fix: tighten types of MergeStrategy in object-schema

[lumirlumir]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request?

Which packages would you like to change?

• @eslint/compat
• @eslint/config-array
• @eslint/config-helpers
• @eslint/core
• @eslint/mcp
• @eslint/migrate-config
• @eslint/object-schema
• @eslint/plugin-kit

What problem do you want to solve?

In the public API of @eslint/object-schema, the return types of MergeStrategy.overwrite, MergeStrategy.replace, and MergeStrategy.assign are currently typed as any, which feels too broad and could be narrowed to a clearer type.

What do you think is the correct solution?

For example:

MergeStrategy.overwrite always returns value2; MergeStrategy.replace always returns either value1 or value2; and MergeStrategy.assign always returns the merged object produced by Object.assign().

I think if we use a template for these static functions, we can make the types clearer.

What changes did you make? (Give an overview)

In this PR, I've tightened the types for MergeStrategy in object-schema.

The previous return type any was too broad, and from a TypeScript perspective, using any is generally not recommended.

By using templates, the return types can be narrowed, so I went ahead with this approach.

Related Issues

N/A

Is there anything you'd like reviewers to focus on?

N/A

[lumirlumir]

Since the replace static method always returns value2 when value2 is not undefined, the return type could be narrowed.

However, there are edge cases where value2 may be, for example, string | undefined, and I don’t see a clear way to express a more specific type for this function right now.

Using TValue1 | TValue2 as the return type seems like the most general approach here. If there’s a way to narrow it more precisely, I’m happy to follow it.

[lumirlumir]

According to the JSDoc comments and the README.md, it seems the restriction on value1 and value2 is that they must be objects with properties, but I'm not entirely sure.

rewrite/packages/object-schema/src/merge-strategy.js

Line 39 in b65204d

* Merges two properties by assigning properties from the second to the first.

rewrite/packages/object-schema/README.md

Line 97 in b65204d

- `"assign"` - use `Object.assign()` to merge the two values into one object.

[fasttime]

It looks like value1 can be undefined. This is the relevant test case:

rewrite/packages/object-schema/tests/object-schema.test.js

Lines 264 to 274 in 7255100

	schema = new ObjectSchema({
	foo: {
	merge: "assign",
	validate() {},
	},
	});
	const result = schema.merge(
	{ foo: { bar: true } },
	{ foo: { baz: false } },
	);

Stepping through with the debugger shows that MergeStrategy.assign() is invoked twice:

1. with value1 = undefined and value2 = { bar: true }
2. with value1 = { bar: true } and value2 = { baz: false }

[lumirlumir]

I intentionally wrote this type test a little loosely, per the #348 (comment), in case the types can be narrowed further in the future.

For example, we can use the following test case instead to ensure its behavior:

- MergeStrategy.replace(1, 2) satisfies number
+ MergeStrategy.replace(1, 2) satisfies 1 | 2

(FYI: this type test is also ensured and fails if we revert this change.)

However, If narrowing the type test is needed here as well, I'm happy to do that.

[Copilot]

Pull request overview

This PR improves type safety in the @eslint/object-schema package by replacing the overly broad any return types in MergeStrategy methods with more precise generic types. This enhances the developer experience by providing better type inference and compile-time checking when using these merge strategies.

• Uses TypeScript generics to narrow return types from any to specific inferred types
• Adds comprehensive type tests to verify the new type behavior
• Maintains backward compatibility as the runtime behavior is unchanged

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
packages/object-schema/src/merge-strategy.js	Updated JSDoc type annotations to use generic templates for overwrite, replace, and assign methods, providing more precise return types
packages/object-schema/tests/types/types.test.ts	Added comprehensive type tests to validate that the new generic types correctly infer return types and properly reject invalid type assertions

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