Adds support for structs in interfaces.

Closes #4733.

test/libsolidity/SolidityEndToEndTest.cpp

@@ -4755,6 +4755,68 @@ BOOST_AUTO_TEST_CASE(constructing_enums_from_ints)
 	ABI_CHECK(callContractFunction("test()"), encodeArgs(1));
 }
 
+BOOST_AUTO_TEST_CASE(struct_referencing)
+{
+	static char const* sourceCode = R"(
+		pragma experimental ABIEncoderV2;
+		interface I {
+			struct S { uint a; }
+		}
+		library L {
+			struct S { uint b; uint a; }
+			function f() public pure returns (S) {
+				S memory s;
+				s.a = 3;
+				return s;
+			}
+			function g() public pure returns (I.S) {
+				I.S memory s;
+				s.a = 4;
+				return s;
+			}
+			// argument-dependant lookup tests
+			function a(I.S memory) public pure returns (uint) { return 1; }
+			function a(S memory) public pure returns (uint) { return 2; }
+		}
+		contract C is I {
+			function f() public pure returns (S) {
+				S memory s;
+				s.a = 1;
+				return s;
+			}
+			function g() public pure returns (I.S) {
+				I.S memory s;
+				s.a = 2;
+				return s;
+			}
+			function h() public pure returns (L.S) {
+				L.S memory s;
+				s.a = 5;
+				return s;
+			}
+			function x() public pure returns (L.S) {
+				return L.f();
+			}
+			function y() public pure returns (I.S) {
+				return L.g();
+			}
+			function a1() public pure returns (uint) { S memory s; return L.a(s); }
+			function a2() public pure returns (uint) { L.S memory s; return L.a(s); }
+		}
+	)";
+	compileAndRun(sourceCode, 0, "L");
+	ABI_CHECK(callContractFunction("f()"), encodeArgs(0, 3));
+	ABI_CHECK(callContractFunction("g()"), encodeArgs(4));
+	compileAndRun(sourceCode, 0, "C", bytes(), map<string, Address>{ {"L", m_contractAddress}});
+	ABI_CHECK(callContractFunction("f()"), encodeArgs(1));
+	ABI_CHECK(callContractFunction("g()"), encodeArgs(2));
+	ABI_CHECK(callContractFunction("h()"), encodeArgs(0, 5));
+	ABI_CHECK(callContractFunction("x()"), encodeArgs(0, 3));
+	ABI_CHECK(callContractFunction("y()"), encodeArgs(4));
+	ABI_CHECK(callContractFunction("a1()"), encodeArgs(1));
+	ABI_CHECK(callContractFunction("a2()"), encodeArgs(2));
+}
+
 BOOST_AUTO_TEST_CASE(enum_referencing)
 {
 	char const* sourceCode = R"(

test/libsolidity/syntaxTests/nameAndTypeResolution/419_interface_structs.sol

@@ -1,9 +1,6 @@
 interface I {
     struct A {
-        // This is currently expected to break, but it *may* change in the future.
         int dummy;
     }
 }
 // ----
-// TypeError: (18-136): Structs cannot be defined in interfaces.
-// TypeError: (120-129): Variables cannot be declared in interfaces.