Invoking TypeScript with --outDir src makes some source files invisible #60867

🔎 Search Terms

outDir globalThis declarations exclude include not found error

TS7017: Element implicitly has an 'any' type because type 'typeof globalThis' has no index signature

🕗 Version & Regression Information

• This changed between versions 5.5.0-dev.20240426 and 5.5.0-dev.20240427
• This changed in commit or PR Exclude outDir and declarationDir even if they come from extended config #58335

⏯ Playground Link

https://github.com/jthemphill/ts-5.5-repro

💻 Code

Run with tsc --outDir src.

tsconfig.json:

{
  "compilerOptions": {
    "noEmit": true,
    "noImplicitAny": true
  },
  "files": ["src/test.spec.ts"],
  "include": ["./src/**/*"]
}

src/test.spec.ts:

globalThis.__IS_CI__ = true;

common.ts:

declare global {
  namespace globalThis {
    var __IS_CI__: boolean;
  }
}
export {};

🙁 Actual behavior

On TS 5.4, the command passes.

On TS 5.5 or newer, the command fails with the error:

src/test.spec.ts:5:12 - error TS7017: Element implicitly has an 'any' type because type 'typeof globalThis' has no index signature.

5 globalThis.__IS_CI__ = true;

🙂 Expected behavior

I expect both commands to pass.

Additional information about the issue

• I'm trying to migrate a large codebase off of Typescript 5.3, and this is the last blocker remaining.
• --outDir src is passed to tsc via a Bazel build rule.
• If {"outDir": "src"} is added to tsconfig.json, then typechecking will fail on both versions of TypeScript.
  • Therefore, I believe that Exclude outDir and declarationDir even if they come from extended config #58335 is a legitimate bugfix which stopped TypeScript from ignoring --outDir src on the command line.

As I understand it, according to #58327, outDir causes source files to be excluded because otherwise, we'll overwrite the .d.ts files we're using as source files.

But if you don't specify outDir at all, your output directory will be the same as your source directory anyway, and somehow this won't be a problem. According to the TypeScript docs:

If [outDir is] not specified, .js files will be emitted in the same directory as the .ts files they were generated from.

Since we already have default behavior where the source and output directories are the same, why does specifying outDir cause such a different behavior?