Require unimplemented functions to be virtual.

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

@@ -1,4 +1,4 @@
-abstract contract X { function test() internal returns (uint256); }
+abstract contract X { function test() internal virtual returns (uint256); }
 contract Y is X {
     uint256 public test = 42;
 }
@@ -6,4 +6,4 @@ contract T {
     constructor() public { new Y(); }
 }
 // ----
-// DeclarationError: (90-114): Identifier already declared.
+// DeclarationError: (98-122): Identifier already declared.

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

@@ -1,4 +1,4 @@
-abstract contract X { function test() private returns (uint256); }
+abstract contract X { function test() private returns (uint256) {} }
 contract Y is X {
     uint256 public test = 42;
 }

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

@@ -1,4 +1,4 @@
-abstract contract X { function test() public returns (uint256); }
+abstract contract X { function test() public virtual returns (uint256); }
 contract Y is X {
     uint256 public test = 42;
 }
@@ -6,4 +6,4 @@ contract T {
     constructor() public { new Y(); }
 }
 // ----
-// DeclarationError: (88-112): Identifier already declared.
+// DeclarationError: (96-120): Identifier already declared.

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

@@ -3,9 +3,9 @@ abstract contract A {
 	function test() internal virtual returns (uint256);
 	function test2() internal virtual returns (uint256);
 }
-abstract contract X is A {
+contract X is A {
 	int public override testvar;
-	function test() internal override returns (uint256);
-	function test2() internal override(A) returns (uint256);
+	function test() internal override returns (uint256) {}
+	function test2() internal override(A) returns (uint256) {}
 }
 // ----

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

@@ -3,12 +3,12 @@ abstract contract A {
 	function foo() internal virtual returns (uint256);
 }
 abstract contract B {
-	function foo() internal returns (uint256);
+	function foo() internal virtual returns (uint256);
 	function test() internal virtual returns (uint256);
 }
 abstract contract X is A, B {
 	int public override testvar;
-	function test() internal override returns (uint256);
+	function test() internal override returns (uint256) {}
 }
 // ----
-// TypeError: (218-333): Derived contract must override function "foo". Function with the same name and parameter types defined in two or more base classes.
+// TypeError: (226-343): Derived contract must override function "foo". Function with the same name and parameter types defined in two or more base classes.

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

@@ -14,7 +14,7 @@ abstract contract D is C {
 	function foo() internal override virtual returns (uint256);
 }
 
-abstract contract X is D {
-	function foo() internal override returns (uint256);
+contract X is D {
+	function foo() internal override returns (uint256) {}
 }
 // ----

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

@@ -2,8 +2,8 @@ interface A {
 	function test() external returns (uint256);
 	function test2() external returns (uint256);
 }
-abstract contract X is A {
-	function test() external override returns (uint256);
-	function test2() external override(A) returns (uint256);
+contract X is A {
+	function test() external override returns (uint256) {}
+	function test2() external override(A) returns (uint256) {}
 }
 // ----

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

@@ -7,8 +7,8 @@ interface B {
 	function test() external returns (uint256);
 	function test2() external returns (uint256);
 }
-abstract contract X is A, B {
-	function test() external override(A, B) returns (uint256);
-	function test2() external override(B, A) returns (uint256);
+contract X is A, B {
+	function test() external override(A, B) returns (uint256) {}
+	function test2() external override(B, A) returns (uint256) {}
 }
 // ----

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

@@ -1,11 +1,11 @@
 abstract contract A {
 	function test() external virtual returns (uint256);
-	function test2() external returns (uint256);
+	function test2() external returns (uint256) {}
 }
 abstract contract X is A {
-	function test() external returns (uint256);
-	function test2() external override(A) returns (uint256);
+	function test() external returns (uint256) {}
+	function test2() external override(A) returns (uint256) {}
 }
 // ----
-// TypeError: (151-194): Overriding function is missing 'override' specifier.
-// TypeError: (76-120): Trying to override non-virtual function. Did you forget to add "virtual"?
+// TypeError: (153-198): Overriding function is missing 'override' specifier.
+// TypeError: (76-122): Trying to override non-virtual function. Did you forget to add "virtual"?

