Fallback function has to be external: backwards-compatible changes.

docs/contracts.rst

@@ -543,7 +543,7 @@ Fallback Function
 =================
 
 A contract can have exactly one unnamed function. This function cannot have
-arguments and cannot return anything.
+arguments, cannot return anything and has to have ``external`` visibility.
 It is executed on a call to the contract if none of the other
 functions match the given function identifier (or if no data was supplied at
 all).
@@ -591,7 +591,7 @@ Like any function, the fallback function can execute complex operations as long
         // Sending Ether to this contract will cause an exception,
         // because the fallback function does not have the `payable`
         // modifier.
-        function() public { x = 1; }
+        function() external { x = 1; }
         uint x;
     }
 
@@ -599,7 +599,7 @@ Like any function, the fallback function can execute complex operations as long
     // This contract keeps all Ether sent to it with no way
     // to get it back.
     contract Sink {
-        function() public payable { }
+        function() external payable { }
     }
 
     contract Caller {

docs/security-considerations.rst

@@ -213,7 +213,7 @@ Now someone tricks you into sending ether to the address of this attack wallet:
             owner = msg.sender;
         }
 
-        function() public {
+        function() external {
             TxUserWallet(msg.sender).transferTo(owner, msg.sender.balance);
         }
     }

docs/style-guide.rst

@@ -278,7 +278,7 @@ Yes::
             ...
         }
 
-        function() public {
+        function() external {
             ...
         }
 
@@ -308,6 +308,10 @@ No::
         // External functions
         // ...
 
+        function() external {
+            ...
+        }
+
         // Private functions
         // ...
 
@@ -318,10 +322,6 @@ No::
             ...
         }
 
-        function() public {
-            ...
-        }
-
         // Internal functions
         // ...
     }
@@ -374,13 +374,13 @@ Don't include a whitespace in the fallback function:
 
 Yes::
 
-    function() public {
+    function() external {
         ...
     }
 
 No::
 
-    function () public {
+    function () external {
         ...
     }