fix(lib): reorder Array#reduce/reduceRight overloads in lib.d.ts

[taronsung]

Summary

Reorder the overload declarations for reduce and reduceRight in both ReadonlyArray<T> and Array<T> interfaces so the generic <U> overload (with initialValue: U) is checked before the same-type overload (with initialValue: T).

Problem

When calling reduce with an initial value of a different type than the array elements, TypeScript incorrectly infers the return type. This is because TypeScript checks overloads in declaration order, and the initialValue: T overload matches before the generic <U> overload gets a chance.

Example from #7014:

typeUnaryFunction=(arg1: any)=>any;typeBinaryFunction=(arg1: any,arg2: any)=>any;letbinaryFuncs: BinaryFunction[]=[];letunaryFunc: UnaryFunction=arg1=>{};letreduced=binaryFuncs.reduce((prev,next)=>prev,unaryFunc);// BEFORE: Error - Type 'BinaryFunction' is not assignable to type 'UnaryFunction'// AFTER: Works correctly - reduced is inferred as UnaryFunctionletf: UnaryFunction=reduced;

Solution

Swap the order of the two overloads that take an initialValue parameter:

• Before: reduce(cb, initialValue: T) then reduce<U>(cb, initialValue: U)
• After: reduce<U>(cb, initialValue: U) then reduce(cb, initialValue: T)

The same change is applied to both reduce and reduceRight in both ReadonlyArray<T> and Array<T>.

Fixes#7014

[taronsung]

@microsoft-github-policy-service agree

[RyanCavanaugh]

@typescript-bot test it

[typescript-bot]

Hey @RyanCavanaugh, this PR is in an unmergable state, so is missing a merge commit to run against; please resolve conflicts and try again.

[vrjordant]

Is there something that needs to be done before this can be merged?