From 05baa23e8a9e297584470200365a7d9a79f9c0ab Mon Sep 17 00:00:00 2001
From: Daniel Kirchner <daniel@ekpyron.org>
Date: Mon, 2 Dec 2019 21:39:53 +0100
Subject: [PATCH 1/3] Require unimplemented functions to be virtual.

---
 libsolidity/analysis/TypeChecker.cpp          |  2 ++
 test/libsolidity/SolidityEndToEndTest.cpp     |  2 +-
 .../SolidityNameAndTypeResolution.cpp         |  8 +++---
 .../functions/functions_external_1.sol        |  4 +--
 .../functions/functions_external_2.sol        |  4 +--
 .../functions/functions_external_3.sol        |  4 +--
 .../abstract/unimplemented_functions.sol      |  4 +--
 .../unimplemented_functions_inherited.sol     |  4 +--
 .../storageReturn/unimplemented_internal.sol  |  4 +--
 ...ation_specifier_test_external_calldata.sol |  4 +--
 ...ocation_specifier_test_internal_memory.sol |  4 +--
 ...cation_specifier_test_internal_storage.sol |  4 +--
 ..._location_specifier_test_public_memory.sol |  4 +--
 ...t_internal_function_by_public_variable.sol |  4 +--
 ...nt_private_function_by_public_variable.sol |  2 +-
 ...ent_public_function_by_public_variable.sol |  4 +--
 .../inheritance/override/override.sol         |  6 ++---
 .../override/override_ambiguous.sol           |  6 ++---
 .../override/override_base_base.sol           |  4 +--
 .../override/override_interface.sol           |  6 ++---
 .../override/override_interface_multiple.sol  |  6 ++---
 .../override/override_missing_virtual.sol     | 10 +++----
 .../override/override_multiple.sol            |  8 +++---
 .../override/override_multiple2.sol           |  4 +--
 .../override/override_multiple_duplicated.sol | 10 +++----
 .../override/override_multiple_missing.sol    |  6 ++---
 .../override/override_return_mismatch.sol     |  4 +--
 .../override/override_type_mismatch.sol       |  8 +++---
 .../override/state_variable_function.sol      |  5 ++--
 .../inheritance/override/virtual_private.sol  |  2 +-
 .../unimplemented_without_virtual.sol         |  5 ++++
 ..._on_abstract_functions_no_parser_error.sol |  8 +++---
 .../differentNumberOfComponentsFromReturn.sol | 26 +++++++++----------
 .../disallowWildcardsFromReturn.sol           | 10 +++----
 .../029_create_abstract_contract.sol          |  4 +--
 ...lemented_abstract_function_as_abstract.sol |  5 ++--
 .../268_function_overload_array_type.sol      |  4 +--
 ...6_no_unused_warning_abstract_arguments.sol |  2 +-
 ...members_that_look_like_address_members.sol |  2 +-
 .../natspec/docstring_empty_description.sol   |  4 +--
 .../natspec/docstring_empty_tag.sol           |  2 +-
 .../syntaxTests/parsing/function_no_body.sol  |  2 +-
 .../function_parameter_return_types_fail.sol  | 16 ++++++------
 .../interface_contract_function_default.sol   |  4 +--
 44 files changed, 123 insertions(+), 118 deletions(-)
 create mode 100644 test/libsolidity/syntaxTests/inheritance/unimplemented_without_virtual.sol

diff --git a/libsolidity/analysis/TypeChecker.cpp b/libsolidity/analysis/TypeChecker.cpp
index d32da45b4..960458f24 100644
--- a/libsolidity/analysis/TypeChecker.cpp
+++ b/libsolidity/analysis/TypeChecker.cpp
@@ -433,6 +433,8 @@ bool TypeChecker::visit(FunctionDefinition const& _function)
 		m_errorReporter.typeError(_function.location(), "Constructor must be implemented if declared.");
 	else if (isLibraryFunction)
 		m_errorReporter.typeError(_function.location(), "Library functions must be implemented if declared.");
+	else if (!_function.virtualSemantics())
+		m_errorReporter.typeError(_function.location(), "Functions without implementation must be marked virtual.");
 
