[test] Test types of .spec lab files (#25684)

packages/material-ui-lab/src/DatePicker/DatePicker.spec.tsx

@@ -1,8 +1,7 @@
 import * as React from 'react';
 import moment, { Moment } from 'moment';
 import DatePicker from '@material-ui/lab/DatePicker';
-import AdapterDateFns from '../AdapterDateFns';
-import MomentAdapter from '../AdapterMoment';
+import { expectType } from '@material-ui/types';
 
 // Allows to set date type right with generic JSX syntax
 <DatePicker<Date>
@@ -41,17 +40,26 @@ const InferTest = () => {
   renderInput={() => <input />}
 />;
 
-// Edge case and known issue. When the passed `value` is not a date type
-// We cannot infer the type properly without explicit generic type or `dateAdapter` prop
-// So in this case it is expected that type will be the type of `value` as for now
-<DatePicker
-  value={null}
-  onChange={(date) =>
-    // getDate is any
-    date?.getDate()
-  }
-  renderInput={() => <input />}
-/>;
+// TypeScript can't know the type of the DateAdapter in the React context.
+// So in this case it is expected that type will be the type of `value` as for now.
+// Argueable, this usage doesn't make sense since the component would never reflect the user picked value.
+{
+  <DatePicker
+    value={null}
+    onChange={(date) => {
+      expectType<null, typeof date>(date);
+    }}
+    renderInput={() => <input />}
+  />;
+  // workaround
+  <DatePicker<Date>
+    value={null}
+    onChange={(date) => {
+      expectType<Date | null, typeof date>(date);
+    }}
+    renderInput={() => <input />}
+  />;
+}
 
 {
   <DatePicker

packages/material-ui-lab/tsconfig.json

@@ -4,6 +4,5 @@
     // @date-io libraries produce duplicate type `DateType`
     "skipLibCheck": true
   },
-  "include": ["src/**/*", "test/**/*"],
-  "exclude": ["./**/*.spec.*"]
+  "include": ["src/**/*", "test/**/*"]
 }