In this repository:

src/Lib.js

// @flow
export function f<+T: { x?: "x" }>(a: T => any, b: $Rest<T, {| x?: "x" |}>) {}

src/lib2.js (identical to src/Lib.js)

// @flow
export function f<+T: { x?: "x" }>(a: T => any, b: $Rest<T, {| x?: "x" |}>) {}

src/foo.js (defines a function to be passed in as a to f)

// @flow
export const Foo = ({}: {| foo: string |}) => {};

src/main.js (imports both versions of f, and attempts to call both versions with 'Foo' and 'LocallyDefinedFoo')

// @flow
import * as Lib from "./Lib.js";
import * as lib2 from "./lib2.js";
import { Foo } from "./foo";

const LocallyDefinedFoo = ({}: {| foo: string |}) => {};

Lib.f(Foo, { foo: "hi" });
lib2.f(Foo, { foo: "hi" });
Lib.f(LocallyDefinedFoo, { foo: "hi" });
lib2.f(LocallyDefinedFoo, { foo: "hi" });

Expected behavior:

All four invocations, Lib.f(Foo, ...), lib2.f(Foo, ...), Lib.f(LocallyDefinedFoo, ...), and lib2.f(LocallyDefinedFoo, ...), should either all produce a type error, or all produce no type error.

Observed behavior

Only Lib.f(Foo, ...) produces a type error. The reason for this seems to have something to do with the filename being capitalized -- if I rename the file to something lower case, the type error goes away.

I am running on Mac OS X which has a case insensitive filesystem, which I initially suspected was important -- but the same behavior appears to manifest inside a linux docker container.

$ node_modules/.bin/flow check
Error ┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈ src/main.js:8:12

Cannot call Lib.f with object literal bound to b because property x is missing in object type [1] but exists in object
literal [2].

     src/main.js
      5│
      6│ const LocallyDefinedFoo = ({}: {| foo: string |}) => {};
      7│
 [2]  8│ Lib.f(Foo, { foo: "hi" });
      9│ lib2.f(Foo, { foo: "hi" });
     10│ Lib.f(LocallyDefinedFoo, { foo: "hi" });
     11│ lib2.f(LocallyDefinedFoo, { foo: "hi" });

     src/foo.js
 [1]  2│ export const Foo = ({}: {| foo: string |}): void => {};

Found 1 error

To reproduce

git clone git@github.com:richardm-stripe/flow-casesensitive
cd flow-casesensitive
yarn
yarn flow check

To reproduce w/ Docker

docker build . -t flowbug
docker run flowbug