Override semantics for fallback function.

test/libsolidity/syntaxTests/fallback/fallback_duplicate_returns.sol

@@ -0,0 +1,6 @@
+contract C {
+    fallback(bytes calldata _input) external returns (bytes memory _output) {}
+    fallback() external {}
+}
+// ----
+// DeclarationError 7301: (96-118): Only one fallback function is allowed.

test/libsolidity/syntaxTests/fallback/fallback_duplicate_returns_inheritance.sol

@@ -0,0 +1,9 @@
+contract C {
+    fallback(bytes calldata _input) external returns (bytes memory _output) {}
+}
+contract D is C {
+    fallback() external {}
+}
+// ----
+// TypeError 9456: (116-138): Overriding function is missing "override" specifier.
+// TypeError 4334: (17-91): Trying to override non-virtual function. Did you forget to add "virtual"?

test/libsolidity/syntaxTests/fallback/fallback_duplicate_returns_override.sol

@@ -0,0 +1,7 @@
+contract C {
+    fallback(bytes calldata _input) external virtual returns (bytes memory _output) {}
+}
+contract D is C {
+    fallback() external override {}
+}
+// ----