Merge pull request #10074 from lf-/jade/ban-implicit-fallthrough

Warn on implicit switch case fallthrough

(cherry picked from commit 21282c3c204597641402c6bcff8fc9ee7bc31fa1)
Change-Id: I5ebbdfb6c037d2c55254f37dd391c07c2ce7443e

src/libexpr/lexer.l

@@ -20,6 +20,12 @@
 #pragma clang diagnostic ignored "-Wunneeded-internal-declaration"
 #endif
 
+// yacc generates code that uses unannotated fallthrough.
+#pragma GCC diagnostic ignored "-Wimplicit-fallthrough"
+#ifdef __clang__
+#pragma clang diagnostic ignored "-Wimplicit-fallthrough"
+#endif
+
 #include <boost/lexical_cast.hpp>
 
 #include "nixexpr.hh"