From 67ec5f6b17bd0fb5369350a270d4b9433eafaf4c Mon Sep 17 00:00:00 2001
From: chriseth <chris@ethereum.org>
Date: Wed, 11 Nov 2020 15:24:11 +0100
Subject: [PATCH] Override semantics for fallback function.

---
 libsolidity/analysis/OverrideChecker.cpp      | 25 ++++++++++++-------
 .../fallback/fallback_duplicate_returns.sol   |  6 +++++
 ...fallback_duplicate_returns_inheritance.sol |  9 +++++++
 .../fallback_duplicate_returns_override.sol   |  7 ++++++
 4 files changed, 38 insertions(+), 9 deletions(-)
 create mode 100644 test/libsolidity/syntaxTests/fallback/fallback_duplicate_returns.sol
 create mode 100644 test/libsolidity/syntaxTests/fallback/fallback_duplicate_returns_inheritance.sol
 create mode 100644 test/libsolidity/syntaxTests/fallback/fallback_duplicate_returns_override.sol

diff --git a/libsolidity/analysis/OverrideChecker.cpp b/libsolidity/analysis/OverrideChecker.cpp
index 0bfb032b1..c0cae71e4 100644
--- a/libsolidity/analysis/OverrideChecker.cpp
+++ b/libsolidity/analysis/OverrideChecker.cpp
@@ -394,6 +394,10 @@ bool OverrideProxy::OverrideComparator::operator<(OverrideComparator const& _oth
 	if (functionKind != _other.functionKind)
 		return *functionKind < *_other.functionKind;
 
+	// Parameters do not matter for non-regular functions.
+	if (functionKind != Token::Function)
+		return false;
+
 	if (!parameterTypes || !_other.parameterTypes)
 		return false;
 
@@ -574,16 +578,19 @@ void OverrideChecker::checkOverride(OverrideProxy const& _overriding, OverridePr
 		FunctionType const* functionType = _overriding.functionType();
 		FunctionType const* superType = _super.functionType();
 
-		solAssert(functionType->hasEqualParameterTypes(*superType), "Override doesn't have equal parameters!");
+		if (_overriding.functionKind() != Token::Fallback)
+		{
+			solAssert(functionType->hasEqualParameterTypes(*superType), "Override doesn't have equal parameters!");
 
-		if (!functionType->hasEqualReturnTypes(*superType))
-			overrideError(
-				_overriding,
-				_super,
-				4822_error,
-				"Overriding " + _overriding.astNodeName() + " return types differ.",
-				"Overridden " + _overriding.astNodeName() + " is here:"
-			);
+			if (!functionType->hasEqualReturnTypes(*superType))
+				overrideError(
+					_overriding,
+					_super,
+					4822_error,
+					"Overriding " + _overriding.astNodeName() + " return types differ.",
+					"Overridden " + _overriding.astNodeName() + " is here:"
+				);
+		}
 
 		// Stricter mutability is always okay except when super is Payable
 		if (
diff --git a/test/libsolidity/syntaxTests/fallback/fallback_duplicate_returns.sol b/test/libsolidity/syntaxTests/fallback/fallback_duplicate_returns.sol
new file mode 100644
index 000000000..625c17f98
--- /dev/null
+++ b/test/libsolidity/syntaxTests/fallback/fallback_duplicate_returns.sol
@@ -0,0 +1,6 @@
+contract C {
+    fallback(bytes calldata _input) external returns (bytes memory _output) {}
+    fallback() external {}
+}
+// ----
+// DeclarationError 7301: (96-118): Only one fallback function is allowed.
diff --git a/test/libsolidity/syntaxTests/fallback/fallback_duplicate_returns_inheritance.sol b/test/libsolidity/syntaxTests/fallback/fallback_duplicate_returns_inheritance.sol
new file mode 100644
index 000000000..818006a11
--- /dev/null
+++ b/test/libsolidity/syntaxTests/fallback/fallback_duplicate_returns_inheritance.sol
@@ -0,0 +1,9 @@
+contract C {
+    fallback(bytes calldata _input) external returns (bytes memory _output) {}
+}
+contract D is C {
+    fallback() external {}
+}
+// ----
+// TypeError 9456: (116-138): Overriding function is missing "override" specifier.
+// TypeError 4334: (17-91): Trying to override non-virtual function. Did you forget to add "virtual"?
diff --git a/test/libsolidity/syntaxTests/fallback/fallback_duplicate_returns_override.sol b/test/libsolidity/syntaxTests/fallback/fallback_duplicate_returns_override.sol
new file mode 100644
index 000000000..32638c082
--- /dev/null
+++ b/test/libsolidity/syntaxTests/fallback/fallback_duplicate_returns_override.sol
@@ -0,0 +1,7 @@
+contract C {
+    fallback(bytes calldata _input) external virtual returns (bytes memory _output) {}
+}
+contract D is C {
+    fallback() external override {}
+}
+// ----