 
 	if (_function.isFallback())
diff --git a/test/libsolidity/SolidityEndToEndTest.cpp b/test/libsolidity/SolidityEndToEndTest.cpp
index ea72a946c..8275536a0 100644
--- a/test/libsolidity/SolidityEndToEndTest.cpp
+++ b/test/libsolidity/SolidityEndToEndTest.cpp
@@ -10464,7 +10464,7 @@ BOOST_AUTO_TEST_CASE(mutex)
 BOOST_AUTO_TEST_CASE(calling_nonexisting_contract_throws)
 {
 	char const* sourceCode = R"YY(
-		abstract contract D { function g() public; }
+		abstract contract D { function g() public virtual; }
 		contract C {
 			D d = D(0x1212);
 			function f() public returns (uint) {
diff --git a/test/libsolidity/SolidityNameAndTypeResolution.cpp b/test/libsolidity/SolidityNameAndTypeResolution.cpp
index f744d5b3c..d22d1880f 100644
--- a/test/libsolidity/SolidityNameAndTypeResolution.cpp
+++ b/test/libsolidity/SolidityNameAndTypeResolution.cpp
@@ -49,7 +49,7 @@ BOOST_AUTO_TEST_CASE(function_no_implementation)
 	SourceUnit const* sourceUnit = nullptr;
 	char const* text = R"(
 		abstract contract test {
-			function functionName(bytes32 input) public returns (bytes32 out);
+			function functionName(bytes32 input) public virtual returns (bytes32 out);
 		}
 	)";
 	sourceUnit = parseAndAnalyse(text);
@@ -83,7 +83,7 @@ BOOST_AUTO_TEST_CASE(abstract_contract_with_overload)
 {
 	SourceUnit const* sourceUnit = nullptr;
 	char const* text = R"(
-		abstract contract base { function foo(bool) public; }
+		abstract contract base { function foo(bool) public virtual; }
 		abstract contract derived is base { function foo(uint) public {} }
 	)";
 	sourceUnit = parseAndAnalyse(text);
@@ -100,7 +100,7 @@ BOOST_AUTO_TEST_CASE(implement_abstract_via_constructor)
 {
 	SourceUnit const* sourceUnit = nullptr;
 	char const* text = R"(
-		abstract contract base { function foo() public; }
+		abstract contract base { function foo() public virtual; }
 		abstract contract foo is base { constructor() public {} }
 	)";
 	sourceUnit = parseAndAnalyse(text);
@@ -357,7 +357,7 @@ BOOST_AUTO_TEST_CASE(dynamic_return_types_not_possible)
 {
 	char const* sourceCode = R"(
 		abstract contract C {
-			function f(uint) public returns (string memory);
+			function f(uint) public virtual returns (string memory);
 			function g() public {
 				string memory x = this.f(2);
 				// we can assign to x but it is not usable.
diff --git a/test/libsolidity/smtCheckerTests/functions/functions_external_1.sol b/test/libsolidity/smtCheckerTests/functions/functions_external_1.sol
index a0eccf3cf..4f1a64783 100644
--- a/test/libsolidity/smtCheckerTests/functions/functions_external_1.sol
+++ b/test/libsolidity/smtCheckerTests/functions/functions_external_1.sol
@@ -2,7 +2,7 @@ pragma experimental SMTChecker;
 
 abstract contract D
 {
-	function g(uint x) public;
+	function g(uint x) public virtual;
 }
 
 contract C
@@ -17,4 +17,4 @@ contract C
 	}
 }
 // ----
-// Warning: (249-263): Assertion violation happens here
+// Warning: (257-271): Assertion violation happens here
diff --git a/test/libsolidity/smtCheckerTests/functions/functions_external_2.sol b/test/libsolidity/smtCheckerTests/functions/functions_external_2.sol
index 60795ca7f..a0c59c0b7 100644
--- a/test/libsolidity/smtCheckerTests/functions/functions_external_2.sol
+++ b/test/libsolidity/smtCheckerTests/functions/functions_external_2.sol
@@ -2,7 +2,7 @@ pragma experimental SMTChecker;
 
 abstract contract D
 {
-	function g(uint x) public;
+	function g(uint x) public virtual;
 }
 
 contract C
@@ -17,4 +17,4 @@ contract C
 	}
 }
 // ----
-// Warning: (289-313): Assertion violation happens here
+// Warning: (297-321): Assertion violation happens here
diff --git a/test/libsolidity/smtCheckerTests/functions/functions_external_3.sol b/test/libsolidity/smtCheckerTests/functions/functions_external_3.sol
index 43745155d..0f4de76f6 100644
--- a/test/libsolidity/smtCheckerTests/functions/functions_external_3.sol
+++ b/test/libsolidity/smtCheckerTests/functions/functions_external_3.sol
@@ -2,7 +2,7 @@ pragma experimental SMTChecker;
 
 abstract contract D
 {
-	function g(uint x) public;
+	function g(uint x) public virtual;
 }
 
