From ef3595b0001614c19174c61d29d221844ed83a0e Mon Sep 17 00:00:00 2001
From: Alex Beregszaszi <alex@rtfs.hu>
Date: Tue, 27 Feb 2018 12:02:56 +0100
Subject: [PATCH] Allow overriding external functions in interfaces with public
 in an implementing contract

---
 Changelog.md                                      |  1 +
 libsolidity/analysis/TypeChecker.cpp              | 15 +++++++++++++++
 .../libsolidity/SolidityNameAndTypeResolution.cpp | 14 ++++++++++++++
 3 files changed, 30 insertions(+)

diff --git a/Changelog.md b/Changelog.md
index c5577b86a..7a4149f38 100644
--- a/Changelog.md
+++ b/Changelog.md
@@ -2,6 +2,7 @@
 
 Features:
  * General: Support accessing dynamic return data in post-byzantium EVMs.
+ * Interfaces: Allow overriding external functions in interfaces with public in an implementing contract.
 
 Bugfixes:
  * Code Generator: Allow ``block.blockhash`` without being called.
diff --git a/libsolidity/analysis/TypeChecker.cpp b/libsolidity/analysis/TypeChecker.cpp
index 999a2a97d..487959c5d 100644
--- a/libsolidity/analysis/TypeChecker.cpp
+++ b/libsolidity/analysis/TypeChecker.cpp
@@ -366,6 +366,16 @@ void TypeChecker::checkContractIllegalOverrides(ContractDefinition const& _contr
 	}
 }
 
+namespace {
+
+bool functionIsInInterface(FunctionDefinition const& _function)
+{
+	return dynamic_cast<ContractDefinition const*>(_function.scope()) &&
+		dynamic_cast<ContractDefinition const*>(_function.scope())->contractKind() == ContractDefinition::ContractKind::Interface;
+}
+
+}
+
 void TypeChecker::checkFunctionOverride(FunctionDefinition const& function, FunctionDefinition const& super)
 {
 	FunctionType functionType(function);
@@ -378,7 +388,12 @@ void TypeChecker::checkFunctionOverride(FunctionDefinition const& function, Func
 		function.annotation().superFunction = &super;
 
 	if (function.visibility() != super.visibility())
+	{
+		// visibility is enforced to be external in interfaces, but a contract can override that with public
+		if (functionIsInInterface(super) && !functionIsInInterface(function) && function.visibility() == FunctionDefinition::Visibility::Public)
+			return;
 		overrideError(function, super, "Overriding function visibility differs.");
+	}
 
 	else if (function.stateMutability() != super.stateMutability())
 		overrideError(
diff --git a/test/libsolidity/SolidityNameAndTypeResolution.cpp b/test/libsolidity/SolidityNameAndTypeResolution.cpp
index 50ee2b2e4..e30ea8a4a 100644
--- a/test/libsolidity/SolidityNameAndTypeResolution.cpp
+++ b/test/libsolidity/SolidityNameAndTypeResolution.cpp
@@ -6784,6 +6784,20 @@ BOOST_AUTO_TEST_CASE(using_interface_complex)
 	CHECK_SUCCESS(text);
 }
 
+BOOST_AUTO_TEST_CASE(interface_implement_public_contract)
+{
+	char const* text = R"(
+		interface I {
+			function f() external;
+		}
+		contract C is I {
+			function f() public {
+			}
+		}
+	)";
+	CHECK_SUCCESS_NO_WARNINGS(text);
+}
+
 BOOST_AUTO_TEST_CASE(warn_about_throw)
 {
 	char const* text = R"(