Make fallthroughs explicit and enable warnings around this

CMakeLists.txt

@@ -36,8 +36,8 @@ set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${GLOBAL_OUTPUT_PATH})
 if (${CMAKE_CXX_COMPILER_ID} MATCHES "Clang" OR
         ${CMAKE_CXX_COMPILER_ID} STREQUAL "GNU")
     set(OPT "-O3")
-    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -g -Wall -Wextra -pedantic -std=c99 -O3 ${OPT}")
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -g -Wall -Wextra -Woverloaded-virtual -pedantic -std=c++17 -fPIC ${OPT}")
+    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -g -Wall -Wextra -Wimplicit-fallthrough -pedantic -std=c99 -O3 ${OPT}")
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -g -Wall -Wextra -Wimplicit-fallthrough -Woverloaded-virtual -pedantic -std=c++17 -fPIC ${OPT}")
 else()
     # TODO: Windows support.
     message(FATAL_ERROR "Compiler ${CMAKE_CXX_COMPILER_ID} not supported")

core/desugarer.cpp

@@ -732,7 +732,7 @@ class Desugarer {
                 } break;
 
                 case BOP_MANIFEST_UNEQUAL: invert = true;
-                /* fallthrough */
+                [[fallthrough]];
                 case BOP_MANIFEST_EQUAL: {
                     ast_ = equals(ast->location, ast->left, ast->right);
                     if (invert)

core/vm.cpp

@@ -2390,7 +2390,7 @@ class Interpreter {
                     stack.top().val2 = scratch;
                     stack.top().kind = FRAME_BINARY_OP;
                 }
-                // Falls through.
+                [[fallthrough]];
                 case FRAME_BINARY_OP: {
                     const auto &ast = *static_cast<const Binary *>(f.ast);
                     const Value &lhs = stack.top().val;