First attempt at explicit fallthrough annotation.

Cython/Compiler/Nodes.py

@@ -3623,6 +3623,7 @@ class DefNodeWrapper(FuncDefNode):
                     if i >= min_positional_args-1:
                         code.put('case %2d: ' % (i+1))
                     code.putln("values[%d] = PyTuple_GET_ITEM(%s, %d);" % (i, Naming.args_cname, i))
+                    code.putln('CYTHON_FALLTHROUGH')
                 if min_positional_args == 0:
                     code.put('case  0: ')
                 code.putln('break;')
@@ -3749,6 +3750,7 @@ class DefNodeWrapper(FuncDefNode):
             code.put('case %2d: ' % (i+1))
             code.putln("values[%d] = PyTuple_GET_ITEM(%s, %d);" % (
                 i, Naming.args_cname, i))
+            code.putln('CYTHON_FALLTHROUGH')
         code.putln('case  0: break;')
         if not self.star_arg:
             code.put('default: ') # more arguments than allowed
@@ -3817,6 +3819,8 @@ class DefNodeWrapper(FuncDefNode):
                             self.name, pystring_cname))
                         code.putln(code.error_goto(self.pos))
                         code.putln('}')
+                if max_positional_args > 0 and i < last_required_arg:
+                    code.putln('CYTHON_FALLTHROUGH')
             if max_positional_args > 0:
                 code.putln('}')
 

Cython/Utility/ModuleSetupCode.c

@@ -356,6 +356,14 @@
   #define __Pyx_PyMethod_New(func, self, klass) PyMethod_New(func, self, klass)
 #endif
 
+#ifndef __has_attribute
+  #define __has_attribute(x) 0
+#endif
+
+#ifndef __has_cpp_attribute
+  #define __has_cpp_attribute(x) 0
+#endif
+
 // backport of PyAsyncMethods from Py3.5 to older Py3.x versions
 // (mis-)using the "tp_reserved" type slot which is re-activated as "tp_as_async" in Py3.5
 #if CYTHON_USE_ASYNC_SLOTS
@@ -434,6 +442,19 @@
    #include <stdint.h>
 #endif
 
+
+#ifndef CYTHON_FALLTHROUGH
+  #if __has_cpp_attribute(fallthrough)
+    #define CYTHON_FALLTHROUGH [[fallthrough]]
+  #elif __has_cpp_attribute(clang::fallthrough)
+    #define CYTHON_FALLTHROUGH [[clang::fallthrough]]
+  #elif __has_attribute(fallthrough) || (defined(__GNUC__) && defined(__attribute__))
+    #define CYTHON_FALLTHROUGH __attribute__((fallthrough))
+  #else
+    #define CYTHON_FALLTHROUGH
+  #endif
+#endif
+
 /////////////// CInitCode ///////////////
 
 // inline attribute