 contract C
@@ -18,4 +18,4 @@ contract C
 	}
 }
 // ----
-// Warning: (347-371): Assertion violation happens here
+// Warning: (355-379): Assertion violation happens here
diff --git a/test/libsolidity/syntaxTests/abstract/unimplemented_functions.sol b/test/libsolidity/syntaxTests/abstract/unimplemented_functions.sol
index 6ec4c2c8e..efad376d8 100644
--- a/test/libsolidity/syntaxTests/abstract/unimplemented_functions.sol
+++ b/test/libsolidity/syntaxTests/abstract/unimplemented_functions.sol
@@ -1,5 +1,5 @@
 contract A {
-    function a() public;
+    function a() public virtual;
 }
 // ----
-// TypeError: (0-39): Contract "A" should be marked as abstract.
+// TypeError: (0-47): Contract "A" should be marked as abstract.
diff --git a/test/libsolidity/syntaxTests/abstract/unimplemented_functions_inherited.sol b/test/libsolidity/syntaxTests/abstract/unimplemented_functions_inherited.sol
index cb4640c3a..fc7f6c829 100644
--- a/test/libsolidity/syntaxTests/abstract/unimplemented_functions_inherited.sol
+++ b/test/libsolidity/syntaxTests/abstract/unimplemented_functions_inherited.sol
@@ -1,7 +1,7 @@
 abstract contract A {
-    function a() public;
+    function a() public virtual;
 }
 contract B is A {
 }
 // ----
-// TypeError: (49-68): Contract "B" should be marked as abstract.
+// TypeError: (57-76): Contract "B" should be marked as abstract.
diff --git a/test/libsolidity/syntaxTests/controlFlow/storageReturn/unimplemented_internal.sol b/test/libsolidity/syntaxTests/controlFlow/storageReturn/unimplemented_internal.sol
index 70cd3fcad..3c8986a6b 100644
--- a/test/libsolidity/syntaxTests/controlFlow/storageReturn/unimplemented_internal.sol
+++ b/test/libsolidity/syntaxTests/controlFlow/storageReturn/unimplemented_internal.sol
@@ -1,5 +1,5 @@
 abstract contract C {
-    function f() internal returns(uint[] storage);
-    function g() internal returns(uint[] storage s);
+    function f() internal virtual returns(uint[] storage);
+    function g() internal virtual returns(uint[] storage s);
 }
 // ----
diff --git a/test/libsolidity/syntaxTests/dataLocations/externalFunction/function_argument_location_specifier_test_external_calldata.sol b/test/libsolidity/syntaxTests/dataLocations/externalFunction/function_argument_location_specifier_test_external_calldata.sol
index cf5839d38..b75fc05c7 100644
--- a/test/libsolidity/syntaxTests/dataLocations/externalFunction/function_argument_location_specifier_test_external_calldata.sol
+++ b/test/libsolidity/syntaxTests/dataLocations/externalFunction/function_argument_location_specifier_test_external_calldata.sol
@@ -1,4 +1,4 @@
-abstract contract test {
-    function f(bytes calldata) external;
+contract test {
+    function f(bytes calldata) external {}
 }
 // ----
diff --git a/test/libsolidity/syntaxTests/dataLocations/internalFunction/function_argument_location_specifier_test_internal_memory.sol b/test/libsolidity/syntaxTests/dataLocations/internalFunction/function_argument_location_specifier_test_internal_memory.sol
index 0f1b642cc..dde99d56c 100644
--- a/test/libsolidity/syntaxTests/dataLocations/internalFunction/function_argument_location_specifier_test_internal_memory.sol
+++ b/test/libsolidity/syntaxTests/dataLocations/internalFunction/function_argument_location_specifier_test_internal_memory.sol
@@ -1,4 +1,4 @@
-abstract contract test {
-    function f(bytes memory) internal;
+contract test {
+    function f(bytes memory) internal {}
 }
 // ----
diff --git a/test/libsolidity/syntaxTests/dataLocations/internalFunction/function_argument_location_specifier_test_internal_storage.sol b/test/libsolidity/syntaxTests/dataLocations/internalFunction/function_argument_location_specifier_test_internal_storage.sol
index fdc07a452..706d22bce 100644
--- a/test/libsolidity/syntaxTests/dataLocations/internalFunction/function_argument_location_specifier_test_internal_storage.sol
+++ b/test/libsolidity/syntaxTests/dataLocations/internalFunction/function_argument_location_specifier_test_internal_storage.sol
@@ -1,4 +1,4 @@
-abstract contract test {
-    function f(bytes storage) internal;
+contract test {
+    function f(bytes storage) internal {}
 }
 // ----
diff --git a/test/libsolidity/syntaxTests/dataLocations/publicFunction/function_argument_location_specifier_test_public_memory.sol b/test/libsolidity/syntaxTests/dataLocations/publicFunction/function_argument_location_specifier_test_public_memory.sol
index 3535d3d97..7cea3cb49 100644
--- a/test/libsolidity/syntaxTests/dataLocations/publicFunction/function_argument_location_specifier_test_public_memory.sol
+++ b/test/libsolidity/syntaxTests/dataLocations/publicFunction/function_argument_location_specifier_test_public_memory.sol
@@ -1,4 +1,4 @@
-abstract contract test {
-    function f(bytes memory) public;
+contract test {
+    function f(bytes memory) public {}
 }
 // ----
diff --git a/test/libsolidity/syntaxTests/inheritance/override/implement_internal_function_by_public_variable.sol b/test/libsolidity/syntaxTests/inheritance/override/implement_internal_function_by_public_variable.sol
index 15b1288b8..ff07bae09 100644
--- a/test/libsolidity/syntaxTests/inheritance/override/implement_internal_function_by_public_variable.sol
+++ b/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.
diff --git a/test/libsolidity/syntaxTests/inheritance/override/implement_private_function_by_public_variable.sol b/test/libsolidity/syntaxTests/inheritance/override/implement_private_function_by_public_variable.sol
index 979f84f23..6d467e07c 100644
--- a/test/libsolidity/syntaxTests/inheritance/override/implement_private_function_by_public_variable.sol
+++ b/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;
 }
