Reorder Array#reduce overloads for better type inference (#7014)

The generic overload `reduce<U>(callbackfn, initialValue: U): U` is now
placed before `reduce(callbackfn, initialValue: T): T` to ensure better
type inference when the initial value type differs from the array element type.

This fixes cases where `U` is assignable to `T` (e.g., `number` assignable
to `{}`), which previously caused the non-generic overload to match incorrectly.

Fixes #7014

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: majiayu000

src/lib/es5.d.ts

@@ -1272,27 +1272,27 @@ interface ReadonlyArray<T> {
      * @param callbackfn A function that accepts up to four arguments. The reduce method calls the callbackfn function one time for each element in the array.
      * @param initialValue If initialValue is specified, it is used as the initial value to start the accumulation. The first call to the callbackfn function provides this value as an argument instead of an array value.
      */
-    reduce(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: readonly T[]) => T): T;
-    reduce(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: readonly T[]) => T, initialValue: T): T;
+    reduce<U>(callbackfn: (previousValue: U, currentValue: T, currentIndex: number, array: readonly T[]) => U, initialValue: U): U;
     /**
      * Calls the specified callback function for all the elements in an array. The return value of the callback function is the accumulated result, and is provided as an argument in the next call to the callback function.
      * @param callbackfn A function that accepts up to four arguments. The reduce method calls the callbackfn function one time for each element in the array.
      * @param initialValue If initialValue is specified, it is used as the initial value to start the accumulation. The first call to the callbackfn function provides this value as an argument instead of an array value.
      */
-    reduce<U>(callbackfn: (previousValue: U, currentValue: T, currentIndex: number, array: readonly T[]) => U, initialValue: U): U;
+    reduce(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: readonly T[]) => T): T;
+    reduce(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: readonly T[]) => T, initialValue: T): T;
     /**
      * Calls the specified callback function for all the elements in an array, in descending order. The return value of the callback function is the accumulated result, and is provided as an argument in the next call to the callback function.
      * @param callbackfn A function that accepts up to four arguments. The reduceRight method calls the callbackfn function one time for each element in the array.
      * @param initialValue If initialValue is specified, it is used as the initial value to start the accumulation. The first call to the callbackfn function provides this value as an argument instead of an array value.
      */
-    reduceRight(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: readonly T[]) => T): T;
-    reduceRight(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: readonly T[]) => T, initialValue: T): T;
+    reduceRight<U>(callbackfn: (previousValue: U, currentValue: T, currentIndex: number, array: readonly T[]) => U, initialValue: U): U;
     /**
      * Calls the specified callback function for all the elements in an array, in descending order. The return value of the callback function is the accumulated result, and is provided as an argument in the next call to the callback function.
      * @param callbackfn A function that accepts up to four arguments. The reduceRight method calls the callbackfn function one time for each element in the array.
      * @param initialValue If initialValue is specified, it is used as the initial value to start the accumulation. The first call to the callbackfn function provides this value as an argument instead of an array value.
      */
-    reduceRight<U>(callbackfn: (previousValue: U, currentValue: T, currentIndex: number, array: readonly T[]) => U, initialValue: U): U;
+    reduceRight(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: readonly T[]) => T): T;
+    reduceRight(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: readonly T[]) => T, initialValue: T): T;
 
