Add formik to the user test suite

[DanielRosenwasser]

It seems that #20891 affects users of Formik, a React forms library.

• Compilation error with TypeScript in strict mode on jaredpalmer/formik#283
• FormikConfig has a generic type of an empty object jaredpalmer/formik#314

It'd be good to have
some extra coverage https://github.com/jaredpalmer/formik

[weswigham]

@DanielRosenwasser Do you have an actual TS example project for that library? I don't think the reported issue surfaces when just checking the .d.ts file.

[DanielRosenwasser]

I think both of the two examples will error

// (1) - explicit parameter annotation
interface FormTypes { firstName: string }
let el1 = <Formik
    initialValues={{ firstName: "Daniel"}}
    onSubmit={(values: FormTypes) => console.log("First name", values.firstName)} />


// (2) - parameter should be inferred, ends up as {}
let el2 = <Formik
    initialValues={{ firstName: "Daniel"}}
    onSubmit={values => console.log("First name", values.firstName)} />

[sandersn]

If the only value of adding formik is as an end-to-end equivalent to the test for a fix to #20891, then I don't think it's worth it. Are there other properties of formik as a dependency that make it useful?

[jaredpalmer]

@sandersn IMHO this is more of a DX thing. On the one hand, adding tests for Apollo, Formik, etc might seem redundant. On the other hand, seems like a good idea to guarantee that the most popular libraries on GitHub are always compatible with the latest version of TypeScript (and are using the latest and greatest TS features).

[sandersn]

Good point. I do think that we’ll need more code than our typical import x from "library" for this test.

[mhegazy]

@sandersn are the two issues (jaredpalmer/formik#283 and jaredpalmer/formik#314) fixed in master now? or we are still breaking formik?

[mhegazy]

@weswigham can we ensure formik runs clean on master after #20995 is merged.