isdigit() needs an unsigned char argument.

Per the C standard, the routine should be passed an int, with a value that's
representable as an unsigned char or EOF. Passing a signed char is wrong,
because a negative value is not representable as an unsigned char.
Unfortunately no compiler warns about that.

src/backend/utils/adt/timestamp.c

@@ -511,7 +511,7 @@ parse_sane_timezone(struct pg_tm *tm, text *zone)
 	 * as invalid, it's enough to disallow having a digit in the first
 	 * position of our input string.
 	 */
-	if (isdigit(*tzname))
+	if (isdigit((unsigned char) *tzname))
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 				 errmsg("invalid input syntax for numeric time zone: \"%s\"",