diff --git a/distrib/sets/lists/tests/mi b/distrib/sets/lists/tests/mi
index a95f479d54fd..0d59a73d1164 100644
--- a/distrib/sets/lists/tests/mi
+++ b/distrib/sets/lists/tests/mi
@@ -1,4 +1,4 @@
-# $NetBSD: mi,v 1.1025 2021/03/06 17:56:33 rillig Exp $
+# $NetBSD: mi,v 1.1026 2021/03/07 19:42:54 rillig Exp $
 #
 # Note: don't delete entries from here - mark them as "obsolete" instead.
 #
@@ -6730,6 +6730,8 @@
 ./usr/tests/usr.bin/xlint/lint1/msg_338.exp			tests-usr.bin-tests	compattestfile,atf
 ./usr/tests/usr.bin/xlint/lint1/msg_339.c			tests-usr.bin-tests	compattestfile,atf
 ./usr/tests/usr.bin/xlint/lint1/msg_339.exp			tests-usr.bin-tests	compattestfile,atf
+./usr/tests/usr.bin/xlint/lint1/msg_340.c			tests-usr.bin-tests	compattestfile,atf
+./usr/tests/usr.bin/xlint/lint1/msg_340.exp			tests-usr.bin-tests	compattestfile,atf
 ./usr/tests/usr.bin/xlint/lint1/t_integration			tests-usr.bin-tests	compattestfile,atf
 ./usr/tests/usr.bin/ztest				tests-usr.bin-tests	compattestfile,atf
 ./usr/tests/usr.bin/ztest/Atffile			tests-usr.bin-tests	compattestfile,atf
diff --git a/tests/usr.bin/xlint/lint1/Makefile b/tests/usr.bin/xlint/lint1/Makefile
index 30f4313c4095..2c730538af36 100644
--- a/tests/usr.bin/xlint/lint1/Makefile
+++ b/tests/usr.bin/xlint/lint1/Makefile
@@ -1,4 +1,4 @@
-# $NetBSD: Makefile,v 1.34 2021/02/28 22:12:16 rillig Exp $
+# $NetBSD: Makefile,v 1.35 2021/03/07 19:42:54 rillig Exp $
 
 NOMAN=		# defined
 
@@ -96,7 +96,7 @@ FILES+=		d_type_question_colon.c
 FILES+=		d_typefun.c
 FILES+=		d_typename_as_var.c
 FILES+=		d_zero_sized_arrays.c
-FILES+=		${:U0 ${:U:range=339}:C,^.$,0&,:C,^..$,0&,:@msg@msg_${msg}.c msg_${msg}.exp@}
+FILES+=		${:U0 ${:U:range=340}:C,^.$,0&,:C,^..$,0&,:@msg@msg_${msg}.c msg_${msg}.exp@}
 
 # Note: only works for adding tests.
 # To remove a test, the $$mi file must be edited manually.
diff --git a/tests/usr.bin/xlint/lint1/msg_340.c b/tests/usr.bin/xlint/lint1/msg_340.c
new file mode 100644
index 000000000000..715f7dc889f0
--- /dev/null
+++ b/tests/usr.bin/xlint/lint1/msg_340.c
@@ -0,0 +1,19 @@
+/*	$NetBSD: msg_340.c,v 1.1 2021/03/07 19:42:54 rillig Exp $	*/
+# 3 "msg_340.c"
+
+// Test for message: initialization with '[a...b]' is a GNU extension [340]
+
+/*
+ * In strict C mode, GNU extensions are flagged as such.
+ */
+
+/* lint1-flags: -Ssw */
+
+int
+example(void)
+{
+	int numbers[] = {
+		[2 ... 3] = 12	/* expect: 340 */
+	};
+	return numbers[0];
+}
diff --git a/tests/usr.bin/xlint/lint1/msg_340.exp b/tests/usr.bin/xlint/lint1/msg_340.exp
new file mode 100644
index 000000000000..b4ea08683597
--- /dev/null
+++ b/tests/usr.bin/xlint/lint1/msg_340.exp
@@ -0,0 +1 @@
+msg_340.c(16): initialization with '[a...b]' is a GNU extension [340]
diff --git a/tests/usr.bin/xlint/lint1/t_integration.sh b/tests/usr.bin/xlint/lint1/t_integration.sh
index 4e197c99bf28..836f9b29b7d2 100644
--- a/tests/usr.bin/xlint/lint1/t_integration.sh
+++ b/tests/usr.bin/xlint/lint1/t_integration.sh
@@ -1,4 +1,4 @@
-# $NetBSD: t_integration.sh,v 1.32 2021/02/28 22:12:16 rillig Exp $
+# $NetBSD: t_integration.sh,v 1.33 2021/03/07 19:42:54 rillig Exp $
 #
 # Copyright (c) 2008, 2010 The NetBSD Foundation, Inc.
 # All rights reserved.
@@ -187,7 +187,7 @@ all_messages_body()
 	srcdir="$(atf_get_srcdir)"
 	ok="true"
 
