Improve handling of the roff(7) "\t" escape sequence:

* Parsing macro arguments has to be done in copy mode,
  which implies replacing "\t" by a literal tab character.
* Otherwise, render "\t" as the empty string, not as a 't' character.

This fixes formatting of the distfile example in the oldrdist(1) manual.
This also shows up in the unzip(1) manual as one of several issues
preventing the removal of USE_GROFF from the archivers/unzip port.
Thanks to espie@ for attracting my attention to the unzip(1) manual.

3 files changed, 25 insertions, 6 deletions

diff --git a/chars.c b/chars.c
index e7947f51..ad3ffd40 100644
--- a/chars.c
+++ b/chars.c
@@ -37,7 +37,7 @@ struct	ln {
 	int		  unicode;
 };
 
-#define	LINES_MAX	  328
+#define	LINES_MAX	  329
 
 #define CHAR(in, ch, code) \
 	{ NULL, (in), (ch), (code) },
diff --git a/chars.in b/chars.in
index 0d6959de..28a37eaa 100644
--- a/chars.in
+++ b/chars.in
@@ -42,6 +42,7 @@ CHAR("&",			"",		0)
 CHAR("^",			"",		0)
 CHAR("|",			"",		0)
 CHAR("}",			"",		0)
+CHAR("t",			"",		0)
 
 /* Accents. */
 CHAR("a\"",			"\"",		779)
diff --git a/mandoc.c b/mandoc.c
index 55419dc1..156bd689 100644
--- a/mandoc.c
+++ b/mandoc.c
@@ -432,17 +432,35 @@ mandoc_getarg(struct mparse *parse, char **cpp, int ln, int *pos)
 	pairs = 0;
 	white = 0;
 	for (cp = start; '\0' != *cp; cp++) {
-		/* Move left after quoted quotes and escaped backslashes. */
+
+		/*
+		 * Move the following text left
+		 * after quoted quotes and after "\\" and "\t".
+		 */
 		if (pairs)
 			cp[-pairs] = cp[0];
+
 		if ('\\' == cp[0]) {
-			if ('\\' == cp[1]) {
-				/* Poor man's copy mode. */
+			/*
+			 * In copy mode, translate double to single
+			 * backslashes and backslash-t to literal tabs.
+			 */
+			switch (cp[1]) {
+			case ('t'):
+				cp[0] = '\t';
+				/* FALLTHROUGH */
+			case ('\\'):
 				pairs++;
 				cp++;
-			} else if (0 == quoted && ' ' == cp[1])
+				break;
+			case (' '):
 				/* Skip escaped blanks. */
-				cp++;
+				if (0 == quoted)
+					cp++;
+				break;
+			default:
+				break;
+			}
 		} else if (0 == quoted) {
 			if (' ' == cp[0]) {
 				/* Unescaped blanks end unquoted args. */