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Inconsistent enum type merging #32621
Description
TypeScript Version: typescript@3.6.0-dev.20190730 (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)
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.
Activity
jack-williams commentedon Jul 30, 2019
This regressed between 3.0.1 -> 3.1.6.
And to add some keywords for searching:
enum subtype reduction
getUnionType
RyanCavanaugh commentedon Jul 30, 2019
Does this have observable effects other than quick info? I don't think it does?
jack-williams commentedon Jul 30, 2019
I guess this should only have observable effects up to the extent that
SomeEnumandSomeEnum.APPLE | SomeEnum.ORANGE | SomeEnum.BANANAare distinguishable.RyanCavanaugh commentedon Aug 7, 2019
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.