Add implicit convertibility to function pointer with higher state mutability

7 files changed, 263 insertions, 23 deletions

diff --git a/docs/types.rst b/docs/types.rst
index c216fd83..251f1edd 100644
--- a/docs/types.rst
+++ b/docs/types.rst
@@ -426,8 +426,26 @@ function type should not return anything, the whole ``returns (<return types>)``
 part has to be omitted.
 
 By default, function types are internal, so the ``internal`` keyword can be
-omitted. In contrast, contract functions themselves are public by default,
-only when used as the name of a type, the default is internal.
+omitted. Note that this only applies to function types. Visibility has
+to be specified explicitly for functions defined in contracts, they
+do not have a default.
+
+A function type ``A`` is implicitly convertible to a function type ``B`` if and only if
+their parameter types are identical, their return types are identical,
+their internal/external property is identical and the state mutability of ``A``
+is not more restrictive than the state mutability of ``B``. In particular:
+
+ - ``pure`` functions can be converted to ``view`` and ``non-payable`` functions
+ - ``view`` functions can be converted to ``non-payable`` functions
+ - ``payable`` functions can be converted to ``non-payable`` functions
+
+No other conversions are possible.
+
+The rule about ``payable`` and ``non-payable`` might be a little
+confusing, but in essence, if a function is ``payable``, this means that it
+also accepts a payment of zero Ether, so it also is ``non-payable``.
+On the other hand, a ``non-payable`` function will reject Ether sent to it,
+so ``non-payable`` functions cannot be converted to ``payable`` functions.
 
 If a function type variable is not initialized, calling it will result
 in an exception. The same happens if you call a function after using ``delete``
diff --git a/libsolidity/ast/Types.cpp b/libsolidity/ast/Types.cpp
index 0d69ae1a..9e815ca8 100644
--- a/libsolidity/ast/Types.cpp
+++ b/libsolidity/ast/Types.cpp
@@ -2569,28 +2569,10 @@ bool FunctionType::operator==(Type const& _other) const
 {
 	if (_other.category() != category())
 		return false;
-
 	FunctionType const& other = dynamic_cast<FunctionType const&>(_other);
-	if (
-		m_kind != other.m_kind ||
-		m_stateMutability != other.stateMutability() ||
-		m_parameterTypes.size() != other.m_parameterTypes.size() ||
-		m_returnParameterTypes.size() != other.m_returnParameterTypes.size()
-	)
-		return false;
-
-	auto typeCompare = [](TypePointer const& _a, TypePointer const& _b) -> bool { return *_a == *_b; };
-	if (
-		!equal(m_parameterTypes.cbegin(), m_parameterTypes.cend(), other.m_parameterTypes.cbegin(), typeCompare) ||
-		!equal(m_returnParameterTypes.cbegin(), m_returnParameterTypes.cend(), other.m_returnParameterTypes.cbegin(), typeCompare)
-	)
-		return false;
-	//@todo this is ugly, but cannot be prevented right now
-	if (m_gasSet != other.m_gasSet || m_valueSet != other.m_valueSet)
-		return false;
-	if (bound() != other.bound())
+	if (!equalExcludingStateMutability(other))
 		return false;
-	if (bound() && *selfType() != *other.selfType())
+	if (m_stateMutability != other.stateMutability())
 		return false;
 	return true;
 }
@@ -2606,6 +2588,31 @@ bool FunctionType::isExplicitlyConvertibleTo(Type const& _convertTo) const
 	return _convertTo.category() == category();
 }
 
+bool FunctionType::isImplicitlyConvertibleTo(Type const& _convertTo) const
+{
+	if (_convertTo.category() != category())
+		return false;
+
+	FunctionType const& convertTo = dynamic_cast<FunctionType const&>(_convertTo);
+
+	if (!equalExcludingStateMutability(convertTo))
+		return false;
+
+	// non-payable should not be convertible to payable
+	if (m_stateMutability != StateMutability::Payable && convertTo.stateMutability() == StateMutability::Payable)
+		return false;
+
+	// payable should be convertible to non-payable, because you are free to pay 0 ether
+	if (m_stateMutability == StateMutability::Payable && convertTo.stateMutability() == StateMutability::NonPayable)
+		return true;
+
+	// e.g. pure should be convertible to view, but not the other way around.
