Flow: incorrect "Missing type annotation for `U`." flow error triggered by adding `export`.

[DianaSuvorova]

Here is a try flow

/* @flow */

type Thing = {
  prop: string
};

// no errors 
function myFunction(things: Array<Thing>) {
  return things.map(a => a);
}

// Missing type annotation for `U`. error
export function myFunction(things: Array<Thing>) {
  return things.map(a => a);
}

[swac]

I think it's referring to the generic U in map's definition: https://github.com/facebook/flow/blob/master/lib/core.js#L247.

Changing the call to things.map<Thing>(a =>a) or specifying a return type of Array<Thing> for myFunction seems to fix it. It does seem weird that it only shows the error when the export is present though.

[mdugue]

got this too recently, not sure what that means

[ujwal-setlur]

me too

[jbrown215]

Flow requires annotations on types in input positions reachable through exports. Until now, we were missing a lot of these cases. This is an example of a case we started catching recently. Even more recent changes, which will be shipped in v0.81.0, will hopefully make these errors clearer. I am also currently working on documentation for fixing these new errors when they appear.

[dnalborczyk]

just got this exception. for anyone else, explained here in more detail: https://medium.com/flow-type/asking-for-required-annotations-64d4f9c1edf8