Summary

Reorder the overload declarations forreduce andreduceRight in bothReadonlyArray<T> andArray<T> interfaces so the generic<U> overload (withinitialValue: U) is checked before the same-type overload (withinitialValue: T).

Problem

When callingreduce 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 theinitialValue: 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 aninitialValue parameter:

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

The same change is applied to bothreduce andreduceRight in bothReadonlyArray<T> andArray<T>.

Fixes#7014