From 784facca7c1c8f5da0a2c707f32ac400ed7b42f4 Mon Sep 17 00:00:00 2001
From: ghaaj <91141283+ghaaj@users.noreply.github.com>
Date: Thu, 23 Jan 2025 21:26:01 +0900
Subject: [PATCH] `Split`: Just return `string[]` when `S` or `Delimiter` is
 non-literal

* Use `IsStringLiteral` for conditional branches, which is more appropriate than `string extends ..`
* Add test cases where `S` or `Delimiter` is not `string` itself but also not a literal (e.g., Uppercase<string>)

Co-authored-by: Som Shekhar Mukherjee <49264891+som-sm@users.noreply.github.com>
---
 source/split.d.ts | 12 +++++-------
 test-d/split.ts   | 14 ++++----------
 2 files changed, 9 insertions(+), 17 deletions(-)

diff --git a/source/split.d.ts b/source/split.d.ts
index 24e58fcb4..e572b377c 100644
--- a/source/split.d.ts
+++ b/source/split.d.ts
@@ -1,5 +1,5 @@
-import type {ArrayValues} from './array-values';
-import type {Substrings} from './internal';
+import type {IsStringLiteral} from './is-literal';
+
 /**
 Represents an array of strings split using a given character or character set.
 
@@ -24,11 +24,9 @@ array = split(items, ',');
 export type Split<
 	S extends string,
 	Delimiter extends string,
-> = string extends S
-	? string[]
-	: string extends Delimiter
-		? [S] | SplitHelper<S, ArrayValues<Substrings<S>>>
-		: SplitHelper<S, Delimiter>;
+> = IsStringLiteral<S | Delimiter> extends true
+	? SplitHelper<S, Delimiter>
+	: string[];
 
 type SplitHelper<
 	S extends string,
diff --git a/test-d/split.ts b/test-d/split.ts
index 4941acc50..41491c555 100644
--- a/test-d/split.ts
+++ b/test-d/split.ts
@@ -45,21 +45,15 @@ expectType<
 
 // Split non-literal string by literal string.
 expectType<string[]>(split(null! as string, 'foobar'));
+expectType<string[]>(split(null! as Uppercase<string>, 'foobar'));
 
 // Split non-literal string by non-literal string.
 expectType<string[]>(split(null! as string, null! as string));
+expectType<string[]>(split(null! as `pre${string}`, null! as `${number}`));
 
 // Split literal string by non-literal string.
-expectType<
-| ['ABCDEF']
-| ['', '']
-| ['', 'F'] | ['A', '']
-| ['', 'EF'] | ['A', 'F'] | ['AB', '']
-| ['', 'DEF'] | ['A', 'EF'] | ['AB', 'F'] | ['ABC', '']
-| ['', 'CDEF'] | ['A', 'DEF'] | ['AB', 'EF'] | ['ABC', 'F'] | ['ABCD', '']
-| ['', 'BCDEF'] | ['A', 'CDEF'] | ['AB', 'DEF'] | ['ABC', 'EF'] | ['ABCD', 'F'] | ['ABCDE', '']
-| ['A', 'B', 'C', 'D', 'E', 'F']
->(split('ABCDEF', null! as string));
+expectType<string[]>(split('ABCDEF', null! as string));
+expectType<string[]>(split('ABCDEF', null! as Capitalize<string>));
 
 // Recursion depth at which a non-tail recursive implementation starts to fail.
 const fiftyZeroes = '00000000000000000000000000000000000000000000000000';