But with numeric enums, you can pass a raw number in places where the enum is expected. What?

If you're desperate to use enums, I'd strongly recommend using string enums only.

const renderMapping: { [l in letters]: renderFunction<l>; } = { 'a': (a: 'a') => 'alpha', 'b': (b: 'b') => 'bravo', }; type renderFunction<l extends letters> = (letter: l) => string; function renderLetter<l extends letters>(letter: l): renderFunction<l> { return renderMapping[letter]; }