Add strong return type to 'invoke' command (cypress-io#4022)

cli/types/index.d.ts

@@ -811,14 +811,10 @@ declare namespace Cypress {
 
  /**
  * Invoke a function on the previously yielded subject.
- * This isn't possible to strongly type without generic override yet.
- * If called on an object you can do this instead: `.then(s => s.show())`.
- * If called on an array you can do this instead: `.each(s => s.show())`.
- * From there the subject will be properly typed.
  *
  * @see https://on.cypress.io/invoke
  */
- invoke(functionName: keyof Subject, ...args: any[]): Chainable<Subject> // don't have a way to express return types yet
+ invoke<K extends { [K in keyof Subject]: Subject[K] extends Function ? K : never }[keyof Subject], R extends Subject[K] extends (...args: any[]) => infer R ? R : never>(functionName: K, ...args: any[]): Chainable<R>;
 
  /**
  * Get a property’s value on the previously yielded subject.

cli/types/tests/chainer-examples.ts

@@ -301,6 +301,9 @@ cy.wrap('foobar').should('not.have.string', 'baz')
 
 cy.wrap('foobar').should('not.include', 'baz')
 
+cy.wrap({ foo: (a:string) => 1, bar: 2 }).invoke('foo').should('equal', 1)
+// cy.wrap({ foo: () => 1, bar: 2 }).invoke('bar') // compile ERROR
+
 ;
 () => {
  let val = 1