Crash in getLocalModuleSpecifier when formatting a type: normalizeSlashes receives undefined (regression in 6.0)

The crash occurs in getLocalModuleSpecifier when normalizeSlashes receives undefined. Start by examining the TypeScript source at src/compiler/moduleSpecifiers.ts and src/compiler/utilities.ts around the normalizeSlashes call. Reproduce the issue with the provided ESLint setup to see the stack trace. Look for where the importing file's path becomes undefined during the second parse after a fix is applied. Check how SourceFile paths are handled in the language service or program reuse scenarios.

🔎 Search Terms

normalizeSlashes, getLocalModuleSpecifier, computeModuleSpecifiers, getSpecifierForModuleSymbol, symbolToNode, "Cannot read properties of undefined (reading 'includes')"

🕗 Version & Regression Information

Crashes on [email protected]. The identical project on [email protected] does not crash — nothing else changed, only the typescript dependency. So this is a regression between 5.9 and 6.0.

⏯ Playground Link

Not reproducible in the playground: it needs a Program created from a tsconfig with a package in node_modules, and a second parse of modified file text.

💻 Code

Four files. npm install, then two commands.

package.json

{
  "name": "ts6-fix-crash-repro",
  "private": true,
  "type": "module",
  "devDependencies": {
    "eslint": "10.10.0",
    "typescript": "6.0.3",
    "typescript-eslint": "8.70.1",
    "vitest": "5.0.0"
  }
}

tsconfig.json

{
  "compilerOptions": {
    "target": "ES2022",
    "module": "NodeNext",
    "moduleResolution": "NodeNext",
    "strict": true,
    "noEmit": true
  },
  "include": ["src"]
}

eslint.config.mjs

import tseslint from 'typescript-eslint';

export default tseslint.config({
  files: ['src/**/*.ts'],
  extends: [...tseslint.configs.recommendedTypeChecked],
  languageOptions: {
    parserOptions: { project: './tsconfig.json', tsconfigRootDir: import.meta.dirname },
  },
});

src/index.ts

/* eslint-disable @typescript-eslint/no-explicit-any */
import { vi } from 'vitest';

export const f = vi.fn();

Run:

npx eslint src                 # exit 0 — one warning, the unused disable directive
npx eslint src --fix-dry-run   # exit 2 — crash

The eslint-disable comment on line 1 is deliberately unused. It is the only fixable thing in the project, so it is what makes the second command apply a fix and re-parse the file. Delete that line and --fix-dry-run exits cleanly.

🙁 Actual behavior

TypeError: Cannot read properties of undefined (reading 'includes')
Occurred while linting .../src/index.ts:4
Rule: "@typescript-eslint/no-unsafe-assignment"
    at normalizeSlashes (typescript/lib/typescript.js:8853:15)
    at getNormalizedAbsolutePath (typescript/lib/typescript.js:8899:12)
    at getLocalModuleSpecifier (typescript/lib/typescript.js:50210:25)
    at computeModuleSpecifiers (typescript/lib/typescript.js:50155:19)
    at getModuleSpecifiersWithCacheInfo (typescript/lib/typescript.js:50089:18)
    at getModuleSpecifiers (typescript/lib/typescript.js:50056:10)
    at getSpecifierForModuleSymbol (typescript/lib/typescript.js:57728:27)
    at createExpressionFromSymbolChain (typescript/lib/typescript.js:57987:29)
    at symbolToExpression (typescript/lib/typescript.js:57974:14)
    at symbolToNode (typescript/lib/typescript.js:55769:14)

normalizeSlashes is called with undefined. The named rule is incidental — it is whichever type-aware rule reaches the file first. Disabling no-unsafe-assignment moves the crash to no-misused-promises, and disabling that moves it to no-unnecessary-type-assertion.

🙂 Expected behavior

getLocalModuleSpecifier should not dereference an absent path on the importing file, or the caller should not reach it with one. Formatting a type should not throw.

Additional information

Conditions, each verified by changing one thing and re-running:

change	result
[email protected]	crash
[email protected], everything else identical	no crash
parserOptions.project	crash
parserOptions.projectService: true instead	no crash
TSESTREE_SINGLE_RUN=false in the environment	no crash
remove the unused eslint-disable line (so no fix is applied)	no crash
recommendedTypeChecked or strictTypeChecked	crash either way
npm or pnpm for the install	crash either way

A fix being applied is necessary: without one there is no second parse and no crash.

I could not reduce this below the ESLint harness. Calling checker.typeToString(type, enclosingDeclaration, TypeFormatFlags.UseFullyQualifiedType) directly does not reproduce it, including when enclosingDeclaration is a SourceFile from a different Program, or one built with ts.createSourceFile whose path I removed — those all format fine. So the pathless importing file appears to arise from something more specific than a detached SourceFile, and I have not identified what.

Related: typescript-eslint#12749 reported the same stack from a different trigger (CI=true, so the same single-run Program inference that TSESTREE_SINGLE_RUN=false disables above). It was closed as external on the grounds that the stack is inside TypeScript, with a request to file here and to include a reproduction. This is that reproduction.

Environment: macOS 27.0 arm64, Node v26.8.1.