+	if (m_stateMutability > convertTo.stateMutability())
+		return false;
+
+	return true;
+}
+
 TypePointer FunctionType::unaryOperatorResult(Token::Value _operator) const
 {
 	if (_operator == Token::Value::Delete)
@@ -2863,6 +2870,38 @@ bool FunctionType::hasEqualParameterTypes(FunctionType const& _other) const
 	);
 }
 
+bool FunctionType::hasEqualReturnTypes(FunctionType const& _other) const
+{
+	if (m_returnParameterTypes.size() != _other.m_returnParameterTypes.size())
+		return false;
+	return equal(
+		m_returnParameterTypes.cbegin(),
+		m_returnParameterTypes.cend(),
+		_other.m_returnParameterTypes.cbegin(),
+		[](TypePointer const& _a, TypePointer const& _b) -> bool { return *_a == *_b; }
+	);
+}
+
+bool FunctionType::equalExcludingStateMutability(FunctionType const& _other) const
+{
+	if (m_kind != _other.m_kind)
+		return false;
+
+	if (!hasEqualParameterTypes(_other) || !hasEqualReturnTypes(_other))
+		return false;
+
+	//@todo this is ugly, but cannot be prevented right now
+	if (m_gasSet != _other.m_gasSet || m_valueSet != _other.m_valueSet)
+		return false;
+
+	if (bound() != _other.bound())
+		return false;
+
+	solAssert(!bound() || *selfType() == *_other.selfType(), "");
+
+	return true;
+}
+
 bool FunctionType::isBareCall() const
 {
 	switch (m_kind)
diff --git a/libsolidity/ast/Types.h b/libsolidity/ast/Types.h
index 8bae3d41..cc96ac4b 100644
--- a/libsolidity/ast/Types.h
+++ b/libsolidity/ast/Types.h
@@ -1012,6 +1012,7 @@ public:
 
 	virtual std::string richIdentifier() const override;
 	virtual bool operator==(Type const& _other) const override;
+	virtual bool isImplicitlyConvertibleTo(Type const& _convertTo) const override;
 	virtual bool isExplicitlyConvertibleTo(Type const& _convertTo) const override;
 	virtual TypePointer unaryOperatorResult(Token::Value _operator) const override;
 	virtual TypePointer binaryOperatorResult(Token::Value, TypePointer const&) const override;
@@ -1041,8 +1042,12 @@ public:
 	/// @param _selfType if the function is bound, this has to be supplied and is the type of the
 	/// expression the function is called on.
 	bool canTakeArguments(TypePointers const& _arguments, TypePointer const& _selfType = TypePointer()) const;
-	/// @returns true if the types of parameters are equal (doesn't check return parameter types)
+	/// @returns true if the types of parameters are equal (does not check return parameter types)
 	bool hasEqualParameterTypes(FunctionType const& _other) const;
+	/// @returns true iff the return types are equal (does not check parameter types)
+	bool hasEqualReturnTypes(FunctionType const& _other) const;
+	/// @returns true iff the function type is equal to the given type, ignoring state mutability differences.
