Don't crash type-aware linting when the project's tsbuildinfo doesn't exist yet

src/parser/ts-patch.js

@@ -36,8 +36,16 @@ try {
       readFile(fname) {
         let fileName = fname;
         let content = '';
-        if (fileName.endsWith('tsconfig.tsbuildinfo')) {
-          return content;
+        if (fileName.endsWith('.tsbuildinfo')) {
+          // Incremental build state is optional and never a .gts/.gjs file, so
+          // delegate to the real host: it returns the content when present and
+          // reports a missing file as absent (undefined) instead of throwing.
+          // The watch program reads `tsBuildInfoFile` without a fileExists
+          // probe (readBuilderProgram -> host.readFile), so a fresh/cleaned
+          // project with a custom buildinfo path (e.g.
+          // "declarations/.tsbuildinfo") would otherwise abort linting of
+          // every file with a Parsing error: ENOENT.
+          return sys.readFile(fileName);
         }
 
         try {

tests/ts-patch.test.js

@@ -0,0 +1,45 @@
+import { describe, expect, it } from 'vitest';
+import { createRequire } from 'node:module';
+import fs from 'node:fs';
+import os from 'node:os';
+import path from 'node:path';
+
+import { patchTs } from '../src/parser/ts-patch.js';
+
+// Resolve the same typescript instance ts-patch patches (the one
+// @typescript-eslint/parser depends on), so we observe the patched ts.sys.
+const require = createRequire(import.meta.url);
+const parserPath = require.resolve('@typescript-eslint/parser');
+const ts = require(require.resolve('typescript', { paths: [parserPath] }));
+
+// Incremental build state is optional input: when `tsBuildInfoFile` points at
+// a file that has not been produced yet (fresh checkout, cleaned repo), the
+// watch program used for type-aware linting reads it WITHOUT a fileExists
+// probe (readBuilderProgram -> host.readFile). ts.sys.readFile's contract is
+// to report a missing file as absent, not to throw — a throw here aborts
+// linting of every file in the project with
+// `Parsing error: ENOENT ... <project>/declarations/.tsbuildinfo`.
+describe('patched ts.sys.readFile — .tsbuildinfo handling', () => {
+  patchTs();
+
+  it('treats a missing custom-named .tsbuildinfo as absent instead of throwing', () => {
+    // e.g. { "tsBuildInfoFile": "declarations/.tsbuildinfo" } — only the
+    // default `tsconfig.tsbuildinfo` name was guarded before.
+    const missing = path.join(os.tmpdir(), 'ee-parser-no-such-dir', 'declarations', '.tsbuildinfo');
+
+    expect(() => ts.sys.readFile(missing)).not.toThrow();
+    expect(ts.sys.readFile(missing)).toBeUndefined();
+  });
+
+  it('returns the real content when the .tsbuildinfo exists', () => {
+    const dir = fs.mkdtempSync(path.join(os.tmpdir(), 'ee-parser-buildinfo-'));
+    const buildInfo = path.join(dir, 'custom.tsbuildinfo');
+    fs.writeFileSync(buildInfo, '{"version":"5.9.3"}');
+
+    try {
+      expect(ts.sys.readFile(buildInfo)).toBe('{"version":"5.9.3"}');
+    } finally {
+      fs.rmSync(dir, { recursive: true, force: true });
+    }
+  });
+});