Allow overrides to have a more strict mutability than super

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

@@ -1,5 +1,3 @@
 contract B { function f() virtual public {} }
-contract C is B { function f() public view {} }
+contract C is B { function f() override public view {} }
 // ----
-// TypeError 9456: (64-91): Overriding function is missing "override" specifier.
-// TypeError 6959: (64-91): Overriding function changes state mutability from "nonpayable" to "view".

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

@@ -0,0 +1,24 @@
+contract A {
+	function foo() external pure virtual returns (uint256) {}
+}
+contract B is A {
+	function foo() external pure override virtual returns (uint256) {}
+}
+contract C is A {
+	function foo() external view override virtual returns (uint256) {}
+}
+contract D is B, C {
+	function foo() external override(B, C) virtual returns (uint256) {}
+}
+contract E is C, B {
+	function foo() external pure override(B, C) virtual returns (uint256) {}
+}
+contract F is C, B {
+	function foo() external payable override(B, C) virtual returns (uint256) {}
+}
+// ----
+// TypeError 6959: (181-247): Overriding function changes state mutability from "pure" to "view".
+// TypeError 6959: (272-339): Overriding function changes state mutability from "pure" to "nonpayable".
+// TypeError 6959: (272-339): Overriding function changes state mutability from "view" to "nonpayable".
+// TypeError 6959: (461-536): Overriding function changes state mutability from "view" to "payable".
+// TypeError 6959: (461-536): Overriding function changes state mutability from "pure" to "payable".

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

@@ -0,0 +1,16 @@
+contract A {
+	function foo() internal view virtual returns (uint256) {}
+}
+contract B is A {
+	function foo() internal pure override virtual returns (uint256) {}
+}
+contract C is A {
+	function foo() internal view override virtual returns (uint256) {}
+}
+contract D is B, C {
+	function foo() internal pure override(B, C) virtual returns (uint256) {}
+}
+contract E is C, B {
+	function foo() internal pure override(B, C) virtual returns (uint256) {}
+}
+// ----

test/libsolidity/syntaxTests/inheritance/override/override_stricter_mutability1.sol

@@ -0,0 +1,8 @@
+contract A {
+	function foo() public payable virtual returns (uint256) {}
+}
+contract B is A {
+	function foo() public override virtual returns (uint256) {}
+}
+// ----
+// TypeError 6959: (94-153): Overriding function changes state mutability from "payable" to "nonpayable".

test/libsolidity/syntaxTests/inheritance/override/override_stricter_mutability2.sol

@@ -0,0 +1,7 @@
+contract A {
+	function foo() internal virtual returns (uint256) {}
+}
+contract B is A {
+	function foo() internal view override virtual returns (uint256) {}
+}
+// ----

test/libsolidity/syntaxTests/inheritance/override/override_stricter_mutability3.sol

@@ -0,0 +1,7 @@
+contract A {
+	function foo() internal view virtual returns (uint256) {}
+}
+contract B is A {
+	function foo() internal pure override virtual returns (uint256) {}
+}
+// ----

test/libsolidity/syntaxTests/inheritance/override/override_stricter_mutability4.sol

@@ -0,0 +1,8 @@
+contract A {
+	function foo() public payable virtual returns (uint256) {}
+}
+contract B is A {
+	function foo() public view override virtual returns (uint256) {}
+}
+// ----
+// TypeError 6959: (94-158): Overriding function changes state mutability from "payable" to "view".

test/libsolidity/syntaxTests/inheritance/override/override_stricter_mutability5.sol

@@ -0,0 +1,8 @@
+contract A {
+	function foo() public payable virtual returns (uint256) {}
+}
+contract B is A {
+	function foo() public pure override virtual returns (uint256) {}
+}
+// ----
+// TypeError 6959: (94-158): Overriding function changes state mutability from "payable" to "pure".

test/libsolidity/syntaxTests/inheritance/override/override_stricter_mutability6.sol

@@ -0,0 +1,8 @@
+contract A {
+	function foo() public virtual returns (uint256) {}
+}
+contract B is A {
+	function foo() public payable override virtual returns (uint256) {}
+}
+// ----
+// TypeError 6959: (86-153): Overriding function changes state mutability from "nonpayable" to "payable".

test/libsolidity/syntaxTests/inheritance/override/override_stricter_mutability7.sol

@@ -0,0 +1,8 @@
+contract A {
+	function foo() public view virtual returns (uint256) {}
+}
+contract B is A {
+	function foo() public payable override virtual returns (uint256) {}
+}
+// ----
+// TypeError 6959: (91-158): Overriding function changes state mutability from "view" to "payable".