Function lacks ending return statement and return type does not include 'undefined'

🔎 Search Terms

switch default

🕗 Version & Regression Information

The tsc complains that the the function return undefined if a add a default case, actually the default case will never be executed, just only for exhausive check.

⏯ Playground Link

https://www.typescriptlang.org/play?jsx=0&ts=5.4.4#code/C4TwDgpgBAyglgL2gXigcgDYEMBOBzCNKAH3QDsB7HAWywyNLQGdaN6T1q4y40AoAMYUyTYFCaIUUAN4BfKFiaxJg4aKgAjDBQEBrKKgA8AJQB8ACgBmALijmAlAdNRjj5M+l8o3qDgjAAVxwyKEsHPlk+VRExASYlVC0dXXMHWzIA6g0IHCcZLx8mAHc4YAEACzsJJEdPH3qoAUVoNEoaOjRbOoaev0DgqABGAp7Inu8mphbsfEIukfHvPqCQgCYF+rHxyZauHk78xfrlgYBaQagNny2enfQWOnp5o58TkIAGK+8bhoATCEsWACGGAzxejTUYjI6QgADccgZxCpFj9IrJ7EA

💻 Code

type Size = 'large' | 'normal' | 'small' | 'mini'
const size = {} as Size
const block = <R>(f: () => R) => {
    return f()
}

const css = block((): number => {
    switch (size) {
        case 'normal': {
            return 1
        }
        case 'large': {
            return 2
        }
        case 'mini': {
            return -1 
        }
        case 'small': {
            return 0
        }
        default: {
            const n: never = size
        }
    }
})

🙁 Actual behavior

The tsc think that the function of css block returns number | undefined.

🙂 Expected behavior

The tsc should know that the default case is never returned, and the function of css block should returns number

Additional information about the issue

No response

Start with the linked TypeScript Playground reproduction and the switch statement in the issue body. Trace how the type checker handles the exhaustive default branch and the enclosing arrow function's return analysis. Done means the example's inferred return type is number rather than number | undefined, with regression coverage for this behavior.