Fixes #8657: Handles union typed React component.

[evansb]

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Fixes #8657

This pull request handles special case of union typed React component, namely type U = React.StatelessComponent<any> | React.ComponentClass<any>. Since StatelessComponent has call but no signature and ComponentClass has no call signature but no construct signature, the union between these two will result in a type with no call or construct signature.

Previously <U /> will fail because the type checker cannot find any call/construct signature on it, which is a false negative.

This patch handles union typed React component separately: we first unfold the type of U and apply the old construct/call signature + return type check.

This involves refactoring JSX Element type resolver to a new function getResolvedJsxType so that it can be applied recursively to each member of the union types.

Evan Sebastian.

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[RyanCavanaugh]

This doesn't seem like the right approach. What if you had a property of type string | ComponentClass ? It shouldn't be legal to use that as a React component.

[evansb]

You are right, I should've raised the error as soon as a type doesn't have constructor/call signature instead.

Anyway this is still pretty much WIP and doesn't solve the issue quite yet, we will have another problem with the different return type of StatelessComponent call and ComponentClass constructor.

At the moment StatelessComponent return type is React.Element. It isn't related with the current ElementClass, which fails to type check when you try to render it with JSX tags.

[evansb]

I think I'm done, would you mind reviewing it? @RyanCavanaugh

See description for patch summary

[RyanCavanaugh]

Can you split out the test so there's a non-erroring test that shows the behavior you're trying to enable? When there's an error we don't generate a type baseline so it's harder to confirm the right thing is happening.

[RyanCavanaugh]

👍

[mhegazy]

thanks!