Throw away old shallow renderer state on type change

This worked in function components but was broken for classes. It incorrectly retained the old instance even if the type was different.

packages/react-test-renderer/src/ReactShallowRenderer.js

@@ -178,6 +178,10 @@ class ReactShallowRenderer {
   };
 
   constructor() {
+    this._reset();
+  }
+
+  _reset() {
     this._context = null;
     this._element = null;
     this._instance = null;
@@ -514,6 +518,9 @@ class ReactShallowRenderer {
     if (this._rendering) {
       return;
     }
+    if (this._element != null && this._element.type !== element.type) {
+      this._reset();
+    }
 
     const elementType = isMemo(element.type) ? element.type.type : element.type;
     const previousElement = this._element;

packages/react-test-renderer/src/__tests__/ReactShallowRenderer-test.js

@@ -1565,4 +1565,46 @@ describe('ReactShallowRenderer', () => {
       'forwardRef requires a render function but was given object.',
     );
   });
+
+  it('should let you change type', () => {
+    function Foo({prop}) {
+      return <div>Foo {prop}</div>;
+    }
+    function Bar({prop}) {
+      return <div>Bar {prop}</div>;
+    }
+
+    const shallowRenderer = createRenderer();
+    shallowRenderer.render(<Foo prop="foo1" />);
+    expect(shallowRenderer.getRenderOutput()).toEqual(<div>Foo {'foo1'}</div>);
+    shallowRenderer.render(<Foo prop="foo2" />);
+    expect(shallowRenderer.getRenderOutput()).toEqual(<div>Foo {'foo2'}</div>);
+    shallowRenderer.render(<Bar prop="bar1" />);
+    expect(shallowRenderer.getRenderOutput()).toEqual(<div>Bar {'bar1'}</div>);
+    shallowRenderer.render(<Bar prop="bar2" />);
+    expect(shallowRenderer.getRenderOutput()).toEqual(<div>Bar {'bar2'}</div>);
+  });
+
+  it('should let you change class type', () => {
+    class Foo extends React.Component {
+      render() {
+        return <div>Foo {this.props.prop}</div>;
+      }
+    }
+    class Bar extends React.Component {
+      render() {
+        return <div>Bar {this.props.prop}</div>;
+      }
+    }
+
+    const shallowRenderer = createRenderer();
+    shallowRenderer.render(<Foo prop="foo1" />);
+    expect(shallowRenderer.getRenderOutput()).toEqual(<div>Foo {'foo1'}</div>);
+    shallowRenderer.render(<Foo prop="foo2" />);
+    expect(shallowRenderer.getRenderOutput()).toEqual(<div>Foo {'foo2'}</div>);
+    shallowRenderer.render(<Bar prop="bar1" />);
+    expect(shallowRenderer.getRenderOutput()).toEqual(<div>Bar {'bar1'}</div>);
+    shallowRenderer.render(<Bar prop="bar2" />);
+    expect(shallowRenderer.getRenderOutput()).toEqual(<div>Bar {'bar2'}</div>);
+  });
 });