Do not require overriding for functions in common base with unique implementation.

test/libsolidity/syntaxTests/inheritance/override/ambiguous_base_and_unique_implementation.sol

@@ -0,0 +1,20 @@
+interface I {
+    function f() external;
+    function g() external;
+}
+interface J {
+	function f() external;
+}
+abstract contract A is I, J {
+    function f() external override (I, J) {}
+    function g() external override virtual;
+}
+abstract contract B is I {
+    function f() external override virtual;
+    function g() external override {}
+}
+contract C is A, B {
+}
+// ----
+// TypeError: (342-364): Derived contract must override function "f". Function with the same name and parameter types defined in two or more base classes.
+// TypeError: (342-364): Derived contract must override function "g". Function with the same name and parameter types defined in two or more base classes.

test/libsolidity/syntaxTests/inheritance/override/ambiguous_base_and_unique_mention.sol

@@ -0,0 +1,17 @@
+interface I {
+    function f() external;
+    function g() external;
+}
+interface J {
+	function f() external;
+}
+abstract contract A is I, J {
+    function f() external override (I, J) {}
+}
+abstract contract B is I {
+    function g() external override {}
+}
+contract C is A, B {
+}
+// ----
+// TypeError: (254-276): Derived contract must override function "f". Function with the same name and parameter types defined in two or more base classes.

test/libsolidity/syntaxTests/inheritance/override/ambiguous_base_functions_overridden_in_intermediate_base.sol

@@ -0,0 +1,12 @@
+contract A {
+	function f() external virtual {}
+}
+contract B {
+	function f() external virtual {}
+}
+contract C is A, B {
+	function f() external override (A, B) {}
+}
+contract X is C {
+}
+// ----

test/libsolidity/syntaxTests/inheritance/override/common_base_and_unique_implementation.sol

@@ -0,0 +1,17 @@
+interface I {
+    function f() external;
+    function g() external;
+}
+abstract contract A is I {
+    function f() external override {}
+    function g() external override virtual;
+}
+abstract contract B is I {
+    function g() external override {}
+    function f() external override virtual;
+}
+contract C is A, B {
+}
+// ----
+// TypeError: (292-314): Derived contract must override function "f". Function with the same name and parameter types defined in two or more base classes.
+// TypeError: (292-314): Derived contract must override function "g". Function with the same name and parameter types defined in two or more base classes.

test/libsolidity/syntaxTests/inheritance/override/common_base_and_unique_mention.sol

@@ -0,0 +1,12 @@
+interface I {
+    function f() external;
+    function g() external;
+}
+abstract contract A is I {
+    function f() external override {}
+}
+abstract contract B is I {
+    function g() external override {}
+}
+contract C is A, B {
+}

test/libsolidity/syntaxTests/inheritance/override/no_common_base_and_unique_implementation.sol

@@ -0,0 +1,13 @@
+abstract contract A {
+    function f() external {}
+    function g() external virtual;
+}
+abstract contract B {
+    function g() external {}
+    function f() external virtual;
+}
+contract C is A, B {
+}
+// ----
+// TypeError: (176-198): Derived contract must override function "f". Function with the same name and parameter types defined in two or more base classes.
+// TypeError: (176-198): Derived contract must override function "g". Function with the same name and parameter types defined in two or more base classes.

test/libsolidity/syntaxTests/inheritance/override/nonintermediate_common_base_and_unique_implementation.sol

@@ -0,0 +1,19 @@
+interface I {
+	function f() external;
+	function g() external;
+}
+interface J {
+	function f() external;
+}
+abstract contract IJ is I, J {
+	function f() external virtual override (I, J);
+}
+abstract contract A is IJ
+{
+	function f() external override {}
+}
+abstract contract B is IJ
+{
+	function g() external override {}
+}
+contract C is A, B {}

test/libsolidity/syntaxTests/inheritance/override/triangle_impl.sol

@@ -0,0 +1,6 @@
+contract A { function f() public pure virtual {} }
+contract B is A { function f() public pure virtual override {} }
+contract C is A, B { }
+contract D is A, B { function f() public pure override(A, B) {} }
+// ----
+// TypeError: (116-138): Derived contract must override function "f". Function with the same name and parameter types defined in two or more base classes.

test/libsolidity/syntaxTests/inheritance/override/triangle_no_impl.sol

@@ -0,0 +1,5 @@
+abstract contract A { function f() public pure virtual; }
+contract B is A { function f() public pure virtual override {} }
+contract C is A, B { }
+contract D is A, B { function f() public pure override(A, B) {} }
+// ----