Function loadEffectiveOptions

src/resolution/path-aliases.ts:175–219

* Read `filePath` and fold its `extends` chain into a single set of * effective options. Parents are applied first and the nearest config * wins — tsc replaces `paths` wholesale rather than merging it. * * `stack` holds the configs currently being resolved, so a cycle * (`a extends b extends a`

(
  filePath: string,
  stack: Set<string>,
  depth: number
)

 * (`a extends b extends a`) stops instead of recursing forever.
 */
function loadEffectiveOptions(
  filePath: string,
  stack: Set<string>,
  depth: number
): EffectiveOptions | null {
  const abs = path.resolve(filePath);
  if (stack.has(abs) || depth > MAX_EXTENDS_DEPTH) {
    logDebug('path-aliases: extends chain cycle or too deep', { filePath: abs, depth });
    return null;
  }
  const raw = readTsconfigLike(abs);
  if (!raw) return null;

  stack.add(abs);
  const dir = path.dirname(abs);
  const effective: EffectiveOptions = {};

  const parents = typeof raw.extends === 'string' ? [raw.extends] : (raw.extends ?? []);
  for (const spec of parents) {
    if (typeof spec !== 'string') continue;
    const target = resolveExtendsTarget(spec, dir);
    if (!target) {
      logDebug('path-aliases: unresolved extends', { from: abs, spec });
      continue;
    }
    const inherited = loadEffectiveOptions(target, stack, depth + 1);
    if (!inherited) continue;
    if (inherited.baseUrl !== undefined) effective.baseUrl = inherited.baseUrl;
    if (inherited.paths !== undefined) {
      effective.paths = inherited.paths;
      effective.pathsDir = inherited.pathsDir;
    }
  }
  stack.delete(abs);

  const co = raw.compilerOptions ?? {};
  // Both are relative to the file that declared them, not to whichever
  // config started the chain.
  if (typeof co.baseUrl === 'string') effective.baseUrl = path.resolve(dir, co.baseUrl);
  if (co.paths && typeof co.paths === 'object') {
    effective.paths = co.paths;
    effective.pathsDir = dir;
  }
  return effective;
}

function readTsconfigLike(filePath: string): RawTsconfig | null {
  try {