Changing baseUrl causes compiler not to look at @types subdirectory

When baseUrl is set in tsconfig.json, the compiler will not look into baseUrl/@types to try and find type definitions

TypeScript Version: 2.2.1

Code

Example is available at:

https://github.com/spion/ts-base-url-issue

If the "paths" entry is removed from tsconfig.json, the compiler is unable to find react-dom

Expected behavior:

Compiles fine even without a "paths" entry.

Actual behavior:

Compiler doesn't find the definition file if the paths config option is removed. --trace-resolution shows that it never tries baseUrl/@types/react-dom

======== Resolving module 'react-dom' from '/Users/spion/Documents/tests/ts-modules-broken/app/index.ts'. ========
Explicitly specified module resolution kind: 'NodeJs'.
'baseUrl' option is set to '/Users/spion/Documents/tests/ts-modules-broken/vendor/node_modules', using this value to resolve non-relative module name 'react-dom'
Resolving module name 'react-dom' relative to base url '/Users/spion/Documents/tests/ts-modules-broken/vendor/node_modules' - '/Users/spion/Documents/tests/ts-modules-broken/vendor/node_modules/react-dom'.
Loading module as file / folder, candidate module location '/Users/spion/Documents/tests/ts-modules-broken/vendor/node_modules/react-dom', target file type 'TypeScript'.
File '/Users/spion/Documents/tests/ts-modules-broken/vendor/node_modules/react-dom.ts' does not exist.
File '/Users/spion/Documents/tests/ts-modules-broken/vendor/node_modules/react-dom.tsx' does not exist.
File '/Users/spion/Documents/tests/ts-modules-broken/vendor/node_modules/react-dom.d.ts' does not exist.
Found 'package.json' at '/Users/spion/Documents/tests/ts-modules-broken/vendor/node_modules/react-dom/package.json'.
'package.json' does not have a 'typings' field.
'package.json' does not have a 'types' field.
File '/Users/spion/Documents/tests/ts-modules-broken/vendor/node_modules/react-dom/index.ts' does not exist.
File '/Users/spion/Documents/tests/ts-modules-broken/vendor/node_modules/react-dom/index.tsx' does not exist.
File '/Users/spion/Documents/tests/ts-modules-broken/vendor/node_modules/react-dom/index.d.ts' does not exist.
Loading module 'react-dom' from 'node_modules' folder, target file type 'TypeScript'.
Directory '/Users/spion/Documents/tests/ts-modules-broken/app/node_modules' does not exist, skipping all lookups in it.
Directory '/Users/spion/Documents/tests/ts-modules-broken/node_modules' does not exist, skipping all lookups in it.
Directory '/Users/spion/Documents/tests/node_modules' does not exist, skipping all lookups in it.
Directory '/Users/spion/Documents/node_modules' does not exist, skipping all lookups in it.
Directory '/Users/spion/node_modules' does not exist, skipping all lookups in it.
Directory '/Users/node_modules' does not exist, skipping all lookups in it.
Directory '/node_modules' does not exist, skipping all lookups in it.
'baseUrl' option is set to '/Users/spion/Documents/tests/ts-modules-broken/vendor/node_modules', using this value to resolve non-relative module name 'react-dom'
Resolving module name 'react-dom' relative to base url '/Users/spion/Documents/tests/ts-modules-broken/vendor/node_modules' - '/Users/spion/Documents/tests/ts-modules-broken/vendor/node_modules/react-dom'.
Loading module as file / folder, candidate module location '/Users/spion/Documents/tests/ts-modules-broken/vendor/node_modules/react-dom', target file type 'JavaScript'.
File '/Users/spion/Documents/tests/ts-modules-broken/vendor/node_modules/react-dom.js' does not exist.
File '/Users/spion/Documents/tests/ts-modules-broken/vendor/node_modules/react-dom.jsx' does not exist.
Found 'package.json' at '/Users/spion/Documents/tests/ts-modules-broken/vendor/node_modules/react-dom/package.json'.
'package.json' has 'main' field 'index.js' that references '/Users/spion/Documents/tests/ts-modules-broken/vendor/node_modules/react-dom/index.js'.
File '/Users/spion/Documents/tests/ts-modules-broken/vendor/node_modules/react-dom/index.js' exist - use it as a name resolution result.
======== Module name 'react-dom' was successfully resolved to '/Users/spion/Documents/tests/ts-modules-broken/vendor/node_modules/react-dom/index.js'. ========
app/index.ts(1,22): error TS7016: Could not find a declaration file for module 'react-dom'. '/Users/spion/Documents/tests/ts-modules-broken/vendor/node_modules/react-dom/index.js' implicitly has an 'any' type.

Workaround

Adding a paths entry gets the @types directory into the lookup list again:

    "baseUrl": "./vendor/node_modules",
    "paths": {
      "*": ["@types/*", "*"]
    }

Start with the linked ts-base-url-issue reproduction, especially tsconfig.json and app/index.ts, and run the compiler with --trace-resolution. Compare resolution with and without the paths entry, then trace the module-resolution entry point responsible for declaration lookup. Done means baseUrl/@types/react-dom is searched without requiring a paths configuration and the reproduction compiles successfully.