+# Checking the warning message guarantees that the trigraph "??!" isn't
+# unnecessarily escaped here even though it would need to be if encoded in a
+# C-string literal. Also notice that unnecessary escaping, such as "\?", from
+# the user specification is eliminated.
+AT_BISON_CHECK([-o input.c input.y], [[0]], [[]],
+[[input.y:22.8-14: warning: symbol SPECIAL redeclared
+input.y:22.8-63: warning: symbol `"\\'?\"\a\b\f\n\r\t\v\001\201\001\201??!"' used more than once as a literal string
+]])