tests/lint: add test for warning about zero-bits in '&'

tests/usr.bin/xlint/lint1/msg_309.c

@@ -1,7 +1,39 @@
-/*	$NetBSD: msg_309.c,v 1.2 2021/02/21 09:07:58 rillig Exp $	*/
+/*	$NetBSD: msg_309.c,v 1.3 2021/05/16 10:08:01 rillig Exp $	*/
 # 3 "msg_309.c"
 
 // Test for message: extra bits set to 0 in conversion of '%s' to '%s', op %s [309]
 
-TODO: "Add example code that triggers the above message." /* expect: 249 */
-TODO: "Add example code that almost triggers the above message."
+int
+scale(unsigned long long x) {
+
+	/*
+	 * Both operands of '&' have the same type, therefore no conversion
+	 * is necessary and no bits can get lost.
+	 */
+	if ((x & 0xffffffff00000000ULL) != 0)
+		return 32;
+
+	/*
+	 * The constant has type 'unsigned 32-bit'.  The usual arithmetic
+	 * conversions of '&' convert this constant to unsigned 64-bit.
+	 * The programmer may or may not have intended to sign-extend the
+	 * bit mask here.  This situation may occur during migration from a
+	 * 32-bit to a 64-bit platform.
+	 */
+	if ((x & 0xffff0000) != 0)	/* expect: 309 */
+		return 16;
+
+	/*
+	 * In the remaining cases, the constant does not have its most
+	 * significant bit set, therefore there is no ambiguity.
+	 */
+	if ((x & 0xff00) != 0)
+		return 8;
+	if ((x & 0xf0) != 0)
+		return 4;
+	if ((x & 0xc) != 0)
+		return 2;
+	if ((x & 0x2) != 0)
+		return 1;
+	return (int)(x & 0x1);
+}

tests/usr.bin/xlint/lint1/msg_309.exp

@@ -1 +1 @@
-msg_309.c(6): error: syntax error ':' [249]
+msg_309.c(23): warning: extra bits set to 0 in conversion of 'unsigned int' to 'unsigned long long', op & [309]