CFA: control flow for switch is defeated by outer if-else

[ghost]

TypeScript Version: nightly (2.1.0-dev.20161012)

Code

enum E { A, B }

function f(e: E): number {
    switch (e) {
        case E.A: return 0
        case E.B: return 1
    }
}

function g(e: E): number {
    if (!true)
        return -1
    else
        switch (e) {
            case E.A: return 0
            case E.B: return 1
        }
}

Expected behavior:

Both functions succeed.

Actual behavior:

a.ts(10,19): error TS2366: Function lacks ending return statement and return type does not include 'undefined'.

If I remove the else keyword (which makes an equivalent program), it compiles without errors.

If I put an e access under the switch, I see (parameter) e: never on hover, so that part is working.

[mhegazy]

This is working as intended. the two checks, all-code-paths-result-in-explicit-return and exhaustive-switch-with-respect-to-type happen in two different phases, once in the binder and the other in the checker. we special case one, and only one case where the switch statement is the last statement in the function body and skip the error if it is exhaustive, see functionHasImplicitReturn.