Add const type parameters for literal type inference

[gjbkz]

Summary

This PR adds TypeScript 5.0's const type parameters to the typeChecker, isOptionalOf, isArrayOf, isDictionaryOf, and ensure functions. This enables the compiler to infer the most specific (literal) types for type arguments instead of widening them, allowing Set-based definitions to produce literal union types rather than generic string or number types.

Key Changes

• Added const modifier to type parameters in typeChecker(), isOptionalOf(), isArrayOf(), isDictionaryOf(), and ensure() functions
• Added comprehensive test file (constTypeParams.test.ts) with:
  • Compile-time type assertions verifying literal union type inference
  • Runtime tests validating the behavior of all modified functions
  • Tests covering string literals, numeric literals, and optional/array/dictionary variants

Implementation Details

The const T modifier ensures that when a Set like new Set(["a", "b"]) is passed to these functions, TypeScript infers the type as the literal union "a" | "b" rather than the widened type string. This provides better type safety and more precise type checking at compile time while maintaining full runtime compatibility.

https://claude.ai/code/session_014UV4gEoodBgPAE8d4jcoJ8

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.19%. Comparing base (cae2eb5) to head (813eebe).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #425      +/-   ##
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+ Coverage   96.86%   97.19%   +0.33%     
==========================================
  Files          45       45              
  Lines        2104     2107       +3     
  Branches      209      210       +1     
==========================================
+ Hits         2038     2048      +10     
+ Misses         66       59       -7     

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