fix(openapi): stop JsonifiedValue from turning interface outputs into unknown

[dinwwwh]

JsonifiedClient typed interface outputs as unknown, while the same shape written as a type alias worked. Interface outputs, and interfaces nested inside other outputs, now resolve to their JSON types (e.g. Date becomes string). Type-checking stays as cheap as before.

interface User { id: number, createdAt: Date }

type Before = JsonifiedValue<User> // unknown
type After = JsonifiedValue<User>  // { id: number, createdAt: string }

Why

Plain objects were detected with T extends Record<string, unknown>. TypeScript gives an implicit index signature only to object literal types, never to interfaces or class instances (microsoft/TypeScript#15300). So interfaces fell through every branch to unknown.

Fixes

• Interface and class-instance outputs map their properties the same way type aliases do.
• readonly arrays and tuples, ReadonlyMap, and ReadonlySet resolve like their mutable counterparts instead of becoming unknown.
• Functions and constructors still resolve to unknown, and built-ins (Date, Map, Set, Blob, event iterators) behave as before.
• Other objects without a dedicated branch (class instances, Error, RegExp, ...) are mapped by their properties like interfaces, since TypeScript cannot tell them apart structurally.

Performance

Plain objects skip the costly infer branches, so resolving outputs costs about the same as before: in a 400-object benchmark, ~92k instantiations vs ~95k on main. Without the extra check the fix would have taken ~185k.

Testing

• New type tests for interfaces at the top level, nested in objects and arrays, in JsonifiedClient outputs, for callable interfaces, and for readonly arrays, tuples, maps, and sets. They fail on main and pass here.
• pnpm type:check passes; eslint is clean on the changed files.

[pkg-pr-new]

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commit: cf48d62

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[codspeed]

Merging this PR will not alter performance

✅ 30 untouched benchmarks

---

_{Comparing dinwwwh:claude/jsonified-value-type-issue-0c9b63 (cf48d62) with main (dfa4e1e)}

[pullfrog]

ℹ️ Minor suggestions only — the interface fix itself is correct and well tested.

Reviewed changes

• Rewrote JsonifiedValue branch order — primitives, Date | bigint | URL, and Array are checked first, then a non-inferring allowlist guard (Blob | Map | Set | AsyncIteratorObject | Function) selects the old infer branches, then T extends object maps the rest. This replaces T extends Record<string, unknown>, which interfaces never satisfied.
• New type tests — interface at the top level, nested in objects/arrays, a JsonifiedClient interface output, and callable-interface → unknown. Verified these fail on the base rev (only types.ts reverted) and pass here.
• pnpm type:check passes (root tsc covers *.test-d.ts); eslint is clean on both files.

ℹ️ Nitpicks

• JsonifiedValue<readonly Date[]> now resolves to readonly string[] (the PR body calls this out), but no test covers readonly arrays. Consider adding one next to the interface cases.
• readonly tuples bypass JsonifiedArray's undefined → null normalization: JsonifiedValue<readonly [Date, undefined]> is readonly [string, undefined], while the mutable [Date, undefined] becomes [string, null]. Harmless but inconsistent.

  ^{｜ Fix all ➔ ｜ Fix 👍s ➔ ｜ View workflow run ｜ Using DeepSeek Flash (free via Pullfrog for OSS) ｜ 𝕏}

[pullfrog]

✅ No new issues found.

Reviewed changes

This run re-reviews the delta since the prior pullfrog review (122ac80), which was the single commit cf48d62 adding readonly-type coverage on top of the interface fix.

• Readonly primitives now follow their mutable counterparts — the Array guard became ReadonlyArray, so readonly Date[] resolves to string[] instead of falling through to the mapped branch.
• ReadonlyMap/ReadonlySet now serialize to entry arrays — the allowlist and inner infer branches widened from Map/Set to ReadonlyMap/ReadonlySet, matching the runtime identity of those values.
• JsonifiedArray constraint widened to ReadonlyArray — readonly tuples route through the same path as mutable ones, so readonly [Date, undefined] gets the undefined → null normalization ([string, null]) that mutable tuples already had.
• Tests added for ReadonlyMap, ReadonlySet, readonly Date[], and readonly [Date, undefined]; they fail on the parent commit and pass here. pnpm type:check passes at head.

The remaining structural mapping of Error/RegExp/Promise is an intentional, documented decision (plain data can structurally extend Error, so special-casing it would break ordinary outputs), so the prior inline thread on built-ins is now resolved.

  ^{｜ View workflow run ｜ Using DeepSeek Flash (free via Pullfrog for OSS) ｜ 𝕏}