--- /dev/null
+/* Test for bit-field widths not integer constant expressions but
+ folding to integer constants: PR 42439. */
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+
+void
+f (void)
+{
+ const int m = 1;
+ ((void)(sizeof(struct { int i:!!m; })));
+}