From 74cefa848c2aa50484f465e019194daef77f5791 Mon Sep 17 00:00:00 2001
From: Jake Bailey <5341706+jakebailey@users.noreply.github.com>
Date: Fri, 26 Jul 2024 10:08:40 -0700
Subject: [PATCH] Tighten signature of append (#59426)

---
 src/compiler/core.ts                          | 25 ++++++++-----------
 src/server/session.ts                         |  2 +-
 .../tsbuildWatch/watchEnvironment.ts          |  2 +-
 3 files changed, 12 insertions(+), 17 deletions(-)

diff --git a/src/compiler/core.ts b/src/compiler/core.ts
index e2238766ee5..fc692f1a65d 100644
--- a/src/compiler/core.ts
+++ b/src/compiler/core.ts
@@ -396,7 +396,7 @@ export function flatten<T>(array: T[][] | readonly (T | readonly T[] | undefined
  *
  * @internal
  */
-export function flatMap<T, U>(array: readonly T[] | undefined, mapfn: (x: T, i: number) => U | readonly U[] | undefined): readonly U[] {
+export function flatMap<T, U extends {}>(array: readonly T[] | undefined, mapfn: (x: T, i: number) => U | readonly U[] | undefined): readonly U[] {
     let result: U[] | undefined;
     if (array !== undefined) {
         for (let i = 0; i < array.length; i++) {
@@ -917,18 +917,13 @@ export function relativeComplement<T>(arrayA: T[] | undefined, arrayB: T[] | und
  *
  * @internal
  */
-export function append<TArray extends any[] | undefined, TValue extends NonNullable<TArray>[number] | undefined>(to: TArray, value: TValue): [undefined, undefined] extends [TArray, TValue] ? TArray : NonNullable<TArray>[number][];
+export function append<T extends {}>(to: T[], value: T | undefined): T[];
 /** @internal */
-export function append<T>(to: T[], value: T | undefined): T[];
+export function append<T extends {}>(to: T[] | undefined, value: T): T[];
 /** @internal */
-export function append<T>(to: T[] | undefined, value: T): T[];
-/** @internal */
-export function append<T>(to: T[] | undefined, value: T | undefined): T[] | undefined;
-/** @internal */
-export function append<T>(to: T[], value: T | undefined): void;
-/** @internal */
-export function append<T>(to: T[] | undefined, value: T | undefined): T[] | undefined {
-    if (value === undefined) return to as T[];
+export function append<T extends {}>(to: T[] | undefined, value: T | undefined): T[] | undefined;
+export function append<T extends {}>(to: T[] | undefined, value: T | undefined): T[] | undefined {
+    if (value === undefined) return to;
     if (to === undefined) return [value];
     to.push(value);
     return to;
@@ -948,13 +943,13 @@ export function append<T>(to: T[] | undefined, value: T | undefined): T[] | unde
  *
  * @internal
  */
-export function combine<T>(xs: T[] | undefined, ys: T[] | undefined): T[] | undefined;
+export function combine<T extends {}>(xs: T[] | undefined, ys: T[] | undefined): T[] | undefined;
 /** @internal */
-export function combine<T>(xs: T | readonly T[] | undefined, ys: T | readonly T[] | undefined): T | readonly T[] | undefined;
+export function combine<T extends {}>(xs: T | readonly T[] | undefined, ys: T | readonly T[] | undefined): T | readonly T[] | undefined;
 /** @internal */
-export function combine<T>(xs: T | T[] | undefined, ys: T | T[] | undefined): T | T[] | undefined;
+export function combine<T extends {}>(xs: T | T[] | undefined, ys: T | T[] | undefined): T | T[] | undefined;
 /** @internal */
-export function combine<T>(xs: T | T[] | undefined, ys: T | T[] | undefined) {
+export function combine<T extends {}>(xs: T | T[] | undefined, ys: T | T[] | undefined) {
     if (xs === undefined) return ys;
     if (ys === undefined) return xs;
     if (isArray(xs)) return isArray(ys) ? concatenate(xs, ys) : append(xs, ys);
diff --git a/src/server/session.ts b/src/server/session.ts
index 40640d052be..fd0ed26586d 100644
--- a/src/server/session.ts
+++ b/src/server/session.ts
@@ -486,7 +486,7 @@ type Projects = readonly Project[] | {
 /**
  * This helper function processes a list of projects and return the concatenated, sortd and deduplicated output of processing each project.
  */
-function combineProjectOutput<T, U>(
+function combineProjectOutput<T, U extends {}>(
     defaultValue: T,
     getValue: (path: Path) => T,
     projects: Projects,
diff --git a/src/testRunner/unittests/tsbuildWatch/watchEnvironment.ts b/src/testRunner/unittests/tsbuildWatch/watchEnvironment.ts
index ad830d0d92a..d8c76df25df 100644
--- a/src/testRunner/unittests/tsbuildWatch/watchEnvironment.ts
+++ b/src/testRunner/unittests/tsbuildWatch/watchEnvironment.ts
@@ -80,7 +80,7 @@ describe("unittests:: tsbuildWatch:: watchEnvironment:: tsbuild:: watchMode:: wi
             watchOrSolution: solutionBuilder,
         });
 
-        function flatArray<T>(arr: T[][]): readonly T[] {
+        function flatArray<T extends {}>(arr: T[][]): readonly T[] {
             return ts.flatMap(arr, ts.identity);
         }
         function pkgs<T>(cb: (index: number) => T): T[] {