Flow argument variance with object's enum value

example.flow.js

// @flow
// https://flow.org/try/#0PTAEAEDMBsHsHcBQiAuBPADgU1AUQHYCuAtgIIDGKAlrPqALygDkAhk6AD7MBG7XT5JgG5k6bKACS+AG4sATlRb4UFarQagA3h0ShQxLCgAWsACYAuPETKUa+ADSIOAXxGpMOAMKxZCpStt1Rm1dUABqA2MzSwISVTtHFzdIQnxAulNYKV9FZQAVIyp8AHMAChZ0y2z5XIC1fABKLWdEFLT60EzvHP8CorKK+stumv942ibNFtEPUDyPAHluACsNbX1DEwtmNk5XRHJaAGcUUGN+y3nsJdXgjajt1nZ9xEzqv3zCktLzkoaRLo+Uaffo-L7Ff6IIA

type EnumAction = 'a' | 'b' | 'c';

type InvariantAction = {|
  method: EnumAction,
|};

type CovariantAction = {|
  +method: EnumAction,
|};

function doInvariantThing(action: InvariantAction) {}
function doCovariantThing(action: CovariantAction) {}


type TypeObj = {| method: 'a' |};
const thing: TypeObj = { method: 'a' };

doInvariantThing(thing);
doCovariantThing(thing);

// 20: doInvariantThing(thing);
//                      ^ Cannot call `doInvariantThing` with `thing` bound to `action` because string literal `a` [1] is incompatible with string literal `b` [2] in property `method`. [incompatible-call]
// References:
// 17: type TypeObj = {| method: 'a' |};
//                               ^ [1]
// 6:   method: EnumAction,
//              ^ [2]
// 20: doInvariantThing(thing);
//                      ^ Cannot call `doInvariantThing` with `thing` bound to `action` because string literal `a` [1] is incompatible with string literal `c` [2] in property `method`. [incompatible-call]
// References:
// 17: type TypeObj = {| method: 'a' |};
//                               ^ [1]
// 6:   method: EnumAction,
//              ^ [2]