Consider re-ordering Array#reduce overloads in lib.d.ts

Example code:

type UnaryFunction = (arg1) => any;
type BinaryFunction = (arg1, arg2) => any;

let binaryFuncs: BinaryFunction[] = [];
let unaryFunc = arg1 => {};
let reduced = binaryFuncs.reduce((prev, next) => prev, unaryFunc);

// ACTUAL:
let f: UnaryFunction = reduced;     // ERROR binary not assignable to unary

// EXPECTED:
let f: UnaryFunction = reduced;     // OK - both lhs and rhs really are unary

The call to Array#reduce in the above example definitely returns a unary function, but the type system erroneously infers the return type as a binary function.

This seems to be caused by the declaration order of the two overloads of Array#reduce in lib.d.ts. If the declaration order is reversed, the problem is solved.

The two overloaded declarations in lib.d.ts are as follows:

reduce(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T, initialValue?: T): T;
reduce<U>(callbackfn: (previousValue: U, currentValue: T, currentIndex: number, array: T[]) => U, initialValue: U): U;

The first overload matches the example with T=BinaryFunction, since it satisfies the compiler's assignability checks. So the second overload is never considered, even though it is a strictly better match, with T=BinaryFunction and U=UnaryFunction.

Would it be possible to swap the overload order for Array#reduce (and Array#reduceRight) in lib.d.ts to resolve this issue?

Bắt đầu trong lib.d.ts bằng cách xác định hai overload của Array#reduce và các overload tương ứng của Array#reduceRight. Tái hiện ví dụ trong issue, sau đó xác minh rằng việc đảo ngược thứ tự các overload khiến kết quả được suy luận có thể gán cho UnaryFunction đối với cả hai phương thức.