diff --git a/test/libsolidity/syntaxTests/inheritance/override/implement_public_function_by_public_variable.sol b/test/libsolidity/syntaxTests/inheritance/override/implement_public_function_by_public_variable.sol
index f70f7608d..b68caffba 100644
--- a/test/libsolidity/syntaxTests/inheritance/override/implement_public_function_by_public_variable.sol
+++ b/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.
diff --git a/test/libsolidity/syntaxTests/inheritance/override/override.sol b/test/libsolidity/syntaxTests/inheritance/override/override.sol
index 66fef0da3..bf79c702d 100644
--- a/test/libsolidity/syntaxTests/inheritance/override/override.sol
+++ b/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) {}
 }
 // ----
diff --git a/test/libsolidity/syntaxTests/inheritance/override/override_ambiguous.sol b/test/libsolidity/syntaxTests/inheritance/override/override_ambiguous.sol
index d77aff87b..966c18f70 100644
--- a/test/libsolidity/syntaxTests/inheritance/override/override_ambiguous.sol
+++ b/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.
diff --git a/test/libsolidity/syntaxTests/inheritance/override/override_base_base.sol b/test/libsolidity/syntaxTests/inheritance/override/override_base_base.sol
index 1df19f9ac..a3f5ed2c7 100644
--- a/test/libsolidity/syntaxTests/inheritance/override/override_base_base.sol
+++ b/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) {}
 }
 // ----
diff --git a/test/libsolidity/syntaxTests/inheritance/override/override_interface.sol b/test/libsolidity/syntaxTests/inheritance/override/override_interface.sol
index 06a846666..8b70bdbb6 100644
--- a/test/libsolidity/syntaxTests/inheritance/override/override_interface.sol
+++ b/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) {}
 }
 // ----
diff --git a/test/libsolidity/syntaxTests/inheritance/override/override_interface_multiple.sol b/test/libsolidity/syntaxTests/inheritance/override/override_interface_multiple.sol
index ad4f31fd5..9b82be89f 100644
--- a/test/libsolidity/syntaxTests/inheritance/override/override_interface_multiple.sol
+++ b/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) {}
 }
 // ----
diff --git a/test/libsolidity/syntaxTests/inheritance/override/override_missing_virtual.sol b/test/libsolidity/syntaxTests/inheritance/override/override_missing_virtual.sol
index 563947f88..bf4b15d46 100644
--- a/test/libsolidity/syntaxTests/inheritance/override/override_missing_virtual.sol
+++ b/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"?
diff --git a/test/libsolidity/syntaxTests/inheritance/override/override_multiple.sol b/test/libsolidity/syntaxTests/inheritance/override/override_multiple.sol
index 2923a5d66..d8daaf406 100644
--- a/test/libsolidity/syntaxTests/inheritance/override/override_multiple.sol
+++ b/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) {}
 }
 // ----
diff --git a/test/libsolidity/syntaxTests/inheritance/override/override_multiple2.sol b/test/libsolidity/syntaxTests/inheritance/override/override_multiple2.sol
index 46c7e1051..6cec3c2e7 100644
--- a/test/libsolidity/syntaxTests/inheritance/override/override_multiple2.sol
+++ b/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);
 }
 // ----
diff --git a/test/libsolidity/syntaxTests/inheritance/override/override_multiple_duplicated.sol b/test/libsolidity/syntaxTests/inheritance/override/override_multiple_duplicated.sol
index 947eb707b..a2c05bbd0 100644
--- a/test/libsolidity/syntaxTests/inheritance/override/override_multiple_duplicated.sol
+++ b/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".
diff --git a/test/libsolidity/syntaxTests/inheritance/override/override_multiple_missing.sol b/test/libsolidity/syntaxTests/inheritance/override/override_multiple_missing.sol
index e989af6b0..e7b493dfd 100644
--- a/test/libsolidity/syntaxTests/inheritance/override/override_multiple_missing.sol
+++ b/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.
diff --git a/test/libsolidity/syntaxTests/inheritance/override/override_return_mismatch.sol b/test/libsolidity/syntaxTests/inheritance/override/override_return_mismatch.sol
index 82cf5d5bc..e4aac5871 100644
--- a/test/libsolidity/syntaxTests/inheritance/override/override_return_mismatch.sol
+++ b/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.
diff --git a/test/libsolidity/syntaxTests/inheritance/override/override_type_mismatch.sol b/test/libsolidity/syntaxTests/inheritance/override/override_type_mismatch.sol
index 7d448e829..ab9a4388b 100644
--- a/test/libsolidity/syntaxTests/inheritance/override/override_type_mismatch.sol
+++ b/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.
diff --git a/test/libsolidity/syntaxTests/inheritance/override/state_variable_function.sol b/test/libsolidity/syntaxTests/inheritance/override/state_variable_function.sol
index fb7f3fbdd..bb4b28be8 100644
--- a/test/libsolidity/syntaxTests/inheritance/override/state_variable_function.sol
+++ b/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.
diff --git a/test/libsolidity/syntaxTests/inheritance/override/virtual_private.sol b/test/libsolidity/syntaxTests/inheritance/override/virtual_private.sol
index 95690e7db..1d3b24478 100644
--- a/test/libsolidity/syntaxTests/inheritance/override/virtual_private.sol
+++ b/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.
diff --git a/test/libsolidity/syntaxTests/inheritance/unimplemented_without_virtual.sol b/test/libsolidity/syntaxTests/inheritance/unimplemented_without_virtual.sol
new file mode 100644
index 000000000..7635e0297
--- /dev/null
+++ b/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.
diff --git a/test/libsolidity/syntaxTests/modifiers/modifiers_on_abstract_functions_no_parser_error.sol b/test/libsolidity/syntaxTests/modifiers/modifiers_on_abstract_functions_no_parser_error.sol
index 534a4d1a8..a3ffd7c17 100644
