Recreate unkeyed functional components when they change position.

With keyed children we can confidently track them as they move
throughout an array of components. When this happens with unkeyed
functional components we should not risk reusing the state as that
could possibly make us re-use state in unrelated components, as
seen in #2949.

src/diff/children.js

@@ -398,8 +398,9 @@ function findMatchingIndex(
 	// if the oldVNode was null or matched, then there could needs to be at least
 	// 1 (aka `remainingOldChildren > 0`) children to find and compare against.
 	let shouldSearch =
+		(typeof type !== 'function' || type === Fragment || key) &&
 		remainingOldChildren >
-		(oldVNode != null && (oldVNode._flags & MATCHED) === 0 ? 1 : 0);
+			(oldVNode != null && (oldVNode._flags & MATCHED) === 0 ? 1 : 0);
 
 	if (
 		oldVNode === null ||

test/browser/render.test.js

@@ -1786,6 +1786,37 @@ describe('render()', () => {
 		);
 	});
 
+	// #2949
+	it('should not swap unkeyed chlildren', () => {
+		class X extends Component {
+			constructor(props) {
+				super(props);
+				this.name = props.name;
+			}
+			render() {
+				return <p>{this.name}</p>;
+			}
+		}
+
+		function Foo({ condition }) {
+			return (
+				<div>
+					{condition ? '' : <X name="A" />}
+					{condition ? <X name="B" /> : ''}
+				</div>
+			);
+		}
+
+		render(<Foo />, scratch);
+		expect(scratch.textContent).to.equal('A');
+
+		render(<Foo condition />, scratch);
+		expect(scratch.textContent).to.equal('B');
+
+		render(<Foo />, scratch);
+		expect(scratch.textContent).to.equal('A');
+	});
+
 	it('handle shuffled (stress test)', () => {
 		function randomize(arr) {
 			for (let i = arr.length - 1; i > 0; i--) {