     readonly [n: number]: T;
 }
@@ -1467,27 +1467,27 @@ interface Array<T> {
      * @param callbackfn A function that accepts up to four arguments. The reduce method calls the callbackfn function one time for each element in the array.
      * @param initialValue If initialValue is specified, it is used as the initial value to start the accumulation. The first call to the callbackfn function provides this value as an argument instead of an array value.
      */
-    reduce(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T): T;
-    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;
     /**
      * Calls the specified callback function for all the elements in an array. The return value of the callback function is the accumulated result, and is provided as an argument in the next call to the callback function.
      * @param callbackfn A function that accepts up to four arguments. The reduce method calls the callbackfn function one time for each element in the array.
      * @param initialValue If initialValue is specified, it is used as the initial value to start the accumulation. The first call to the callbackfn function provides this value as an argument instead of an array value.
      */
-    reduce<U>(callbackfn: (previousValue: U, currentValue: T, currentIndex: number, array: T[]) => U, initialValue: U): U;
+    reduce(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T): T;
+    reduce(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T, initialValue: T): T;
     /**
      * Calls the specified callback function for all the elements in an array, in descending order. The return value of the callback function is the accumulated result, and is provided as an argument in the next call to the callback function.
      * @param callbackfn A function that accepts up to four arguments. The reduceRight method calls the callbackfn function one time for each element in the array.
      * @param initialValue If initialValue is specified, it is used as the initial value to start the accumulation. The first call to the callbackfn function provides this value as an argument instead of an array value.
      */
-    reduceRight(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T): T;
-    reduceRight(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T, initialValue: T): T;
+    reduceRight<U>(callbackfn: (previousValue: U, currentValue: T, currentIndex: number, array: T[]) => U, initialValue: U): U;
     /**
      * Calls the specified callback function for all the elements in an array, in descending order. The return value of the callback function is the accumulated result, and is provided as an argument in the next call to the callback function.
      * @param callbackfn A function that accepts up to four arguments. The reduceRight method calls the callbackfn function one time for each element in the array.
      * @param initialValue If initialValue is specified, it is used as the initial value to start the accumulation. The first call to the callbackfn function provides this value as an argument instead of an array value.
      */
-    reduceRight<U>(callbackfn: (previousValue: U, currentValue: T, currentIndex: number, array: T[]) => U, initialValue: U): U;
+    reduceRight(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T): T;
+    reduceRight(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T, initialValue: T): T;
 
     [n: number]: T;
 }

src/lib/esnext.iterator.d.ts

@@ -66,15 +66,14 @@ declare global {
          * @param callbackfn A function that accepts up to three arguments. The reduce method calls the callbackfn function one time for each element in the iterator.
          * @param initialValue If initialValue is specified, it is used as the initial value to start the accumulation. The first call to the callbackfn function provides this value as an argument instead of a value from the iterator.
          */
-        reduce(callbackfn: (previousValue: T, currentValue: T, currentIndex: number) => T): T;
-        reduce(callbackfn: (previousValue: T, currentValue: T, currentIndex: number) => T, initialValue: T): T;
-
+        reduce<U>(callbackfn: (previousValue: U, currentValue: T, currentIndex: number) => U, initialValue: U): U;
         /**
          * Calls the specified callback function for all the elements in this iterator. The return value of the callback function is the accumulated result, and is provided as an argument in the next call to the callback function.
          * @param callbackfn A function that accepts up to three arguments. The reduce method calls the callbackfn function one time for each element in the iterator.
          * @param initialValue If initialValue is specified, it is used as the initial value to start the accumulation. The first call to the callbackfn function provides this value as an argument instead of a value from the iterator.
          */
-        reduce<U>(callbackfn: (previousValue: U, currentValue: T, currentIndex: number) => U, initialValue: U): U;
+        reduce(callbackfn: (previousValue: T, currentValue: T, currentIndex: number) => T): T;
+        reduce(callbackfn: (previousValue: T, currentValue: T, currentIndex: number) => T, initialValue: T): T;
 
         /**
          * Creates a new array from the values yielded by this iterator.