--- a/test/libsolidity/syntaxTests/modifiers/modifiers_on_abstract_functions_no_parser_error.sol
+++ b/test/libsolidity/syntaxTests/modifiers/modifiers_on_abstract_functions_no_parser_error.sol
@@ -5,9 +5,9 @@
 abstract contract C
 {
     modifier only_owner() { _; }
-    function foo() only_owner public;
-    function bar() public only_owner;
+    function foo() only_owner public virtual;
+    function bar() public only_owner virtual;
 }
 // ----
-// SyntaxError: (212-245): Functions without implementation cannot have modifiers.
-// SyntaxError: (250-283): Functions without implementation cannot have modifiers.
+// SyntaxError: (212-253): Functions without implementation cannot have modifiers.
+// SyntaxError: (258-299): Functions without implementation cannot have modifiers.
diff --git a/test/libsolidity/syntaxTests/multiVariableDeclaration/differentNumberOfComponentsFromReturn.sol b/test/libsolidity/syntaxTests/multiVariableDeclaration/differentNumberOfComponentsFromReturn.sol
index 332ac6b14..dead52486 100644
--- a/test/libsolidity/syntaxTests/multiVariableDeclaration/differentNumberOfComponentsFromReturn.sol
+++ b/test/libsolidity/syntaxTests/multiVariableDeclaration/differentNumberOfComponentsFromReturn.sol
@@ -1,4 +1,4 @@
-abstract contract C {
+contract C {
     function f() public {
         uint a = two();
         uint b = three();
@@ -13,17 +13,17 @@ abstract contract C {
         (uint a1, uint b1, uint c1, uint d1) = three();
         (uint a2, uint b2, uint c2) = four();
     }
-    function one() public pure returns (uint);
-    function two() public pure returns (uint, uint);
-    function three() public pure returns (uint, uint, uint);
-    function four() public pure returns (uint, uint, uint, uint);
+    function one() public pure returns (uint) {}
+    function two() public pure returns (uint, uint) {}
+    function three() public pure returns (uint, uint, uint) {}
+    function four() public pure returns (uint, uint, uint, uint) {}
 }
 // ----
-// TypeError: (56-70): Different number of components on the left hand side (1) than on the right hand side (2).
-// TypeError: (80-96): Different number of components on the left hand side (1) than on the right hand side (3).
-// TypeError: (106-121): Different number of components on the left hand side (1) than on the right hand side (4).
-// TypeError: (163-207): Different number of components on the left hand side (4) than on the right hand side (1).
-// TypeError: (217-252): Different number of components on the left hand side (3) than on the right hand side (1).
-// TypeError: (262-288): Different number of components on the left hand side (2) than on the right hand side (1).
-// TypeError: (330-376): Different number of components on the left hand side (4) than on the right hand side (3).
-// TypeError: (386-422): Different number of components on the left hand side (3) than on the right hand side (4).
+// TypeError: (47-61): Different number of components on the left hand side (1) than on the right hand side (2).
+// TypeError: (71-87): Different number of components on the left hand side (1) than on the right hand side (3).
+// TypeError: (97-112): Different number of components on the left hand side (1) than on the right hand side (4).
+// TypeError: (154-198): Different number of components on the left hand side (4) than on the right hand side (1).
+// TypeError: (208-243): Different number of components on the left hand side (3) than on the right hand side (1).
+// TypeError: (253-279): Different number of components on the left hand side (2) than on the right hand side (1).
+// TypeError: (321-367): Different number of components on the left hand side (4) than on the right hand side (3).
+// TypeError: (377-413): Different number of components on the left hand side (3) than on the right hand side (4).
diff --git a/test/libsolidity/syntaxTests/multiVariableDeclaration/disallowWildcardsFromReturn.sol b/test/libsolidity/syntaxTests/multiVariableDeclaration/disallowWildcardsFromReturn.sol
index 969b6883a..ee141495c 100644
--- a/test/libsolidity/syntaxTests/multiVariableDeclaration/disallowWildcardsFromReturn.sol
+++ b/test/libsolidity/syntaxTests/multiVariableDeclaration/disallowWildcardsFromReturn.sol
@@ -12,11 +12,11 @@ abstract contract C {
         (,uint m, uint n,) = five();
         a;b;c;d;e;f;g;h;i;j;k;l;m;n;
     }
-    function one() public pure returns (uint);
-    function two() public pure returns (uint, uint);
-    function three() public pure returns (uint, uint, uint);
-    function four() public pure returns (uint, uint, uint, uint);
-    function five() public pure returns (uint, uint, uint, uint, uint);
+    function one() public pure returns (uint) {}
+    function two() public pure returns (uint, uint) {}
+    function three() public pure returns (uint, uint, uint) {}
+    function four() public pure returns (uint, uint, uint, uint) {}
+    function five() public pure returns (uint, uint, uint, uint, uint) {}
 }
 // ----
 // TypeError: (62-81): Different number of components on the left hand side (2) than on the right hand side (3).
diff --git a/test/libsolidity/syntaxTests/nameAndTypeResolution/029_create_abstract_contract.sol b/test/libsolidity/syntaxTests/nameAndTypeResolution/029_create_abstract_contract.sol
index ba901a358..85c78bf30 100644
--- a/test/libsolidity/syntaxTests/nameAndTypeResolution/029_create_abstract_contract.sol
+++ b/test/libsolidity/syntaxTests/nameAndTypeResolution/029_create_abstract_contract.sol
@@ -1,7 +1,7 @@
-contract base { function foo() public; }
+contract base { function foo() public virtual; }
 contract derived {
     base b;
     function foo() public { b = new base(); }
 }
 // ----
