When evaluating the conditional type in both cases, it selects the true branch because T extends any[] is true. The distinction lies in how TypeScript infers the generic type argument T when calling either func1() or func2(). In both instances, the argument passed is [1, 2, 3], and the compiler has the flexibility to infer anything compatible with that argument. For func1(), it infers the tuple type [number, number, number], while for func2() it infers the unordered arbitrary-length array type number[]. Both inferences are valid, but different heuristics influence the preference.
The specific workings of this inference process are not extensively documented, so any explanation provided here serves more as an illustration rather than a completely accurate depiction of the process.
TypeScript 4.0 brought about variadic tuple types through microsoft/TypeScript#39094, which allow defining a preference for tuple type inference simply by using [...T] where T is an array-like type parameter.
By defining functions like
<T extends any[]>(t: [...T]) => t
, tuples are favored, whereas
<T extends any[]>(t: T) => t
leans towards arrays.
Moreover, with
<T,>(t: T extends any[] ? [...T] : never) => t
, preferece is shown for tuples. However,
<T,>(t: T extends any[] ? [...T] : T) => t
does not exhibit the same preference due to interference caused by the presence of
T. The exact nature of this interference remains unclear, with speculation pointing towards the "strength" of inference candidates affecting the outcome. Nonetheless, playing around with variadic tuples and conditional types can lead to significant inference challenges.
In conclusion, delving into the details of how inference of variadic tuples works through conditional types might shed light on these complexities in the future. Meanwhile, avoiding intricate manipulations involving variadic tuples and conditional types can simplify the inference process. Opting for const type parameters like <const T,>(t: T) => t could be a smoother alternative for inferring tuples in certain scenarios, although its applicability should be assessed based on individual use cases.
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