From 799849cc41f00d93a82d63d79dfc3cf2ad98fc45 Mon Sep 17 00:00:00 2001
From: technicalcandidate-stack <technicalcandidate@harperinsure.com>
Date: Fri, 30 Jan 2026 11:43:16 -0800
Subject: [PATCH] fix: preserve literal types in Infer by using const type
 parameter

- Change literal<T extends any> to literal<const T> for proper type inference
- Add type-level tests to verify literal types are preserved
- Add TypeScript 5.0+ requirement to README
- Add optional peerDependency on TypeScript >=5.0.0

The const type parameter modifier (TS 5.0+) prevents type widening,
so Infer<typeof literal('text')> now correctly produces 'text' instead of string.
---
 README.md                      |  7 +++++++
 package.json                   |  8 ++++++++
 src/predicates/literal.spec.ts | 13 +++++++++++++
 src/predicates/literal.ts      |  2 +-
 4 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/README.md b/README.md
index 7e3ee9c..90ff6ce 100644
--- a/README.md
+++ b/README.md
@@ -59,6 +59,13 @@ runtyp is designed for speed. Here's how it compares to other popular programmat
 npm i runtyp
 ```
 
+## Requirements
+
+- **Node.js**: 18+ (or any modern browser)
+- **TypeScript**: 5.0+ (for type inference features)
+
+> **Note**: JavaScript users can use runtyp without TypeScript. The TypeScript requirement only applies if you want type inference via `Infer<>`.
+
 # Usage
 
 ## Example
diff --git a/package.json b/package.json
index 1069b30..9c51497 100644
--- a/package.json
+++ b/package.json
@@ -36,6 +36,14 @@
     "lint": "eslint ./ --ext .js,.ts",
     "build": "tsc"
   },
+  "peerDependencies": {
+    "typescript": ">=5.0.0"
+  },
+  "peerDependenciesMeta": {
+    "typescript": {
+      "optional": true
+    }
+  },
   "devDependencies": {
     "@typescript-eslint/eslint-plugin": "^5.59.2",
     "@typescript-eslint/parser": "^5.59.2",
diff --git a/src/predicates/literal.spec.ts b/src/predicates/literal.spec.ts
index 8f8a003..4e731e1 100644
--- a/src/predicates/literal.spec.ts
+++ b/src/predicates/literal.spec.ts
@@ -1,5 +1,18 @@
 import {test} from 'kizu';
 import {literal} from './literal';
+import type {Infer} from '..';
+
+// Type-level test: Infer should preserve literal types
+// eslint-disable-next-line @typescript-eslint/no-unused-vars
+const textValidator = literal('text');
+
+type TextType = Infer<typeof textValidator>;
+
+// eslint-disable-next-line @typescript-eslint/no-unused-vars, no-underscore-dangle
+const typeTest: TextType = 'text';
+// @ts-expect-error - 'other' is not assignable to 'text'
+// eslint-disable-next-line @typescript-eslint/no-unused-vars, no-underscore-dangle
+const typeTestFail: TextType = 'other';
 
 test('literal(): valid inputs', (assert) => {
 
diff --git a/src/predicates/literal.ts b/src/predicates/literal.ts
index 7e88189..ff3d69d 100644
--- a/src/predicates/literal.ts
+++ b/src/predicates/literal.ts
@@ -1,6 +1,6 @@
 import {Pred, ValidationResult} from '..';
 
-export function literal<T extends any>(expected: T): Pred<T> {
+export function literal<const T>(expected: T): Pred<T> {
 
     return (value: unknown): ValidationResult<T> => {