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

@@ -1,7 +1,7 @@
 abstract contract A {
 	int public testvar;
 	function foo() internal virtual returns (uint256);
-	function test(uint8 _a) internal virtual returns (uint256);
+	function test(uint8 _a) internal virtual returns (uint256) {}
 }
 abstract contract B {
 	function foo() internal virtual returns (uint256);
@@ -13,9 +13,9 @@ abstract contract C {
 abstract contract D {
 	function foo() internal virtual returns (uint256);
 }
-abstract contract X is A, B, C, D {
+contract X is A, B, C, D {
 	int public override testvar;
-	function test() internal override returns (uint256);
-	function foo() internal override(A, B, C, D) returns (uint256);
+	function test() internal override returns (uint256) {}
+	function foo() internal override(A, B, C, D) returns (uint256) {}
 }
 // ----

test/libsolidity/syntaxTests/inheritance/override/override_multiple2.sol

@@ -1,7 +1,7 @@
 abstract contract A {
 	int public testvar;
 	function foo() internal virtual returns (uint256);
-	function test(uint8 _a) internal returns (uint256);
+	function test(uint8 _a) internal virtual returns (uint256);
 }
 abstract contract B {
 	function foo() internal virtual returns (uint256);
@@ -10,6 +10,6 @@ abstract contract B {
 abstract contract C is A {
 }
 abstract contract D is A, B, C {
-	function foo() internal override(A, B) returns (uint256);
+	function foo() internal override(A, B) virtual returns (uint256);
 }
 // ----

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

@@ -16,11 +16,11 @@ abstract contract D {
 }
 abstract contract X is A, B, C, D {
 	int public override testvar;
-	function test() internal override(B, D, D) returns (uint256);
-	function foo() internal override(A, C, B, B, B, D ,D) returns (uint256);
+	function test() internal override(B, D, D) virtual returns (uint256);
+	function foo() internal override(A, C, B, B, B, D ,D) virtual returns (uint256);
 }
 // ----
 // TypeError: (599-600): Duplicate contract "D" found in override list of "test".
-// TypeError: (664-665): Duplicate contract "B" found in override list of "foo".
-// TypeError: (667-668): Duplicate contract "B" found in override list of "foo".
-// TypeError: (673-674): Duplicate contract "D" found in override list of "foo".
+// TypeError: (672-673): Duplicate contract "B" found in override list of "foo".
+// TypeError: (675-676): Duplicate contract "B" found in override list of "foo".
+// TypeError: (681-682): Duplicate contract "D" found in override list of "foo".

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

@@ -16,9 +16,9 @@ abstract contract D {
 }
 abstract contract X is A, B, C, D {
 	int public override testvar;
-	function test() internal override(B, D, C) returns (uint256);
-	function foo() internal override(A, C) returns (uint256);
+	function test() internal override(B, D, C) virtual returns (uint256);
+	function foo() internal override(A, C) virtual returns (uint256);
 }
 // ----
 // TypeError: (584-601): Invalid contract specified in override list: C.
-// TypeError: (646-660): Function needs to specify overridden contracts B and D.
+// TypeError: (654-668): Function needs to specify overridden contracts B and D.

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

@@ -8,7 +8,7 @@ abstract contract B {
 }
 abstract contract X is A, B {
 	int public override testvar;
-	function test() internal override returns (uint256);
+	function test() internal override virtual returns (uint256);
 }
 // ----
-// TypeError: (224-339): Derived contract must override function "foo". Function with the same name and parameter types defined in two or more base classes.
+// TypeError: (224-347): Derived contract must override function "foo". Function with the same name and parameter types defined in two or more base classes.

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

@@ -18,9 +18,9 @@ abstract contract X is A, B, C, D {
 	enum ENUM { F,G,H }
 
 	int public override testvar;
-	function test() internal override returns (uint256);
-	function foo() internal override(MyStruct, ENUM, A, B, C, D) returns (uint256);
+	function test() internal override virtual returns (uint256);
+	function foo() internal override(MyStruct, ENUM, A, B, C, D) virtual returns (uint256);
 }
 // ----
-// TypeError: (645-653): Expected contract but got struct X.MyStruct.
-// TypeError: (655-659): Expected contract but got enum X.ENUM.
+// TypeError: (653-661): Expected contract but got struct X.MyStruct.
+// TypeError: (663-667): Expected contract but got enum X.ENUM.

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

@@ -2,8 +2,7 @@ contract A {
 	uint public x;
 }
 contract C is A {
-	function x() public returns (uint);
+	function x() public returns (uint) {}
 }
 // ----
-// DeclarationError: (50-85): Identifier already declared.
-// TypeError: (50-85): Redeclaring an already implemented function as abstract
+// DeclarationError: (50-87): Identifier already declared.

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

@@ -2,7 +2,7 @@ abstract contract A {
 	function test() private virtual returns (uint256);
 }
 abstract contract X is A {
-	function test() private override returns (uint256);
+	function test() private override returns (uint256) {}
 }
 // ----
 // TypeError: (23-73): "virtual" and "private" cannot be used together.

test/libsolidity/syntaxTests/inheritance/unimplemented_without_virtual.sol

@@ -0,0 +1,5 @@
+abstract contract C {
+	function f() external;
+}
+// ----
+// TypeError: (23-45): Functions without implementation must be marked virtual.