+	bool equalExcludingStateMutability(FunctionType const& _other) const;
 
 	/// @returns true if the ABI is used for this call (only meaningful for external calls)
 	bool isBareCall() const;
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
new file mode 100644
index 00000000..0d4fded1
--- /dev/null
+++ b/test/libsolidity/syntaxTests/types/function_types/function_parameter_return_types_fail.sol
@@ -0,0 +1,42 @@
+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 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; }
+
+    function uint256_uint256_to_uint256(uint256 x, uint256 y) internal pure returns (uint256) { return x + y; }
+    function uint256_uint256_to_string(uint256 x, uint256 y) internal pure returns (string memory) { return x == y ? "a" : "b"; }
+    function string_uint256_to_string(string memory x, uint256 y) internal pure returns (string memory) { return y == 0 ? "a" : x; }
+    function string_string_to_string(string memory x, string memory y) internal pure returns (string memory) { return bytes(x).length == 0 ? y : x; }
+    function uint256_string_to_string(uint256 x, string memory y) internal pure returns (string memory) { return x == 0 ? "a" : y; }
+
+    function tests() internal pure
+    {
+      function (uint256) internal pure returns (uint256) var_uint256_to_uint256 = uint256_to_string;
+      function (uint256) internal pure returns (string memory) var_uint256_to_string = uint256_to_string_storage;
+      function (string memory) internal pure returns (uint256) var_string_to_uint256 = uint256_to_string;
+      function (string memory) internal pure returns (string memory) var_string_to_string = var_uint256_to_string;
+
+      function (uint256, uint256) internal pure returns (uint256) var_uint256_uint256_to_uint256 = uint256_to_uint256;
+      function (string memory, uint256) internal pure returns (string memory) var_string_uint256_to_string = string_to_string;
+      function (string memory, string memory) internal pure returns (string memory) var_string_string_to_string = string_to_string;
+
+      var_uint256_to_uint256(1);
+      var_uint256_to_string(2);
+      var_string_to_uint256("a");
+      var_string_to_string("b");
+      var_uint256_uint256_to_uint256(3, 4);
+      var_string_uint256_to_string("c", 7);
+      var_string_string_to_string("d", "e");
+    }
+}
+// ----
+// TypeError: (1218-1311): Type function (uint256) pure returns (string memory) is not implicitly convertible to expected type function (uint256) pure returns (uint256).
+// TypeError: (1319-1425): Type function (uint256) pure returns (string storage pointer) is not implicitly convertible to expected type function (uint256) pure returns (string memory).
+// TypeError: (1433-1531): Type function (uint256) pure returns (string memory) is not implicitly convertible to expected type function (string memory) pure returns (uint256).
+// TypeError: (1539-1646): Type function (uint256) pure returns (string memory) is not implicitly convertible to expected type function (string memory) pure returns (string memory).
+// TypeError: (1655-1766): Type function (uint256) pure returns (uint256) is not implicitly convertible to expected type function (uint256,uint256) pure returns (uint256).
+// TypeError: (1774-1893): Type function (string memory) pure returns (string memory) is not implicitly convertible to expected type function (string memory,uint256) pure returns (string memory).
+// TypeError: (1901-2025): 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/types/function_types/function_parameter_return_types_success.sol b/test/libsolidity/syntaxTests/types/function_types/function_parameter_return_types_success.sol
new file mode 100644
index 00000000..f750632e
--- /dev/null
+++ b/test/libsolidity/syntaxTests/types/function_types/function_parameter_return_types_success.sol
@@ -0,0 +1,39 @@
+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 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; }
+
+  function uint256_uint256_to_uint256(uint256 x, uint256 y) internal pure returns (uint256) { return x + y; }
+  function uint256_uint256_to_string(uint256 x, uint256 y) internal pure returns (string memory) { return x == y ? "a" : "b"; }
+  function string_uint256_to_string(string memory x, uint256 y) internal pure returns (string memory) { return y == 0 ? "a" : x; }
+  function string_string_to_string(string memory x, string memory y) internal pure returns (string memory) { return bytes(x).length == 0 ? y : x; }
+  function uint256_string_to_string(uint256 x, string memory y) internal pure returns (string memory) { return x == 0 ? "a" : y; }
+
+  function tests() internal pure
+  {
+    function (uint256) internal pure returns (uint256) var_uint256_to_uint256 = uint256_to_uint256;
+    function (uint256) internal pure returns (string memory) var_uint256_to_string = uint256_to_string;