-// TypeError: (0-40): Contract "base" should be marked as abstract.
+// TypeError: (0-48): Contract "base" should be marked as abstract.
diff --git a/test/libsolidity/syntaxTests/nameAndTypeResolution/030_redeclare_implemented_abstract_function_as_abstract.sol b/test/libsolidity/syntaxTests/nameAndTypeResolution/030_redeclare_implemented_abstract_function_as_abstract.sol
index d18a9a0a8..a29c2f24c 100644
--- a/test/libsolidity/syntaxTests/nameAndTypeResolution/030_redeclare_implemented_abstract_function_as_abstract.sol
+++ b/test/libsolidity/syntaxTests/nameAndTypeResolution/030_redeclare_implemented_abstract_function_as_abstract.sol
@@ -1,6 +1,5 @@
 abstract contract base { function foo() public virtual; }
 contract derived is base { function foo() public virtual override {} }
-contract wrong is derived { function foo() public; }
+contract wrong is derived { function foo() public virtual override; }
 // ----
-// TypeError: (157-179): Overriding function is missing 'override' specifier.
-// TypeError: (157-179): Redeclaring an already implemented function as abstract
+// TypeError: (157-196): Redeclaring an already implemented function as abstract
diff --git a/test/libsolidity/syntaxTests/nameAndTypeResolution/268_function_overload_array_type.sol b/test/libsolidity/syntaxTests/nameAndTypeResolution/268_function_overload_array_type.sol
index 659928277..6a713457e 100644
--- a/test/libsolidity/syntaxTests/nameAndTypeResolution/268_function_overload_array_type.sol
+++ b/test/libsolidity/syntaxTests/nameAndTypeResolution/268_function_overload_array_type.sol
@@ -1,5 +1,5 @@
 abstract contract M {
-    function f(uint[] memory) public;
-    function f(int[] memory) public;
+    function f(uint[] memory) public virtual;
+    function f(int[] memory) public virtual;
 }
 // ----
diff --git a/test/libsolidity/syntaxTests/nameAndTypeResolution/446_no_unused_warning_abstract_arguments.sol b/test/libsolidity/syntaxTests/nameAndTypeResolution/446_no_unused_warning_abstract_arguments.sol
index e6ae44ea7..636c9a360 100644
--- a/test/libsolidity/syntaxTests/nameAndTypeResolution/446_no_unused_warning_abstract_arguments.sol
+++ b/test/libsolidity/syntaxTests/nameAndTypeResolution/446_no_unused_warning_abstract_arguments.sol
@@ -1,4 +1,4 @@
 abstract contract C {
-    function f(uint a) pure public returns (uint b);
+    function f(uint a) pure public virtual returns (uint b);
 }
 // ----
diff --git a/test/libsolidity/syntaxTests/nameAndTypeResolution/559_no_warning_for_using_members_that_look_like_address_members.sol b/test/libsolidity/syntaxTests/nameAndTypeResolution/559_no_warning_for_using_members_that_look_like_address_members.sol
index a49a5e358..d4a47ec3c 100644
--- a/test/libsolidity/syntaxTests/nameAndTypeResolution/559_no_warning_for_using_members_that_look_like_address_members.sol
+++ b/test/libsolidity/syntaxTests/nameAndTypeResolution/559_no_warning_for_using_members_that_look_like_address_members.sol
@@ -1,5 +1,5 @@
 abstract contract C {
-    function transfer(uint) public;
+    function transfer(uint) public virtual;
     function f() public {
         this.transfer(10);
     }
diff --git a/test/libsolidity/syntaxTests/natspec/docstring_empty_description.sol b/test/libsolidity/syntaxTests/natspec/docstring_empty_description.sol
index da36f419f..cc8a31f3a 100644
--- a/test/libsolidity/syntaxTests/natspec/docstring_empty_description.sol
+++ b/test/libsolidity/syntaxTests/natspec/docstring_empty_description.sol
@@ -1,6 +1,6 @@
-abstract contract C {
+contract C {
     /// @param id
-    function vote(uint id) public;
+    function vote(uint id) public {}
 }
 // ----
 // DocstringParsingError: No description given for param id
diff --git a/test/libsolidity/syntaxTests/natspec/docstring_empty_tag.sol b/test/libsolidity/syntaxTests/natspec/docstring_empty_tag.sol
index e822e89b1..2e2584407 100644
--- a/test/libsolidity/syntaxTests/natspec/docstring_empty_tag.sol
+++ b/test/libsolidity/syntaxTests/natspec/docstring_empty_tag.sol
@@ -1,6 +1,6 @@
 abstract contract C {
     /// @param
-    function vote(uint id) public;
+    function vote(uint id) public {}
 }
 // ----
 // DocstringParsingError: End of tag @param not found
diff --git a/test/libsolidity/syntaxTests/parsing/function_no_body.sol b/test/libsolidity/syntaxTests/parsing/function_no_body.sol
index 1de881264..5851d2f61 100644
--- a/test/libsolidity/syntaxTests/parsing/function_no_body.sol
+++ b/test/libsolidity/syntaxTests/parsing/function_no_body.sol
@@ -1,4 +1,4 @@
 abstract contract test {
-	function functionName(bytes32 input) public returns (bytes32 out);
+	function functionName(bytes32 input) public virtual returns (bytes32 out);
 }
 // ----
diff --git a/test/libsolidity/syntaxTests/types/function_types/function_parameter_return_types_fail.sol b/test/libsolidity/syntaxTests/types/function_types/function_parameter_return_types_fail.sol
index 692bcf5a0..c722ed814 100644
--- a/test/libsolidity/syntaxTests/types/function_types/function_parameter_return_types_fail.sol
+++ b/test/libsolidity/syntaxTests/types/function_types/function_parameter_return_types_fail.sol
@@ -2,7 +2,7 @@ abstract contract Test
 {
     function uint256_to_uint256(uint256 x) internal pure returns (uint256) { return x; }
     function uint256_to_string(uint256 x) internal pure returns (string memory) { return x == 0 ? "a" : "b"; }
-    function uint256_to_string_storage(uint256) internal pure returns (string storage);
+    function uint256_to_string_storage(uint256) internal pure returns (string storage) {}
     function string_to_uint256(string memory x) internal pure returns (uint256) { return bytes(x).length; }
     function string_to_string(string memory x) internal pure returns (string memory) { return x; }
 
