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Exhaustive switch return type is incorrect without intermediate variable #34661
Description
TypeScript Version: 3.7.0-beta
Search Terms: enum switch
Code
enum Animal {
DOG,
// Add second member to avoid https://github.com/microsoft/TypeScript/issues/23572.
CAT
}
const zoo: { animal: Animal } | undefined = { animal: Animal.DOG }
// OK.
function intermediateVariable(): Animal {
const animal = zoo?.animal ?? Animal.DOG
switch (animal) {
case Animal.DOG: return Animal.DOG
case Animal.CAT: return Animal.CAT
}
}
// Error: Function lacks ending return statement and return type does not include 'undefined'.(2366)
function expression(): Animal {
switch (zoo?.animal ?? Animal.DOG) {
case Animal.DOG: return Animal.DOG
case Animal.CAT: return Animal.CAT
}
}Expected behavior: Inferred type doesn't depend on intermediate state.
Actual behavior: Inferred type differs when an expression is switched on directly versus when the value is assigned to an untyped intermediate variable and then switched on.
Activity
ahejlsberg commentedon Oct 30, 2019
This issue is an effect of switching to the control flow analyzer for exhaustiveness checking. We need to unconditionally include CFA nodes representing a missing default clauses in order for our analysis to be correct. Currently we only include such CFA nodes when the
switchexpression is a narrowing expression.