tests/baselines/reference/genericReduce.types

@@ -42,12 +42,12 @@ var n1 = b.reduce((x, y) => x + y);
 >   : ^^^^^^
 >b.reduce((x, y) => x + y) : number
 >                          : ^^^^^^
->b.reduce : { (callbackfn: (previousValue: number, currentValue: number, currentIndex: number, array: number[]) => number): number; (callbackfn: (previousValue: number, currentValue: number, currentIndex: number, array: number[]) => number, initialValue: number): number; <U>(callbackfn: (previousValue: U, currentValue: number, currentIndex: number, array: number[]) => U, initialValue: U): U; }
->         : ^^^          ^^^             ^^^^^^^^^^            ^^^^^^^^^^            ^^      ^^     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^          ^^^             ^^^^^^^^^^            ^^^^^^^^^^            ^^      ^^     ^^^^^^^^^^^^^^^^^^^^^^^            ^^^^^^^^^^^^^^^^^^^^ ^^          ^^^             ^^^^^            ^^^^^^^^^^            ^^      ^^     ^^^^^^^^^^^^^^^^^^            ^^^^^^^^^^
+>b.reduce : { <U>(callbackfn: (previousValue: U, currentValue: number, currentIndex: number, array: number[]) => U, initialValue: U): U; (callbackfn: (previousValue: number, currentValue: number, currentIndex: number, array: number[]) => number): number; (callbackfn: (previousValue: number, currentValue: number, currentIndex: number, array: number[]) => number, initialValue: number): number; }
+>         : ^^^ ^^          ^^^             ^^^^^            ^^^^^^^^^^            ^^      ^^     ^^^^^^^^^^^^^^^^^^            ^^^^^^^^^^          ^^^             ^^^^^^^^^^            ^^^^^^^^^^            ^^      ^^     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^          ^^^             ^^^^^^^^^^            ^^^^^^^^^^            ^^      ^^     ^^^^^^^^^^^^^^^^^^^^^^^            ^^^^^^^^^^^^^^^^^^^^
 >b : number[]
 >  : ^^^^^^^^
->reduce : { (callbackfn: (previousValue: number, currentValue: number, currentIndex: number, array: number[]) => number): number; (callbackfn: (previousValue: number, currentValue: number, currentIndex: number, array: number[]) => number, initialValue: number): number; <U>(callbackfn: (previousValue: U, currentValue: number, currentIndex: number, array: number[]) => U, initialValue: U): U; }
->       : ^^^          ^^^             ^^^^^^^^^^            ^^^^^^^^^^            ^^      ^^     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^          ^^^             ^^^^^^^^^^            ^^^^^^^^^^            ^^      ^^     ^^^^^^^^^^^^^^^^^^^^^^^            ^^^^^^^^^^^^^^^^^^^^ ^^          ^^^             ^^^^^            ^^^^^^^^^^            ^^      ^^     ^^^^^^^^^^^^^^^^^^            ^^^^^^^^^^
+>reduce : { <U>(callbackfn: (previousValue: U, currentValue: number, currentIndex: number, array: number[]) => U, initialValue: U): U; (callbackfn: (previousValue: number, currentValue: number, currentIndex: number, array: number[]) => number): number; (callbackfn: (previousValue: number, currentValue: number, currentIndex: number, array: number[]) => number, initialValue: number): number; }
+>       : ^^^ ^^          ^^^             ^^^^^            ^^^^^^^^^^            ^^      ^^     ^^^^^^^^^^^^^^^^^^            ^^^^^^^^^^          ^^^             ^^^^^^^^^^            ^^^^^^^^^^            ^^      ^^     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^          ^^^             ^^^^^^^^^^            ^^^^^^^^^^            ^^      ^^     ^^^^^^^^^^^^^^^^^^^^^^^            ^^^^^^^^^^^^^^^^^^^^
 >(x, y) => x + y : (x: number, y: number) => number
 >                : ^ ^^^^^^^^^^ ^^^^^^^^^^^^^^^^^^^
 >x : number
