diff --git a/Makefile b/Makefile
index 1d3d9df5d..313f02ede 100644
--- a/Makefile
+++ b/Makefile
@@ -68,4 +68,4 @@ endif
 
 include mk/lib.mk
 
-GLOBAL_CXXFLAGS += -g -Wall -include $(buildprefix)config.h -std=c++2a -I src
+GLOBAL_CXXFLAGS += -g -Wall -Wimplicit-fallthrough -include $(buildprefix)config.h -std=c++2a -I src
diff --git a/src/libexpr/lexer.l b/src/libexpr/lexer.l
index df2cbd06f..ced1c4c39 100644
--- a/src/libexpr/lexer.l
+++ b/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"
diff --git a/tests/unit/libstore-support/tests/path.cc b/tests/unit/libstore-support/tests/path.cc
index e5f169e94..8bf501ab6 100644
--- a/tests/unit/libstore-support/tests/path.cc
+++ b/tests/unit/libstore-support/tests/path.cc
@@ -33,6 +33,7 @@ Gen<StorePathName> Arbitrary<StorePathName>::arbitrary()
         switch (auto i = *gen::inRange<uint8_t>(0, 10 + 2 * 26 + 6)) {
             case 0 ... 9:
                 pre += '0' + i;
+                break;
             case 10 ... 35:
                 pre += 'A' + (i - 10);
                 break;
@@ -52,6 +53,7 @@ Gen<StorePathName> Arbitrary<StorePathName>::arbitrary()
                     pre += '.';
                     break;
                 }
+                [[fallthrough]];
             case 65:
                 pre += '_';
                 break;