unreachable() is defined in C23 as function-like macro, undefine
This definition conflicts with usage in smatch, as it has a valid
meaning in processing syntactic tree, not a mark that part of the
smatch state itself can't be reached.
Other possible solution would be mass-rename of all usages to prevent
conflict.
https://bugs.gentoo.org/944322
--- a/smatch.h
+++ b/smatch.h
@@ -717,6 +717,10 @@ void __set_fake_cur_stree_fast(struct stree *stree);
 void __pop_fake_cur_stree_fast(void);
 void __merge_stree_into_cur(struct stree *stree);
 
+#if __STDC_VERSION__ > 201710L
+#undef unreachable //defined in C23
+#endif
+
 int unreachable(void);
 void __set_cur_stree_readonly(void);
 void __set_cur_stree_writable(void);