = 0; i + 1 < conf.manpath.sz; i++)
+					printf("%s:", conf.manpath.paths[i]);
+				printf("%s\n", conf.manpath.paths[i]);
+			}
+			manconf_free(&conf);
+			return (int)mandoc_msg_getrc();
+		}
 		for (res = NULL, ressz = 0; argc > 0; argc--, argv++) {
 			(void)mansearch(&search, &conf.manpath,
 			    1, argv, &resn, &resnsz);

usr.bin/mandoc/man.1

@@ -1,9 +1,9 @@
-.\"	$OpenBSD: man.1,v 1.35 2020/02/08 01:09:58 jsg Exp $
+.\"	$OpenBSD: man.1,v 1.36 2020/02/10 13:49:04 schwarze Exp $
 .\"
 .\" Copyright (c) 1989, 1990, 1993
 .\"	The Regents of the University of California.  All rights reserved.
 .\" Copyright (c) 2003, 2007, 2008, 2014 Jason McIntyre <jmc@openbsd.org>
-.\" Copyright (c) 2010, 2011, 2014-2018 Ingo Schwarze <schwarze@openbsd.org>
+.\" Copyright (c) 2010, 2011, 2014-2020 Ingo Schwarze <schwarze@openbsd.org>
 .\"
 .\" Redistribution and use in source and binary forms, with or without
 .\" modification, are permitted provided that the following conditions
@@ -31,7 +31,7 @@
 .\"
 .\"     @(#)man.1	8.2 (Berkeley) 1/2/94
 .\"
-.Dd $Mdocdate: February 8 2020 $
+.Dd $Mdocdate: February 10 2020 $
 .Dt MAN 1
 .Os
 .Sh NAME
@@ -199,6 +199,9 @@ Kernel internals.
 .It Fl w
 List the pathnames of all matching manual pages instead of displaying
 any of them.
+If no
+.Ar name
+is given, list the directories that would be searched.
 .El
 .Pp
 The options