[Fix] Check the state does not actually change when no changes are expected (#1161)

* fix: check the state doesn’t change when the modify closure is nil

* test: add tests for the 'unexpected changes on the state' failure

Author: umbertovolta

Sources/ComposableArchitecture/TestSupport/TestStore.swift

@@ -401,9 +401,10 @@
       file: StaticString,
       line: UInt
     ) throws {
-      guard let modify = modify else { return }
       let current = expected
-      try modify(&expected)
+      if let modify = modify {
+        try modify(&expected)
+      }
 
       if expected != actual {
         let difference =
@@ -417,16 +418,19 @@
           \(String(describing: actual).indent(by: 2))
           """
 
+        let messageHeading = modify != nil ?
+          "A state change does not match expectation" :
+          "State was not expected to change, but a change occurred"
         XCTFail(
           """
-          A state change does not match expectation: …
+          \(messageHeading): …
 
           \(difference)
           """,
           file: file,
           line: line
         )
-      } else if expected == current {
+      } else if expected == current && modify != nil {
         XCTFail(
           """
           Expected state to change, but no change occurred.

Tests/ComposableArchitectureTests/TestStoreFailureTests.swift

@@ -62,6 +62,59 @@ class TestStoreFailureTests: XCTestCase {
     }
   }
 
+  func testUnexpectedStateChangeOnSendFailure() {
+    struct State: Equatable { var count = 0 }
+    let store = TestStore(
+      initialState: .init(),
+      reducer: Reducer<State, Void, Void> { state, action, _ in state.count += 1
+        return .none
+      },
+      environment: ()
+    )
+
+    XCTExpectFailure {
+      store.send(())
+    } issueMatcher: {
+      $0.compactDescription == """
+        State was not expected to change, but a change occurred: …
+
+            − TestStoreFailureTests.State(count: 0)
+            + TestStoreFailureTests.State(count: 1)
+
+        (Expected: −, Actual: +)
+        """
+    }
+  }
+
+  func testUnexpectedStateChangeOnReceiveFailure() {
+    struct State: Equatable { var count = 0 }
+    enum Action { case first, second }
+    let store = TestStore(
+      initialState: .init(),
+      reducer: Reducer<State, Action, Void> { state, action, _ in
+        switch action {
+        case .first: return .init(value: .second)
+        case .second: state.count += 1; return .none
+        }
+      },
+      environment: ()
+    )
+
+    store.send(.first)
+    XCTExpectFailure {
+      store.receive(.second)
+    } issueMatcher: {
+      $0.compactDescription == """
+        State was not expected to change, but a change occurred: …
+
+            − TestStoreFailureTests.State(count: 0)
+            + TestStoreFailureTests.State(count: 1)
+
+        (Expected: −, Actual: +)
+        """
+    }
+  }
+
   func testReceivedActionAfterDeinit() {
     XCTExpectFailure {
       do {