-	for msg in $(seq 0 339); do
+	for msg in $(seq 0 340); do
 		base="$(printf '%s/msg_%03d' "${srcdir}" "${msg}")"
 		flags="$(extract_flags "${base}.c")"
 
diff --git a/usr.bin/xlint/lint1/cgram.y b/usr.bin/xlint/lint1/cgram.y
index b51837f8c3c4..e0c33ac8151d 100644
--- a/usr.bin/xlint/lint1/cgram.y
+++ b/usr.bin/xlint/lint1/cgram.y
@@ -1,5 +1,5 @@
 %{
-/* $NetBSD: cgram.y,v 1.165 2021/03/07 18:02:45 rillig Exp $ */
+/* $NetBSD: cgram.y,v 1.166 2021/03/07 19:42:54 rillig Exp $ */
 
 /*
  * Copyright (c) 1996 Christopher G. Demetriou.  All Rights Reserved.
@@ -35,7 +35,7 @@
 
 #include <sys/cdefs.h>
 #if defined(__RCSID) && !defined(lint)
-__RCSID("$NetBSD: cgram.y,v 1.165 2021/03/07 18:02:45 rillig Exp $");
+__RCSID("$NetBSD: cgram.y,v 1.166 2021/03/07 19:42:54 rillig Exp $");
 #endif
 
 #include <limits.h>
@@ -1353,6 +1353,8 @@ range:
 	| constant T_ELLIPSIS constant {
 		$$.lo = toicon($1, 1);
 		$$.hi = toicon($3, 1);
+		/* initialization with '[a...b]' is a GNU extension */
+		gnuism(340);
 	  }
 	;
 
diff --git a/usr.bin/xlint/lint1/err.c b/usr.bin/xlint/lint1/err.c
index 85d141e51489..69498ebc932b 100644
--- a/usr.bin/xlint/lint1/err.c
+++ b/usr.bin/xlint/lint1/err.c
@@ -1,4 +1,4 @@
-/*	$NetBSD: err.c,v 1.86 2021/02/28 02:45:37 rillig Exp $	*/
+/*	$NetBSD: err.c,v 1.87 2021/03/07 19:42:54 rillig Exp $	*/
 
 /*
  * Copyright (c) 1994, 1995 Jochen Pohl
@@ -37,7 +37,7 @@
 
 #include <sys/cdefs.h>
 #if defined(__RCSID) && !defined(lint)
-__RCSID("$NetBSD: err.c,v 1.86 2021/02/28 02:45:37 rillig Exp $");
+__RCSID("$NetBSD: err.c,v 1.87 2021/03/07 19:42:54 rillig Exp $");
 #endif
 
 #include <sys/types.h>
@@ -399,6 +399,7 @@ const	char *msgs[] = {
 	"right operand of '%s' must not be bool",		      /* 337 */
 	"option '%c' should be handled in the switch",		      /* 338 */
 	"option '%c' should be listed in the options string",	      /* 339 */
+	"initialization with '[a...b]' is a GNU extension",	      /* 340 */
 };
 
 /*