Flow data flow analysis loses info under a lambda

flow-lambda-bug.js

/* @flow */

/* Paste this into https://flow.org/try/ to see the bug: */

const f = (lut: ?{[key: string]: number}): number => {
  
  // This correctly notices that we're doing a null check on lut:
  const testKey = 'fizz'
  const y = (lut != null) ? lut[testKey] : 0	
  
  // Putting the access of lut under a lambda (the arg to map)
  // loses this info:
  const keyList = ['foo','fee','foe'] 
  const zs = (lut != null) ? keyList.map(k => lut[k]) : []
  
  return y
}

Actually this may just be Flow being overly conservative --
See:
https://flow.org/en/docs/lang/refinements/

And, indeed, a workaround is to change first two lines to:

const f = (inLut: ?{[key: string]: number}): number => {
  const lut = inLut

and then it all works.