+    function (string memory) internal pure returns (uint256) var_string_to_uint256 = string_to_uint256;
+    function (string memory) internal pure returns (string memory) var_string_to_string = string_to_string;
+
+    function (uint256, uint256) internal pure returns (uint256) var_uint256_uint256_to_uint256 = uint256_uint256_to_uint256;
+    function (uint256, uint256) internal pure returns (string memory) var_uint256_uint256_to_string = uint256_uint256_to_string;
+    function (string memory, uint256) internal pure returns (string memory) var_string_uint256_to_string = string_uint256_to_string;
+    function (string memory, string memory) internal pure returns (string memory) var_string_string_to_string = string_string_to_string;
+    function (uint256, string memory) internal pure returns (string memory) var_uint256_string_to_string = uint256_string_to_string;
+
+    // Avoid unused variable warnings:
+    var_uint256_to_uint256(1);
+    var_uint256_to_string(2);
+    var_string_to_uint256("a");
+    var_string_to_string("b");
+    var_uint256_uint256_to_uint256(3, 4);
+    var_uint256_uint256_to_string(5, 6);
+    var_string_uint256_to_string("c", 7);
+    var_string_string_to_string("d", "e");
+    var_uint256_string_to_string(8, "f");
+  }
+}
+// ----
diff --git a/test/libsolidity/syntaxTests/types/function_types/function_state_mutability_fail.sol b/test/libsolidity/syntaxTests/types/function_types/function_state_mutability_fail.sol
new file mode 100644
index 00000000..818d7840
--- /dev/null
+++ b/test/libsolidity/syntaxTests/types/function_types/function_state_mutability_fail.sol
@@ -0,0 +1,51 @@
+contract Test
+{
+  function internalPureFunc(uint256 x) internal pure returns (uint256) { return x; }
+  function internalViewFunc(uint256 x) internal view returns (uint256) { return x; }
+  function internalMutableFunc(uint256 x) internal returns (uint256) { return x; }
+
+  function externalPureFunc(uint256 x) external pure returns (uint256) { return x; }
+  function externalViewFunc(uint256 x) external view returns (uint256) { return x; }
+  function externalPayableFunc(uint256 x) external payable returns (uint256) { return x; }
+  function externalMutableFunc(uint256 x) external returns (uint256) { return x; }
+
+  function funcTakesInternalPure(function(uint256) internal pure returns(uint256) a) internal returns (uint256) { return a(4); }
+  function funcTakesInternalView(function(uint256) internal view returns(uint256) a) internal returns (uint256) { return a(4); }
+  function funcTakesInternalMutable(function(uint256) internal returns(uint256) a) internal returns (uint256) { return a(4); }
+
+  function funcTakesExternalPure(function(uint256) external pure returns(uint256) a) internal returns (uint256) { return a(4); }
+  function funcTakesExternalView(function(uint256) external view returns(uint256) a) internal returns (uint256) { return a(4); }
+  function funcTakesExternalPayable(function(uint256) external payable returns(uint256) a) internal returns (uint256) { return a(4); }
+  function funcTakesExternalMutable(function(uint256) external returns(uint256) a) internal returns (uint256) { return a(4); }
+
+  function tests() internal
+  {
+    funcTakesInternalPure(internalViewFunc); // view -> pure should fail
+    funcTakesInternalPure(internalMutableFunc); // mutable -> pure should fail
+
+    funcTakesInternalView(internalMutableFunc); // mutable -> view should fail
+
+    funcTakesExternalPure(this.externalViewFunc); // view -> pure should fail
+    funcTakesExternalPure(this.externalPayableFunc); // payable -> pure should fail
+    funcTakesExternalPure(this.externalMutableFunc); // mutable -> pure should fail
+
+    funcTakesExternalView(this.externalPayableFunc); // payable -> view should fail
+    funcTakesExternalView(this.externalMutableFunc); // mutable -> view should fail
+
+    funcTakesExternalPayable(this.externalPureFunc); // pure -> payable should fail
+    funcTakesExternalPayable(this.externalViewFunc); // view -> payable should fail
+    funcTakesExternalPayable(this.externalMutableFunc); // mutable -> payable should fail
+  }
+}
+// ----
+// TypeError: (1580-1596): Invalid type for argument in function call. Invalid implicit conversion from function (uint256) view returns (uint256) to function (uint256) pure returns (uint256) requested.
+// TypeError: (1653-1672): Invalid type for argument in function call. Invalid implicit conversion from function (uint256) returns (uint256) to function (uint256) pure returns (uint256) requested.
+// TypeError: (1733-1752): Invalid type for argument in function call. Invalid implicit conversion from function (uint256) returns (uint256) to function (uint256) view returns (uint256) requested.
+// TypeError: (1813-1834): Invalid type for argument in function call. Invalid implicit conversion from function (uint256) view external returns (uint256) to function (uint256) pure external returns (uint256) requested.
