Summary

Fixes https://github.com/webpack/webpack/actions/runs/26091453506/job/76736654246?pr=20966

When tsconfig: true and _findTsconfigUpward walks up the directory tree to find a tsconfig.json without baseUrl, tsconfigPathsToResolveOptions was unconditionally adding the tsconfig's directory (configDir) to the resolver's modules list.

This caused bare specifiers to incorrectly resolve against files in the tsconfig root directory. For example, import value from "package" would resolve to <tsconfig_root>/package.json (a JSON module) instead of node_modules/package/index.js.

Root cause

In tsconfigPathsToResolveOptions:

const absoluteBaseUrl = !baseUrl ? configDir : resolver.join(configDir, baseUrl);
// ...
if (absoluteBaseUrl && !modules.includes(absoluteBaseUrl)) {
    modules.push(absoluteBaseUrl);  // ← bug: adds configDir even without baseUrl
}

When baseUrl is undefined, absoluteBaseUrl falls back to configDir. The guard only checks truthiness, so it always adds it.

Fix

Only add absoluteBaseUrl to modules when baseUrl was explicitly set in the tsconfig:

if (baseUrl && absoluteBaseUrl && !modules.includes(absoluteBaseUrl)) {

Reproduction

A subdirectory with node_modules/package/index.js and a parent tsconfig.json (no baseUrl):

root/
├── tsconfig.json          # { "compilerOptions": { "target": "ES2017" } }
├── package.json           # { "name": "myproject" }
└── subdir/
    └── node_modules/
        └── package/
            ├── index.js   # module.exports = 42;
            └── package.json

Resolving "package" from subdir/ with tsconfig: true:

• Before: resolves to root/package.json (wrong)
• After: resolves to subdir/node_modules/package/index.js (correct)

This bug was discovered through a webpack test failure (ConfigTestCases › managedPaths › futureDefaults) where experiments.futureDefaults enables resolve.tsconfig: true.

Use of AI: Claude Code was used to investigate and draft this fix under human review.