Debug Failure: "No error for last overload signature" crash when CustomTypeOptions.resources uses typeof on a large (~10k line) JSON file

[shahanahmed86]

Bug Report

TypeScript version: 5.7.2 (also reproduced on 6.0.0-beta)
i18next version: 24.2.2
react-i18next version: 15.4.0
Node: 22.9.0

---

Summary

TypeScript crashes with an internal Debug Failure when:

1. A .d.ts file augments i18next's CustomTypeOptions.resources using typeof on a large JSON file (~10k lines), and
2. TypeScript resolves the t() function's overloads against that resource type.

This is a compiler crash — not a type error surfaced to the user — so it always represents a broken invariant inside the checker.

---

Reproduction

File structure

src/
  i18next.d.ts
public/assets/locales/en/translation.json   ← ~10,000 lines, deeply nested

src/i18next.d.ts

import type translation from '../public/assets/locales/en/translation.json';

declare module 'i18next' {
  interface CustomTypeOptions {
    defaultNS: 'translation';
    resources: {
      translation: typeof translation;
    };
  }
}

tsconfig.app.json (relevant parts)

{
  "compilerOptions": {
    "moduleResolution": "bundler",
    "resolveJsonModule": true,
    "paths": {
      "@locales/*": ["./public/assets/locales/*"]
    }
  },
  "include": ["src"]
}

Running tsc --noEmit

error TS2Function: Debug Failure. No error for last overload signature.

The process exits non-zero. No useful file/line information is attached to the diagnostic.

---

Key observations

Condition	Crashes?
typeof on the full 10,085-line JSON	✅ Yes
Replaced with a small hand-written type (same shape, ~20 keys)	❌ No
CustomTypeOptions tuned with returnNull: false, returnEmptyString: false, jsonFormat: "v4"	✅ Still crashes
import type instead of import	✅ Still crashes
resolveJsonModule toggled	✅ Still crashes

The crash is scale-triggered: the combination of i18next's multi-overload t() signature and the deeply-nested union type produced by typeof largeJson causes the overload checker to reach an assertion it cannot satisfy.

---

Why this is a TypeScript bug

Debug Failure diagnostics are internal Debug.assert() calls that should never reach the user. Regardless of how complex or unusual the input types are, the compiler must either:

• produce a proper type error, or
• succeed silently.

Crashing with an assertion failure when resolving overloads against a large typeof type is a broken invariant in the checker.

---

Workarounds (all unsatisfactory)

• Commit with --no-verify to skip the typecheck step
• Replace typeof translation with a hand-maintained TranslationKeys type (defeats the purpose of auto-typed i18n)
• Suppress with // @ts-ignore across all call sites

---

Additional context

The t() function in i18next 24.x has dozens of overloads that distribute over the resource namespace union. When the resource type has thousands of deeply-nested keys (as is common in production i18n files), the overload resolution cartesian-products the key space and appears to hit an unchecked branch in the "last overload must produce an error" logic.

A stack trace from the crash (abbreviated):

TypeError: Debug Failure. No error for last overload signature.
    at resolveOverload (typescript/lib/typescript.js)
    at getResolvedSignature (typescript/lib/typescript.js)
    at checkCallExpression (typescript/lib/typescript.js)

[RyanCavanaugh]

We need a concrete repro we can run ourselves