+// TypeError: (1891-1915): Invalid type for argument in function call. Invalid implicit conversion from function (uint256) payable external returns (uint256) to function (uint256) pure external returns (uint256) requested.
+// TypeError: (1975-1999): Invalid type for argument in function call. Invalid implicit conversion from function (uint256) external returns (uint256) to function (uint256) pure external returns (uint256) requested.
+// TypeError: (2060-2084): Invalid type for argument in function call. Invalid implicit conversion from function (uint256) payable external returns (uint256) to function (uint256) view external returns (uint256) requested.
+// TypeError: (2144-2168): Invalid type for argument in function call. Invalid implicit conversion from function (uint256) external returns (uint256) to function (uint256) view external returns (uint256) requested.
+// TypeError: (2232-2253): Invalid type for argument in function call. Invalid implicit conversion from function (uint256) pure external returns (uint256) to function (uint256) payable external returns (uint256) requested.
+// TypeError: (2316-2337): Invalid type for argument in function call. Invalid implicit conversion from function (uint256) view external returns (uint256) to function (uint256) payable external returns (uint256) requested.
+// TypeError: (2400-2424): Invalid type for argument in function call. Invalid implicit conversion from function (uint256) external returns (uint256) to function (uint256) payable external returns (uint256) requested.
diff --git a/test/libsolidity/syntaxTests/types/function_types/function_state_mutability_success.sol b/test/libsolidity/syntaxTests/types/function_types/function_state_mutability_success.sol
new file mode 100644
index 00000000..4ee515fc
--- /dev/null
+++ b/test/libsolidity/syntaxTests/types/function_types/function_state_mutability_success.sol
@@ -0,0 +1,46 @@
+contract Test
+{
+  uint y;
+  function internalPureFunc(uint256 x) internal pure returns (uint256) { return x; }
+  function internalViewFunc(uint256 x) internal view returns (uint256) { return x + y; }
+  function internalMutableFunc(uint256 x) internal returns (uint256) { y = x; return x; }
+
+  function externalPureFunc(uint256 x) external pure returns (uint256) { return x; }
+  function externalViewFunc(uint256 x) external view returns (uint256) { return x + y; }
+  function externalPayableFunc(uint256 x) external payable returns (uint256) { return x + y; }
+  function externalMutableFunc(uint256 x) external returns (uint256) { y = x; return x; }
+
+  function funcTakesInternalPure(function(uint256) internal pure returns(uint256) a) internal pure returns (uint256) { return a(4); }
+  function funcTakesInternalView(function(uint256) internal view returns(uint256) a) internal view returns (uint256) { return a(4); }
+  function funcTakesInternalMutable(function(uint256) internal returns(uint256) a) internal returns (uint256) { return a(4); }
+
+  function funcTakesExternalPure(function(uint256) external pure returns(uint256) a) internal pure returns (uint256) { return a(4); }
+  function funcTakesExternalView(function(uint256) external view returns(uint256) a) internal view returns (uint256) { return a(4); }
+  function funcTakesExternalPayable(function(uint256) external payable returns(uint256) a) internal returns (uint256) { return a(4); }
+  function funcTakesExternalMutable(function(uint256) external returns(uint256) a) internal returns (uint256) { return a(4); }
+
+  function tests() internal
+  {
+    funcTakesInternalPure(internalPureFunc);
+
+    funcTakesInternalView(internalPureFunc);
+    funcTakesInternalView(internalViewFunc);
+
+    funcTakesInternalMutable(internalPureFunc);
+    funcTakesInternalMutable(internalViewFunc);
+    funcTakesInternalMutable(internalMutableFunc);
+
+    funcTakesExternalPure(this.externalPureFunc);
+
+    funcTakesExternalView(this.externalPureFunc);
+    funcTakesExternalView(this.externalViewFunc);
+
+    funcTakesExternalPayable(this.externalPayableFunc);
+
+    funcTakesExternalMutable(this.externalPureFunc);
+    funcTakesExternalMutable(this.externalViewFunc);
+    funcTakesExternalMutable(this.externalPayableFunc);
+    funcTakesExternalMutable(this.externalMutableFunc);
+  }
+}
+// ----