Some more tests.

test/libsolidity/semanticTests/fallback/fallback_override.sol

@@ -0,0 +1,19 @@
+contract A {
+    fallback (bytes calldata _input) virtual external returns (bytes memory) {
+        return _input;
+    }
+}
+contract B is A {
+    fallback (bytes calldata _input) override external returns (bytes memory) {
+        return "xyz";
+    }
+    function f() public returns (bool, bytes memory) {
+        (bool success, bytes memory retval) = address(this).call("abc");
+        return (success, retval);
+    }
+}
+// ====
+// EVMVersion: >=byzantium
+// compileViaYul: also
+// ----
+// f() -> 0x01, 0x40, 0x03, 0x78797a0000000000000000000000000000000000000000000000000000000000

test/libsolidity/semanticTests/fallback/fallback_override2.sol

@@ -0,0 +1,18 @@
+contract A {
+    fallback (bytes calldata _input) virtual external returns (bytes memory) {
+        return _input;
+    }
+}
+contract B is A {
+    fallback () override external {
+    }
+    function f() public returns (bool, bytes memory) {
+        (bool success, bytes memory retval) = address(this).call("abc");
+        return (success, retval);
+    }
+}
+// ====
+// EVMVersion: >=byzantium
+// compileViaYul: also
+// ----
+// f() -> 1, 0x40, 0x00

test/libsolidity/semanticTests/fallback/fallback_override_multi.sol

@@ -0,0 +1,22 @@
+contract A {
+    fallback (bytes calldata _input) virtual external returns (bytes memory) {
+        return _input;
+    }
+}
+contract B {
+    fallback (bytes calldata _input) virtual external returns (bytes memory) {
+        return "xyz";
+    }
+}
+contract C is B, A {
+    fallback () external override (B, A) {}
+    function f() public returns (bool, bytes memory) {
+        (bool success, bytes memory retval) = address(this).call("abc");
+        return (success, retval);
+    }
+}
+// ====
+// EVMVersion: >=byzantium
+// compileViaYul: also
+// ----
+// f() -> 0x01, 0x40, 0x00

test/libsolidity/syntaxTests/fallback/fallback_wrong_data_location.sol

@@ -0,0 +1,13 @@
+contract D {
+    fallback(bytes memory) external returns (bytes memory) {}
+}
+contract E {
+    fallback(bytes memory) external returns (bytes calldata) {}
+}
+contract F {
+    fallback(bytes calldata) external returns (bytes calldata) {}
+}
+// ----
+// TypeError 5570: (57-71): Fallback function either has to have the signature "fallback()" or "fallback(bytes calldata) returns (bytes memory)".
+// TypeError 5570: (134-150): Fallback function either has to have the signature "fallback()" or "fallback(bytes calldata) returns (bytes memory)".
+// TypeError 5570: (215-231): Fallback function either has to have the signature "fallback()" or "fallback(bytes calldata) returns (bytes memory)".

test/libsolidity/syntaxTests/fallback/inheritance_multi_base.sol

@@ -0,0 +1,20 @@
+contract A {
+    fallback (bytes calldata _input) external returns (bytes memory) {
+        return _input;
+    }
+}
+contract B {
+    fallback (bytes calldata _input) external returns (bytes memory) {
+        return "xyz";
+    }
+}
+contract C is B, A {
+     function f() public returns (bool, bytes memory) {
+        (bool success, bytes memory retval) = address(this).call("abc");
+        return (success, retval);
+    }
+}
+// ====
+// EVMVersion: >=byzantium
+// ----
+// TypeError 6480: (229-420): Derived contract must override function "". Two or more base classes define function with same name and parameter types.