Flow's predicate functions' quirk (checked on flow-bin v0.120.1)

data.js

// @flow

export type A = 'A';

export type B = 'B';

export type T = A | B;

export function isA(data: T): %checks {
  return data === 'A';
}

export function isB(data: T): %checks {
  return data === 'B';
}

export const Predicates = {
  isA,
  isB,
};

user.js

// @flow

import * as Data from './data';
import { isA } from './data';

const x: Data.T = 'A';

// Works as expected.
if (isA(x)) {
  (x: Data.A);
} else {
  (x: Data.B);
}

// But Namespace-imported predicates don't refine types.
if (Data.isA(x)) {
  // $FlowFixMe
  (x: Data.A);
} else {
  // $FlowFixMe
  (x: Data.B);
}

// Type refinement ability revives by aliasing predicates (why???)
const isA_ = Data.isA;
if (isA_(x)) {
  (x: Data.A);
} else {
  (x: Data.B);
}