@@ -66,12 +66,12 @@ var n2 = b.reduceRight((x, y) => x + y);
 >   : ^^^^^^
 >b.reduceRight((x, y) => x + y) : number
 >                               : ^^^^^^
->b.reduceRight : { (callbackfn: (previousValue: number, currentValue: number, currentIndex: number, array: number[]) => number): number; (callbackfn: (previousValue: number, currentValue: number, currentIndex: number, array: number[]) => number, initialValue: number): number; <U>(callbackfn: (previousValue: U, currentValue: number, currentIndex: number, array: number[]) => U, initialValue: U): U; }
->              : ^^^          ^^^             ^^^^^^^^^^            ^^^^^^^^^^            ^^      ^^     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^          ^^^             ^^^^^^^^^^            ^^^^^^^^^^            ^^      ^^     ^^^^^^^^^^^^^^^^^^^^^^^            ^^^^^^^^^^^^^^^^^^^^ ^^          ^^^             ^^^^^            ^^^^^^^^^^            ^^      ^^     ^^^^^^^^^^^^^^^^^^            ^^^^^^^^^^
+>b.reduceRight : { <U>(callbackfn: (previousValue: U, currentValue: number, currentIndex: number, array: number[]) => U, initialValue: U): U; (callbackfn: (previousValue: number, currentValue: number, currentIndex: number, array: number[]) => number): number; (callbackfn: (previousValue: number, currentValue: number, currentIndex: number, array: number[]) => number, initialValue: number): number; }
+>              : ^^^ ^^          ^^^             ^^^^^            ^^^^^^^^^^            ^^      ^^     ^^^^^^^^^^^^^^^^^^            ^^^^^^^^^^          ^^^             ^^^^^^^^^^            ^^^^^^^^^^            ^^      ^^     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^          ^^^             ^^^^^^^^^^            ^^^^^^^^^^            ^^      ^^     ^^^^^^^^^^^^^^^^^^^^^^^            ^^^^^^^^^^^^^^^^^^^^
 >b : number[]
 >  : ^^^^^^^^
->reduceRight : { (callbackfn: (previousValue: number, currentValue: number, currentIndex: number, array: number[]) => number): number; (callbackfn: (previousValue: number, currentValue: number, currentIndex: number, array: number[]) => number, initialValue: number): number; <U>(callbackfn: (previousValue: U, currentValue: number, currentIndex: number, array: number[]) => U, initialValue: U): U; }
->            : ^^^          ^^^             ^^^^^^^^^^            ^^^^^^^^^^            ^^      ^^     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^          ^^^             ^^^^^^^^^^            ^^^^^^^^^^            ^^      ^^     ^^^^^^^^^^^^^^^^^^^^^^^            ^^^^^^^^^^^^^^^^^^^^ ^^          ^^^             ^^^^^            ^^^^^^^^^^            ^^      ^^     ^^^^^^^^^^^^^^^^^^            ^^^^^^^^^^
+>reduceRight : { <U>(callbackfn: (previousValue: U, currentValue: number, currentIndex: number, array: number[]) => U, initialValue: U): U; (callbackfn: (previousValue: number, currentValue: number, currentIndex: number, array: number[]) => number): number; (callbackfn: (previousValue: number, currentValue: number, currentIndex: number, array: number[]) => number, initialValue: number): number; }
+>            : ^^^ ^^          ^^^             ^^^^^            ^^^^^^^^^^            ^^      ^^     ^^^^^^^^^^^^^^^^^^            ^^^^^^^^^^          ^^^             ^^^^^^^^^^            ^^^^^^^^^^            ^^      ^^     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^          ^^^             ^^^^^^^^^^            ^^^^^^^^^^            ^^      ^^     ^^^^^^^^^^^^^^^^^^^^^^^            ^^^^^^^^^^^^^^^^^^^^
 >(x, y) => x + y : (x: number, y: number) => number
 >                : ^ ^^^^^^^^^^ ^^^^^^^^^^^^^^^^^^^
 >x : number
