lib: add Math.sumPrecise type definition (ES2026 / esnext)

Math.sumPrecise is part of the TC39 proposal-math-sum which reached
Stage 4 on 2025-07-28, making it part of ES2026.

Already implemented in Firefox 137+, Safari 18.4, and Bun,
but not yet in V8/Node.js.

Adds src/lib/esnext.math.d.ts with the Math interface extension,
and wires it into esnext.d.ts and libs.json.

Fixes #63427

Author: 827652549

src/lib/es2025.d.ts

@@ -2,6 +2,6 @@
 /// <reference lib="es2025.collection" />
 /// <reference lib="es2025.float16" />
 /// <reference lib="es2025.intl" />
-/// <reference lib="es2025.iterator" />
+/// <reference lib="es2025.iterator" />
 /// <reference lib="es2025.promise" />
 /// <reference lib="es2025.regexp" />

src/lib/esnext.d.ts

@@ -9,3 +9,4 @@
 /// <reference lib="esnext.typedarrays" />
 /// <reference lib="esnext.temporal" />
 /// <reference lib="esnext.date" />
+/// <reference lib="esnext.math" />

src/lib/esnext.math.d.ts

@@ -0,0 +1,14 @@
+/// <reference lib="es2015.iterable" />
+
+interface Math {
+    /**
+     * Returns the sum of the values in the iterable using a more precise
+     * summation algorithm than naive floating-point addition.
+     * Returns `-0` if the iterable is empty.
+     * @param numbers An iterable (such as an Array) of numbers.
+     * @throws {TypeError} If `numbers` is not iterable, or if any value in the iterable is not of type `number`.
+     * @throws {RangeError} If the iterable yields 2^53 or more values.
+     * @see https://tc39.es/proposal-math-sum/
+     */
+    sumPrecise(numbers: Iterable<number>): number;
+}

src/lib/libs.json

@@ -83,10 +83,11 @@
         "es2025.collection",
         "es2025.float16",
         "es2025.intl",
-        "es2025.iterator",
+        "es2025.iterator",
         "es2025.promise",
         "es2025.regexp",
-        "esnext.array",
+        "esnext.array",
+        "esnext.math",
         "esnext.collection",
         "esnext.date",
         "esnext.decorators",

tests/cases/conformance/esnext/mathSumPrecise.ts

@@ -0,0 +1,31 @@
+// @target: esnext
+// @lib: esnext
+// @noemit: true
+// @strict: true
+
+// Basic usage with array
+const sum1 = Math.sumPrecise([1, 2, 3]);
+
+// Floating point precision
+const sum2 = Math.sumPrecise([1e20, 0.1, -1e20]);
+
+// Empty iterable returns -0
+const sum3 = Math.sumPrecise([]);
+
+// Works with any Iterable<number>
+const sum4 = Math.sumPrecise(new Set([1, 2, 3]));
+
+function* gen(): Iterable<number> {
+    yield 0.1;
+    yield 0.2;
+}
+const sum5 = Math.sumPrecise(gen());
+
+// Return type is number
+const result: number = Math.sumPrecise([1, 2, 3]);
+
+// @ts-expect-error - BigInt is not a number
+Math.sumPrecise([1n, 2n]);
+
+// @ts-expect-error - string is not a number
+Math.sumPrecise(["a", "b"]);