Allow fallback function to return data.

test/libsolidity/semanticTests/fallback/falback_return.sol

@@ -0,0 +1,18 @@
+contract A {
+    uint public x;
+    fallback () external {
+        if (x == 2) return;
+        x++;
+    }
+}
+// ====
+// compileViaYul: also
+// ----
+// ()
+// x() -> 1
+// ()
+// x() -> 2
+// ()
+// x() -> 2
+// ()
+// x() -> 2

test/libsolidity/semanticTests/fallback/fallback_argument.sol

@@ -0,0 +1,17 @@
+contract A {
+    uint public x;
+    fallback (bytes calldata _input) external returns (bytes memory) {
+        x = _input.length;
+        return "";
+    }
+    function f() public returns (bool, bytes memory) {
+        (bool success, bytes memory retval) = address(this).call("abc");
+        return (success, retval);
+    }
+}
+// ====
+// compileViaYul: also
+// EVMVersion: >=byzantium
+// ----
+// f() -> 0x01, 0x40, 0x00
+// x() -> 3

test/libsolidity/semanticTests/fallback/fallback_argument_to_storage.sol

@@ -0,0 +1,16 @@
+contract A {
+    bytes public x;
+    fallback (bytes calldata _input) external returns (bytes memory) {
+        x = _input;
+        return "";
+    }
+    function f() public returns (bool, bytes memory) {
+        (bool success, bytes memory retval) = address(this).call("abc");
+        return (success, retval);
+    }
+}
+// ====
+// EVMVersion: >=byzantium
+// ----
+// f() -> 0x01, 0x40, 0x00
+// x() -> 0x20, 3, "abc"

test/libsolidity/semanticTests/fallback/fallback_return_data.sol

@@ -0,0 +1,14 @@
+contract A {
+    fallback (bytes calldata _input) external returns (bytes memory) {
+        return _input;
+    }
+    function f() public returns (bool, bytes memory) {
+        (bool success, bytes memory retval) = address(this).call("abc");
+        return (success, retval);
+    }
+}
+// ====
+// compileViaYul: also
+// EVMVersion: >=byzantium
+// ----
+// f() -> 0x01, 0x40, 0x03, 0x6162630000000000000000000000000000000000000000000000000000000000

test/libsolidity/syntaxTests/fallback/arguments.sol

@@ -2,4 +2,4 @@ contract C {
     fallback(uint256) external {}
 }
 // ----
-// TypeError 3978: (25-34): Fallback function cannot take parameters.
+// TypeError 5570: (44-44): Fallback function either has to have the signature "fallback()" or "fallback(bytes calldata) returns (bytes memory)".

test/libsolidity/syntaxTests/fallback/no_input_no_output.sol

@@ -0,0 +1,9 @@
+contract C {
+    fallback() external returns (bytes memory _output) {}
+}
+contract D {
+    fallback(bytes calldata _input) external {}
+}
+// ----
+// TypeError 5570: (45-67): Fallback function either has to have the signature "fallback()" or "fallback(bytes calldata) returns (bytes memory)".
+// TypeError 5570: (131-131): Fallback function either has to have the signature "fallback()" or "fallback(bytes calldata) returns (bytes memory)".

test/libsolidity/syntaxTests/fallback/return_value_number.sol

@@ -2,4 +2,4 @@ contract C {
     fallback() external returns (bytes memory, bytes memory) {}
 }
 // ----
-// TypeError 5570: (45-73): Fallback function can only have a single "bytes memory" return value.
+// TypeError 5570: (45-73): Fallback function either has to have the signature "fallback()" or "fallback(bytes calldata) returns (bytes memory)".

test/libsolidity/syntaxTests/fallback/return_value_type.sol

@@ -2,4 +2,4 @@ contract C {
     fallback() external returns (uint256) {}
 }
 // ----
-// TypeError 5570: (45-54): Fallback function can only have a single "bytes memory" return value.
+// TypeError 5570: (45-54): Fallback function either has to have the signature "fallback()" or "fallback(bytes calldata) returns (bytes memory)".

test/libsolidity/syntaxTests/fallback/return_value_unsupported.sol

@@ -2,4 +2,4 @@ contract C {
     fallback() external returns (bytes memory) {}
 }
 // ----
-// TypeError 6151: (45-59): Return values for fallback functions are not yet implemented.
+// TypeError 5570: (45-59): Fallback function either has to have the signature "fallback()" or "fallback(bytes calldata) returns (bytes memory)".

test/libsolidity/syntaxTests/fallback/returns.sol

@@ -0,0 +1,4 @@
+contract C {
+    fallback(bytes calldata _input) external returns (bytes memory _output) {}
+}
+// ----

test/libsolidity/syntaxTests/nameAndTypeResolution/075_fallback_function_with_arguments.sol

@@ -3,4 +3,4 @@ contract C {
     fallback(uint a) external { x = 2; }
 }
 // ----
-// TypeError 3978: (37-45): Fallback function cannot take parameters.
+// TypeError 5570: (55-55): Fallback function either has to have the signature "fallback()" or "fallback(bytes calldata) returns (bytes memory)".

test/libsolidity/syntaxTests/nameAndTypeResolution/077_fallback_function_with_return_parameters.sol

@@ -2,4 +2,4 @@ contract C {
     fallback() external returns (uint) { }
 }
 // ----
-// TypeError 5570: (45-51): Fallback function can only have a single "bytes memory" return value.
+// TypeError 5570: (45-51): Fallback function either has to have the signature "fallback()" or "fallback(bytes calldata) returns (bytes memory)".