Split fallback function and introduce "fallback()" and "receive()" syntax.

test/libsolidity/syntaxTests/fallback/arguments.sol

@@ -0,0 +1,5 @@
+contract C {
+    fallback(uint256) external {}
+}
+// ----
+// TypeError: (25-34): Fallback function cannot take parameters.

test/libsolidity/syntaxTests/fallback/default_visibility.sol

@@ -1,6 +1,6 @@
 contract C {
     // Check that visibility is also enforced for the fallback function.
-    function() {}
+    fallback() {}
 }
 // ----
 // SyntaxError: (90-103): No visibility specified. Did you intend to add "external"?

test/libsolidity/syntaxTests/fallback/fallback_as_function_name.sol

@@ -0,0 +1,5 @@
+contract C {
+    function fallback() external pure {}
+}
+// ----
+// Warning: (26-34): This function is named "fallback" but is not the fallback function of the contract. If you intend this to be a fallback function, use "fallback(...) { ... }" without the "function" keyword to define it.

test/libsolidity/syntaxTests/fallback/old_syntax.sol

@@ -0,0 +1,5 @@
+contract C {
+    function() external {}
+}
+// ----
+// ParserError: (37-38): Expected a state variable declaration. If you intended this as a fallback function or a function to handle plain ether transactions, use the "fallback" keyword or the "ether" keyword instead.

test/libsolidity/syntaxTests/fallback/payable_fallback_with_inherited_receive.sol

@@ -0,0 +1,6 @@
+contract A {
+    receive() external payable { }
+}
+contract C is A {
+    fallback() external payable { }
+}

test/libsolidity/syntaxTests/fallback/payable_fallback_without_receive_empty.sol

@@ -0,0 +1,3 @@
+contract C {
+    fallback() external payable { }
+}

test/libsolidity/syntaxTests/fallback/payable_fallback_without_receive_nonempty.sol

@@ -0,0 +1,6 @@
+contract C {
+    fallback() external payable { }
+    function f() public pure { }
+}
+// ----
+// Warning: (0-83): This contract has a payable fallback function, but no receive ether function. Consider adding a receive ether function.

test/libsolidity/syntaxTests/fallback/payable_fallback_without_receive_nonempty_by_inheritance.sol

@@ -0,0 +1,8 @@
+contract A {
+    function f() external pure {}
+}
+contract C is A {
+    fallback() external payable { }
+}
+// ----
+// Warning: (49-104): This contract has a payable fallback function, but no receive ether function. Consider adding a receive ether function.

test/libsolidity/syntaxTests/fallback/payable_fallback_without_receive_only_internal.sol

@@ -0,0 +1,4 @@
+contract C {
+    fallback() external payable { }
+    function f() internal pure { }
+}

test/libsolidity/syntaxTests/fallback/pure_modifier.sol

@@ -1,6 +1,6 @@
 contract C {
     uint x;
-    function() external pure { x = 2; }
+    fallback() external pure { x = 2; }
 }
 // ----
 // TypeError: (29-64): Fallback function must be payable or non-payable, but is "pure".

test/libsolidity/syntaxTests/fallback/return_value_number.sol

@@ -0,0 +1,5 @@
+contract C {
+    fallback() external returns (bytes memory, bytes memory) {}
+}
+// ----
+// TypeError: (45-73): Fallback function can only have a single "bytes memory" return value.

test/libsolidity/syntaxTests/fallback/return_value_type.sol

@@ -0,0 +1,5 @@
+contract C {
+    fallback() external returns (uint256) {}
+}
+// ----
+// TypeError: (45-54): Fallback function can only have a single "bytes memory" return value.

test/libsolidity/syntaxTests/fallback/return_value_unsupported.sol

@@ -0,0 +1,5 @@
+contract C {
+    fallback() external returns (bytes memory) {}
+}
+// ----
+// TypeError: (45-59): Return values for fallback functions are not yet implemented.

test/libsolidity/syntaxTests/fallback/view_modifier.sol

@@ -1,6 +1,6 @@
 contract C {
     uint x;
-    function() external view { x = 2; }
+    fallback() external view { x = 2; }
 }
 // ----
 // TypeError: (29-64): Fallback function must be payable or non-payable, but is "view".