@@ -33,10 +33,10 @@ abstract contract Test
     }
 }
 // ----
-// TypeError: (1227-1320): Type function (uint256) pure returns (string memory) is not implicitly convertible to expected type function (uint256) pure returns (uint256).
-// TypeError: (1328-1434): Type function (uint256) pure returns (string storage pointer) is not implicitly convertible to expected type function (uint256) pure returns (string memory).
-// TypeError: (1442-1540): Type function (uint256) pure returns (string memory) is not implicitly convertible to expected type function (string memory) pure returns (uint256).
-// TypeError: (1548-1655): Type function (uint256) pure returns (string memory) is not implicitly convertible to expected type function (string memory) pure returns (string memory).
-// TypeError: (1664-1775): Type function (uint256) pure returns (uint256) is not implicitly convertible to expected type function (uint256,uint256) pure returns (uint256).
-// TypeError: (1783-1902): Type function (string memory) pure returns (string memory) is not implicitly convertible to expected type function (string memory,uint256) pure returns (string memory).
-// TypeError: (1910-2034): Type function (string memory) pure returns (string memory) is not implicitly convertible to expected type function (string memory,string memory) pure returns (string memory).
+// TypeError: (1229-1322): Type function (uint256) pure returns (string memory) is not implicitly convertible to expected type function (uint256) pure returns (uint256).
+// TypeError: (1330-1436): Type function (uint256) pure returns (string storage pointer) is not implicitly convertible to expected type function (uint256) pure returns (string memory).
+// TypeError: (1444-1542): Type function (uint256) pure returns (string memory) is not implicitly convertible to expected type function (string memory) pure returns (uint256).
+// TypeError: (1550-1657): Type function (uint256) pure returns (string memory) is not implicitly convertible to expected type function (string memory) pure returns (string memory).
+// TypeError: (1666-1777): Type function (uint256) pure returns (uint256) is not implicitly convertible to expected type function (uint256,uint256) pure returns (uint256).
+// TypeError: (1785-1904): Type function (string memory) pure returns (string memory) is not implicitly convertible to expected type function (string memory,uint256) pure returns (string memory).
+// TypeError: (1912-2036): Type function (string memory) pure returns (string memory) is not implicitly convertible to expected type function (string memory,string memory) pure returns (string memory).
diff --git a/test/libsolidity/syntaxTests/visibility/interface/interface_contract_function_default.sol b/test/libsolidity/syntaxTests/visibility/interface/interface_contract_function_default.sol
index 8c1aa0fda..c0adb47c8 100644
--- a/test/libsolidity/syntaxTests/visibility/interface/interface_contract_function_default.sol
+++ b/test/libsolidity/syntaxTests/visibility/interface/interface_contract_function_default.sol
@@ -4,9 +4,9 @@ interface I {
     function f();
 }
 abstract contract C {
-    function g();
+    function g() {}
 }
 // ----
 // SyntaxError: (158-171): No visibility specified. Did you intend to add "external"?
-// SyntaxError: (200-213): No visibility specified. Did you intend to add "public"?
+// SyntaxError: (200-215): No visibility specified. Did you intend to add "public"?
 // TypeError: (158-171): Functions in interfaces must be declared external.

