If a .shift request has a negative argument, do not use a negative array

index but use 0 instead of the argument, just like groff.
Warn about the invalid argument.
While here, fix the column number in another warning message.

Segfault reported by tb@, found with afl(1).

3 files changed, 13 insertions, 6 deletions

diff --git a/regress/usr.bin/mandoc/roff/shift/bad.in b/regress/usr.bin/mandoc/roff/shift/bad.in
index 809832defd8..22eff7e0c3c 100644
--- a/regress/usr.bin/mandoc/roff/shift/bad.in
+++ b/regress/usr.bin/mandoc/roff/shift/bad.in
@@ -1,5 +1,5 @@
-.\" $OpenBSD: bad.in,v 1.1 2018/08/23 14:16:12 schwarze Exp $
-.TH SHIFT_BAD 1 "August 23, 2018"
+.\" $OpenBSD: bad.in,v 1.2 2022/04/24 13:34:53 schwarze Exp $
+.TH SHIFT_BAD 1 "April 24, 2022"
 .SH NAME
 .B shift-bad
 \(en wrong usage of macro arguments
@@ -22,6 +22,10 @@ argument used after call: "\$1"
 .de mym
 .shift badarg
 after shift badarg: "\\$1"
+.br
+.shift -1
+after shift \-1: "\\$1"
+.br
 .shift 2
 after excessive shift: \\n(.$ "\\$1"
 ..
diff --git a/regress/usr.bin/mandoc/roff/shift/bad.out_ascii b/regress/usr.bin/mandoc/roff/shift/bad.out_ascii
index 2d6a2922c2f..ddb8c521aff 100644
--- a/regress/usr.bin/mandoc/roff/shift/bad.out_ascii
+++ b/regress/usr.bin/mandoc/roff/shift/bad.out_ascii
@@ -14,8 +14,10 @@ DDEESSCCRRIIPPTTIIOONN
 
        argument used after call: ""
 
-       after shift badarg: "arg2" after excessive shift: 0 ""
+       after shift badarg: "arg2"
+       after shift -1: "arg2"
+       after excessive shift: 0 ""
 
        final text
 
-OpenBSD                         August 23, 2018                   SHIFT_BAD(1)
+OpenBSD                         April 24, 2022                    SHIFT_BAD(1)
diff --git a/regress/usr.bin/mandoc/roff/shift/bad.out_lint b/regress/usr.bin/mandoc/roff/shift/bad.out_lint
index 1f696fc8681..b122415e989 100644
--- a/regress/usr.bin/mandoc/roff/shift/bad.out_lint
+++ b/regress/usr.bin/mandoc/roff/shift/bad.out_lint
@@ -3,5 +3,6 @@ mandoc: bad.in:15:2: ERROR: ignoring request outside macro: shift
 mandoc: bad.in:17:31: ERROR: argument number is not numeric: \$x
 mandoc: bad.in:19:28: ERROR: using macro argument outside macro: \$1
 mandoc: bad.in:20:2: ERROR: ignoring request outside macro: shift
-mandoc: bad.in:28:8: ERROR: argument is not numeric, using 1: shift badarg
-mandoc: bad.in:28:9: ERROR: excessive shift: 2, but max is 1
+mandoc: bad.in:32:8: ERROR: argument is not numeric, using 1: shift badarg
+mandoc: bad.in:32:8: ERROR: negative argument, using 0: shift -1
+mandoc: bad.in:32:8: ERROR: excessive shift: 2, but max is 1