As demonstrated in this example using TypeScript:
type FirstOrSecond<condition, T1, T2> = condition extends never ? T1 : T2
type foo = never extends never ? () => 'hi' : (arg1: never) => 'hi'
type wtf = FirstOrSecond<never, () => 'hi', (arg1: never) => 'hi'>
The generic function FirstOrSecond assigns never to the wtf variable, while the specific definition of foo assigns () => 'hi' as the type.
What causes the generic function to behave unexpectedly?
Understanding distributive conditional types. It's all by design.
A common workaround involves enclosing types in a single-member tuple within the condition:
type EitherOr<condition, X, Y> = [condition] extends [never] ? X : Y
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