From c33d233d881b1dc321e8fe50b04c179c9963e24d Mon Sep 17 00:00:00 2001
From: Daniel Kirchner <daniel@ekpyron.org>
Date: Tue, 3 Dec 2019 10:50:53 +0100
Subject: [PATCH 2/3] Update docs.

---
 docs/abi-spec.rst                     | 6 +++---
 docs/contracts/abstract-contracts.rst | 2 +-
 docs/contracts/inheritance.rst        | 6 +++---
 3 files changed, 7 insertions(+), 7 deletions(-)

diff --git a/docs/abi-spec.rst b/docs/abi-spec.rst
index c9938cb79..b50017429 100644
--- a/docs/abi-spec.rst
+++ b/docs/abi-spec.rst
@@ -529,11 +529,11 @@ As an example, the code
     pragma experimental ABIEncoderV2;
 
 
-    abstract contract Test {
+    contract Test {
         struct S { uint a; uint[] b; T[] c; }
         struct T { uint x; uint y; }
-        function f(S memory s, T memory t, uint a) public;
-        function g() public returns (S memory s, T memory t, uint a);
+        function f(S memory s, T memory t, uint a) public {}
+        function g() public returns (S memory s, T memory t, uint a) {}
     }
 
 would result in the JSON:
diff --git a/docs/contracts/abstract-contracts.rst b/docs/contracts/abstract-contracts.rst
index eab98154f..372c104cb 100644
--- a/docs/contracts/abstract-contracts.rst
+++ b/docs/contracts/abstract-contracts.rst
@@ -15,7 +15,7 @@ provided (no implementation body ``{ }`` was given).::
     pragma solidity >=0.4.0 <0.7.0;
 
     abstract contract Feline {
-        function utterance() public returns (bytes32);
+        function utterance() public virtual returns (bytes32);
     }
 
 Such abstract contracts can not be instantiated directly. This is also true, if an abstract contract itself does implement
diff --git a/docs/contracts/inheritance.rst b/docs/contracts/inheritance.rst
index 81ad06f44..cd2e3f8fc 100644
--- a/docs/contracts/inheritance.rst
+++ b/docs/contracts/inheritance.rst
@@ -53,13 +53,13 @@ Details are given in the following example.
     // without body. If a contract does not implement all
     // functions it can only be used as an interface.
     abstract contract Config {
-        function lookup(uint id) public returns (address adr);
+        function lookup(uint id) public virtual returns (address adr);
     }
 
 
     abstract contract NameReg {
-        function register(bytes32 name) public;
-        function unregister() public;
+        function register(bytes32 name) public virtual;
+        function unregister() public virtual;
     }
 
 

From c1505d3f44b7e6baba62de1c67ed569e976ff044 Mon Sep 17 00:00:00 2001
From: Daniel Kirchner <daniel@ekpyron.org>
Date: Tue, 3 Dec 2019 10:53:11 +0100
Subject: [PATCH 3/3] Fix solc-js tests.

---
 test/externalTests/solc-js/DAO/Token.sol | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/test/externalTests/solc-js/DAO/Token.sol b/test/externalTests/solc-js/DAO/Token.sol
index 10457774d..0e566830f 100644
--- a/test/externalTests/solc-js/DAO/Token.sol
+++ b/test/externalTests/solc-js/DAO/Token.sol
@@ -87,7 +87,7 @@ virtual returns (bool success);
 }
 
 abstract contract tokenRecipient {
-    function receiveApproval(address _from, uint256 _value, address _token, bytes memory _extraData) public;
+    function receiveApproval(address _from, uint256 _value, address _token, bytes memory _extraData) public virtual;
 }
 
 contract Token is TokenInterface {