Simple function with conditional type

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The short a answer is you can’t. No value will be assignable to an unresolved conditional type (a conditional type that still depends on a free generic type variable). The only thing you can do is use a type assertion. 

function test<T extends boolean>(a: T): T extends true ? string : number {
  return (a ? '1' : 1)  as any
}

Conditional types are useful to express relations between parameters but they don’t help when it comes to implementing the function. Another approach would be to use a more permissive implementation signature. 

function test<T extends boolean>(a: T): T extends true ? string : number
function test(a: boolean): number | string {
    return (a ? '1' : 1)
}

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