Adding test case: do (x = y) -> overrides existing x value

test/function_invocation.coffee

@@ -509,29 +509,32 @@ test "#1416: don't omit one 'new' when compiling 'new new fn()()'", ->
 
 test "#1840: accessing the `prototype` after function invocation should compile", ->
   doesNotThrow -> CoffeeScript.compile 'fn()::prop'
-  
+
   nonce = {}
   class Test then id: nonce
 
   dotAccess = -> Test::
   protoAccess = -> Test
-  
+
   eq dotAccess().id, nonce
   eq protoAccess()::id, nonce
 
 test "#960: improved 'do'", ->
-  
+
   do (nonExistent = 'one') ->
     eq nonExistent, 'one'
-  
+
+  overridden = 1
+  do (overridden = 2) ->
+    eq overridden, 2
+
   two = 2
   do (one = 1, two, three = 3) ->
     eq one, 1
     eq two, 2
     eq three, 3
-    
+
   ret = do func = (two) ->
     eq two, 2
     func
   eq ret, func
-