integration: add function return type test (#244)

Closes #236

crates/integration/contracts/FunctionType.sol

@@ -0,0 +1,36 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.28;
+
+/* runner.json
+{
+    "differential": true,
+    "actions": [
+        {
+            "Instantiate": {
+                "code": {
+                    "Solidity": {
+                        "contract": "FunctionType"
+                    }
+                }
+            }
+        },
+        {
+            "Call": {
+                "dest": {
+                    "Instantiated": 0
+                },
+                "data": "b8c9d365"
+            }
+        }
+    ]
+}
+*/
+
+contract FunctionType {
+    uint public immutable x = 42;
+
+    function h() public view returns (function() external view returns (uint)) {
+        return this.x;
+    }
+}

crates/integration/src/tests.rs

@@ -56,6 +56,7 @@ test_spec!(transfer, "Transfer", "Transfer.sol");
 test_spec!(send, "Send", "Send.sol");
 test_spec!(function_pointer, "FunctionPointer", "FunctionPointer.sol");
 test_spec!(mload, "MLoad", "MLoad.sol");
+test_spec!(function_type, "FunctionType", "FunctionType.sol");
 
 fn instantiate(path: &str, contract: &str) -> Vec<SpecsAction> {
     vec![Instantiate {