Fix nested dist when referenced package has a .ts file next to a .js file

[maschwenk]

Reproduction case:

• Repro for weird nesting TS Go does dimitropoulos/tsgo-repros#2

I prompted Cursor with:

I'm having an issue where if we have this structure

package A
   depends on package B and imports from package B
package B
   has a dist folder with both an dist/index.js and dist/index.ts

when I build package A, the dist folder of A becomes nested. the Go implementation behaves this way but the javascript implementation doesn't. Can you figure this out?

and after a few turns got this, so still working out if this is valid or not but it does fix my issue.

Simpler fix for this is just not having .ts files in dist, which is generally a good thing in general, but I was just curious why this was behaving differently in strada/corsa.

[Copilot]

Pull Request Overview

A fix to prevent nested dist directories when referencing a package that ships both .js and .ts files.

• Added findPackageRoot helper to identify package boundaries via package.json.
• Enhanced import resolution to detect cross-package imports and skip TypeScript source files from external dependencies.
• Updated shouldAddFile logic to exclude external .ts/.tsx/.mts/.cts files.

Comments suppressed due to low confidence (3)

internal/compiler/fileloader.go:434

• [nitpick] The isExternalDependency flag is being overloaded to also represent cross-package imports. Consider introducing a separate isCrossPackageDependency flag for clarity and to simplify intent.

			if !isExternalDependency && !tspath.IsExternalModuleNameRelative(moduleName) {

internal/compiler/fileloader.go:458

• Add a unit test for the scenario where a package’s dist contains both .js and .ts files to verify that external TypeScript sources are correctly excluded and no nested output folder is created.

				!isTypeScriptSourceFromExternalDep &&

internal/compiler/fileloader.go:336

• Consider caching results of findPackageRoot (e.g., in a map) to avoid repeated filesystem walks for each import, which could affect performance in large monorepos.

func (p *fileLoader) findPackageRoot(startDir string) string {

[jakebailey]

I'm not sure what's going on here; this code is very much unlike the original code. I think the fix is going to look very different to this.

[maschwenk]

Per Discord, we've determined that the actual root cause of the issue here is the differences in implementation of

_submodules/TypeScript/src/compiler/utilities.ts's sourceFileMayBeEmitted versus

internal/compiler/emitter.go's sourceFileMayBeEmitted

I believe the actual case that should be returning false here is one of these:

    // Source file from node_modules are not emitted
    if (host.isSourceFileFromExternalLibrary(sourceFile)) return false;
    // forcing dts emit => file needs to be emitted
    if (forceDtsEmit) return true;
    // Check other conditions for file emit
    // Source files from referenced projects are not emitted
    if (host.isSourceOfProjectReferenceRedirect(sourceFile.fileName)) return false;

and I'm fairly sure it's isSourceFileFromExternalLibrary which is still not implemented in Go-land

[jakebailey]

All of the code needed to implement isSourceFileFromExternalLibrary is actually there since #1189, actually. We just aren't collecting that list of files.