Possible TypeScript bug

Reproduction

Reproduce with nix

enter a nix shell

nix-shell

follow Reproduce with global npm + node

Reproduce with global npm + node

npx tsc

Outcome

TypeScript will tell you:

testcase.ts:3:39 - error TS7030: Not all code paths return a value.

3 export function f1(q: Q | undefined): string {
                                        ~~~~~~


Found 1 error.