diff --git a/Makefile b/Makefile
index d3542c3e9..f8689c8cf 100644
--- a/Makefile
+++ b/Makefile
@@ -81,7 +81,7 @@ ifdef HOST_WINDOWS
   GLOBAL_LDFLAGS += -Wl,--export-all-symbols
 endif
 
-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
 
 # Include the main lib, causing rules to be defined
 
diff --git a/src/libexpr/lexer.l b/src/libexpr/lexer.l
index 380048c77..5b26d6927 100644
--- a/src/libexpr/lexer.l
+++ b/src/libexpr/lexer.l
@@ -94,6 +94,9 @@ static StringToken unescapeStr(SymbolTable & symbols, char * s, size_t length)
 
 }
 
+// yacc generates code that uses unannotated fallthrough.
+#pragma GCC diagnostic ignored "-Wimplicit-fallthrough"
+
 #define YY_USER_INIT initLoc(yylloc)
 #define YY_USER_ACTION adjustLoc(yylloc, yytext, yyleng);
 
diff --git a/src/libutil/fs-sink.cc b/src/libutil/fs-sink.cc
index 95b6088da..35ce0ac36 100644
--- a/src/libutil/fs-sink.cc
+++ b/src/libutil/fs-sink.cc
@@ -15,6 +15,7 @@ void copyRecursive(
     case SourceAccessor::tSymlink:
     {
         sink.createSymlink(to, accessor.readLink(from));
+        break;
     }
 
     case SourceAccessor::tRegular:
@@ -38,6 +39,7 @@ void copyRecursive(
                 sink, to + "/" + name);
             break;
         }
+        break;
     }
 
     case SourceAccessor::tMisc: