Inconsistent enum type merging

[zenorbi]

TypeScript Version: [email protected] (same with 3.2.4, 3.5.2, 3.5.3)

Search Terms: enum hover ternary union merging
enum subtype reduction
getUnionType

Code

enum SomeEnum {
  APPLE = 'APPLE',
  ORANGE = 'ORANGE',
  BANANA = 'BANANA'
}

function test(inputEnumMember: SomeEnum) {
  // Correctly gives `const a: SomeEnum`
  const a: SomeEnum.BANANA | SomeEnum = inputEnumMember;

  // Incorrectly gives `const b: SomeEnum.APPLE | SomeEnum`
  const b = Math.random() < 0.5 ? SomeEnum.APPLE : inputEnumMember;

  // Incorrectly gives `const c: SomeEnum.APPLE | SomeEnum` (really weird)
  const c = Math.random() < 0.5 ? SomeEnum.ORANGE : inputEnumMember;

  // Incorrectly gives `const d: SomeEnum.BANANA | SomeEnum`
  const d = Math.random() < 0.5 ? SomeEnum.BANANA : inputEnumMember;
}

Expected behavior:
All declared variables have the type SomeEnum or at least are consistently "wrong" (see const c as showing APPLE)

Playground Link:
https://www.typescriptlang.org/play/#code/KYOwrgtgBAyg9hYBRc0DeAoKUCCAFPAGSSgF4oByfIpCgGiygHkAlHAOQHETyLWPu9RgCEOYspVHsxFDAF8MGAGZgQAYwAuASzggoG4AGcNACi0gADmA0pIAWWAQARsABOALlgJkqAJRRMbAB6IKgAYThXV2BNABsATygAcy0ANyMoAAM1XWMoAENPeERbCEzGHJA8wq8S1AA6KXEAH1qfSAlzKxtUB2c3AG5FYNCASXVI6LjElPTDLMq8pyLvUvrqYihW4vayityNKCcJO3yNAAt613yQABMEE38AHigABnqAVigAfja1jZIni61lKfRcriGjBCUHGOSiMQ0CWSaQy2QOUDUKzqkHWBE221WqEyUBMAHdgFpXLdfPsqoc1CczpdrncHs83p8fn8GvwuICoMCevZHODISMYRN4dNkXMFujbljdo0xNIttzIOVsItDrdGRcrjd7hBHlAXu8vr8dmsmqqgZYQb0RYN5BggA

This is a really minor issue as it only applies to hover type information and the reported value while being weird isn't breaking anything.

[jack-williams]

This regressed between 3.0.1 -> 3.1.6.

And to add some keywords for searching:

enum subtype reduction
getUnionType

[RyanCavanaugh]

Does this have observable effects other than quick info? I don't think it does?

[jack-williams]

I guess this should only have observable effects up to the extent that SomeEnum and SomeEnum.APPLE | SomeEnum.ORANGE | SomeEnum.BANANA are distinguishable.

[RyanCavanaugh]

Exactly when to perform subtype reduction is tricky business; there are some compromises we have to make in order for other scenarios to work. "Consistent" type display between different ways of generating observably-same types isn't something we can guarantee.