Exhaustive switch return type is incorrect without intermediate variable

[niedzielski]

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.

Playground Link

Related Issues: #6155 #32905

[ahejlsberg]

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 switch expression is a narrowing expression.