TypeScript: expand ${configDir} in inherited tsconfig paths #13

Problem

tsconfig.json allows the variable ${configDir} in path options such as outDir, rootDir, baseUrl and paths. It stands for the directory of the top-level config file, so a shared base config can point at the project that extends it. The TypeScript reader in #12 reads these options but does not expand the variable.

What happens

sindresorhus/ky at 0d59458a0a extends @sindresorhus/tsconfig, whose tsconfig.json has:

"outDir": "${configDir}/distribution"

With node_modules present, load_typescript_config returns out_dir as:

<repo>/node_modules/@sindresorhus/tsconfig/${configDir}/distribution

That directory does not exist, so the compiled-target mapping from outDir back to rootDir cannot fire.

Why it does not show today

ky's seven entry points are still found, because the fallback added in #12 (a distribution/ target maps to a unique source/ file) covers this case. So the facts are right for ky. A project whose source root is not src/ or source/, or whose base config sets paths with ${configDir}, would get a wrong or missing mapping with no line saying so.

Fix

In typescript_config.py, when reading a path option from any file in the extends chain, replace ${configDir} with the directory of the top-level tsconfig.json before resolving it. The TypeScript documentation for ${configDir} is the reference for the rule.

Acceptance

Written before the change:

• On ky at the pinned commit with node_modules installed, out_dir resolves to <repo>/distribution.
• A fixture whose base config sets "paths": { "@/*": ["${configDir}/src/*"] } resolves @/lib/util to the source module.
• The seven-repository run in bench/typescript_adapter.py produces the same numbers as the run recorded on Add TypeScript extraction, change, and history support #12.

Found while reviewing #12. Not blocking that PR.