Stumble upon some narrowing issues

test-d/inference.test-d.ts

@@ -9,6 +9,17 @@ function isNum(x: unknown): x is number {
   return typeof x === "number";
 }
 
+function isPosNum(x: unknown): x is number {
+  return isNum(x) && x >= 0;
+}
+
+const x = -32 as string | number;
+if (isPosNum(x)) {
+  x.toFixed(2); // x is number
+} else {
+  x.length; // x is number
+}
+
 {
   // partition with type predicate
   const items: unknown[] = [1, 2, null, true, 0, "hi", false, -1];
@@ -38,3 +49,23 @@ function isNum(x: unknown): x is number {
   expectType<unknown[]>(numbers);
   expectType<unknown[]>(others);
 }
+
+{
+  // partitionN with no type predicate
+  const items: unknown[] = [1, 2, null, true, 0, "hi", false, -1];
+  const [strings, numbers, others] = partitionN(
+    items,
+    (x) => String(typeof x) === "string",
+    (x) => String(typeof x) === "number",
+  );
+  expectType<unknown[]>(strings);
+  expectType<unknown[]>(numbers);
+  expectType<unknown[]>(others);
+}
+
+{
+  const values = ["hi", 3, null, "foo", -7];
+  const [nums, rest] = partitionN(values, isPosNum);
+  expectType<number[]>(nums);
+  expectType<(string | number | boolean)[]>(rest);
+}

test/more-itertools.test.ts

@@ -20,6 +20,7 @@ import * as fc from "fast-check";
 const isEven = (x: number) => x % 2 === 0;
 const isEvenIndex = (_: unknown, index: number) => index % 2 === 0;
 const isPositive = (x: number) => x >= 0;
+const isPositiveNum = (x: unknown): x is number => isNum(x) && x >= 0;
 
 function isString(value: unknown): value is string {
   return typeof value === "string";
@@ -318,6 +319,14 @@ describe("partitionN", () => {
     expect(rest).toEqual([null]);
     //     ^^^ (string | null)[]
   });
+
+  it("partitionN retains complex rich type info", () => {
+    const values = ["hi", 3, null, "foo", -7];
+    const [nums, rest] = partitionN(values, isPositiveNum);
+    expect(nums).toEqual([3]);
+    //     ^^^^ number[]
+    expect(rest).toEqual(["hi", null, "foo", -7]);
+  });
 });
 
 describe("roundrobin", () => {