Description

When a child tsconfig extends a parent that defines rootDirs with relative paths, the paths are normalized relative to the child's directory instead of the parent's directory.

Reproduction

project/
├── tsconfig.json          # { "extends": "./.svelte-kit/tsconfig.json" }
├── src/routes/+page.ts    # import type { PageLoad } from './$types'
└── .svelte-kit/
    ├── tsconfig.json       # { "compilerOptions": { "rootDirs": ["..", "./types"] } }
    └── types/src/routes/
        └── $types.d.ts

import { ResolverFactory } from 'oxc-resolver';

// Works: pointing directly at the config that defines rootDirs
const r1 = new ResolverFactory({
  tsconfig: { configFile: 'project/.svelte-kit/tsconfig.json' },
  extensions: ['.ts', '.d.ts'],
});
r1.resolveFileSync('project/src/routes/+page.ts', './$types');
// Resolves to project/.svelte-kit/types/src/routes/$types.d.ts

// Broken: using the child that extends the parent
const r2 = new ResolverFactory({
  tsconfig: { configFile: 'project/tsconfig.json' },
  extensions: ['.ts', '.d.ts'],
});
r2.resolveFileSync('project/src/routes/+page.ts', './$types');
// Cannot find module './$types'

Root cause

In extend_tsconfig(), root_dirs are cloned from the parent as-is (raw relative paths). Then build() normalizes them relative to config_dir (the child's directory), not the parent's directory where they were defined.

Compare with paths which carries paths_base to preserve the correct base directory through the extends chain. rootDirs has no equivalent mechanism.

Real-world impact

This affects SvelteKit projects where tsconfig.json extends .svelte-kit/tsconfig.json which defines rootDirs: ["..", "./types"] for virtual module resolution.