@@ -138,12 +138,12 @@ var n3 = b.reduce<string>( (x, y) => x + y, ""); // Initial value is of type str
 >   : ^^^^^^
 >b.reduce<string>( (x, y) => x + y, "") : string
 >                                       : ^^^^^^
->b.reduce : { (callbackfn: (previousValue: number, currentValue: number, currentIndex: number, array: number[]) => number): number; (callbackfn: (previousValue: number, currentValue: number, currentIndex: number, array: number[]) => number, initialValue: number): number; <U>(callbackfn: (previousValue: U, currentValue: number, currentIndex: number, array: number[]) => U, initialValue: U): U; }
->         : ^^^          ^^^             ^^^^^^^^^^            ^^^^^^^^^^            ^^      ^^     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^          ^^^             ^^^^^^^^^^            ^^^^^^^^^^            ^^      ^^     ^^^^^^^^^^^^^^^^^^^^^^^            ^^^^^^^^^^^^^^^^^^^^ ^^          ^^^             ^^^^^            ^^^^^^^^^^            ^^      ^^     ^^^^^^^^^^^^^^^^^^            ^^^^^^^^^^
+>b.reduce : { <U>(callbackfn: (previousValue: U, currentValue: number, currentIndex: number, array: number[]) => U, initialValue: U): U; (callbackfn: (previousValue: number, currentValue: number, currentIndex: number, array: number[]) => number): number; (callbackfn: (previousValue: number, currentValue: number, currentIndex: number, array: number[]) => number, initialValue: number): number; }
+>         : ^^^ ^^          ^^^             ^^^^^            ^^^^^^^^^^            ^^      ^^     ^^^^^^^^^^^^^^^^^^            ^^^^^^^^^^          ^^^             ^^^^^^^^^^            ^^^^^^^^^^            ^^      ^^     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^          ^^^             ^^^^^^^^^^            ^^^^^^^^^^            ^^      ^^     ^^^^^^^^^^^^^^^^^^^^^^^            ^^^^^^^^^^^^^^^^^^^^
 >b : number[]
 >  : ^^^^^^^^
->reduce : { (callbackfn: (previousValue: number, currentValue: number, currentIndex: number, array: number[]) => number): number; (callbackfn: (previousValue: number, currentValue: number, currentIndex: number, array: number[]) => number, initialValue: number): number; <U>(callbackfn: (previousValue: U, currentValue: number, currentIndex: number, array: number[]) => U, initialValue: U): U; }
->       : ^^^          ^^^             ^^^^^^^^^^            ^^^^^^^^^^            ^^      ^^     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^          ^^^             ^^^^^^^^^^            ^^^^^^^^^^            ^^      ^^     ^^^^^^^^^^^^^^^^^^^^^^^            ^^^^^^^^^^^^^^^^^^^^ ^^          ^^^             ^^^^^            ^^^^^^^^^^            ^^      ^^     ^^^^^^^^^^^^^^^^^^            ^^^^^^^^^^
+>reduce : { <U>(callbackfn: (previousValue: U, currentValue: number, currentIndex: number, array: number[]) => U, initialValue: U): U; (callbackfn: (previousValue: number, currentValue: number, currentIndex: number, array: number[]) => number): number; (callbackfn: (previousValue: number, currentValue: number, currentIndex: number, array: number[]) => number, initialValue: number): number; }
+>       : ^^^ ^^          ^^^             ^^^^^            ^^^^^^^^^^            ^^      ^^     ^^^^^^^^^^^^^^^^^^            ^^^^^^^^^^          ^^^             ^^^^^^^^^^            ^^^^^^^^^^            ^^      ^^     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^          ^^^             ^^^^^^^^^^            ^^^^^^^^^^            ^^      ^^     ^^^^^^^^^^^^^^^^^^^^^^^            ^^^^^^^^^^^^^^^^^^^^
 >(x, y) => x + y : (x: string, y: number) => string
 >                : ^ ^^^^^^^^^^ ^^^^^^^^^^^^^^^^^^^
 >x : string