[TEST] Add edge case tests for `readonly` and `optional` modifiers

[kakasoo]

Feature Request

• Extensions of existing features
• Propose a type that didn't exist before

Type Expectation

Core types like DeepStrictPick, DeepStrictOmit, DeepStrictMerge, and DeepDateToString lack explicit tests for readonly properties and optional(?:) properties. Since modifier preservation is critical to type safety, explicit tests are needed to prevent regressions.

Example Type

// optional preservation test
type Input = { a?: number; b: string; c?: { d: number } };
type Q1 = DeepStrictPick<Input, "a">;
type A1 = Equal<Q1, { a?: number }>;  // verify optional is preserved

// readonly property preservation test
type Input2 = { readonly a: number; b: string };
type Q2 = DeepStrictPick<Input2, "a">;
type A2 = Equal<Q2, { readonly a: number }>;  // verify readonly is preserved

// optional + nesting
type Input3 = { a?: { b?: { c: number } } };
type Q3 = DeepStrictPick<Input3, "a.b.c">;
type A3 = Equal<Q3, { a?: { b?: { c: number } } }>;

Proposed Solution

Add edge case tests to existing test files (DeepStrictPick.ts, DeepStrictOmit.ts, DeepStrictMerge.ts, DeepDateToString.ts).

Test Requirements

All changes must include the following tests:

1. Backward Compatibility

   • All existing tests must pass
   • Add tests to verify that existing type behavior remains unchanged

2. Feature Verification

   • optional properties are preserved after pick/omit
   • readonly properties are preserved after pick/omit
   • How optional/readonly from both sides are handled during merge

3. Complex Type Stability

   • { a?: readonly { b?: number }[] } (optional + readonly + array)
   • { readonly a?: { readonly b: Date } } (readonly + optional + Date)
   • readonly [{ a?: 1 }, { b: 2 }] (readonly tuple + optional)
   • Branded type + optional ({ a?: string & MinLength<1> })
   • Union + optional ({ a?: string | null })
   • 3-level nested optional ({ a?: { b?: { c?: number } } })

How to verify:

npm run build:test && npm run test
npm run prettier