TSServer import path autocomplete reads incorrect directories when a file is not a direct child of a root directory

[PhoebeSzmucer]

Bug Report

My project has the following (simplified) structure:

app/
  build/
    dts/
      a.d.ts
      a.graphql.d.ts # Codegened from a.graphql
      tmp/
        b.d.ts
        b.graphql.d.ts # Codegened from b.graphql
  src/
    a.ts
    a.graphql
    tmp/
      b.ts
      b.graphql

where rootDirs is configured to read from both src/ and build/dts. In other words, I'm codegening some extra .d.ts files during build time which I can then import in my typescript code.

When tsserver computes autocomplete for an import path in the a.ts file, it correctly returns the relevant files:

However, when this is done inside of a directory (in this example one called tmp/, but this works in general), tsserver reads the /tmp system directory instead of the src/tmp folder:

TSServer is still searching the current script directory (in this case src/tmp), but instead of searching build/dts/tmp it then also searches /tmp. This is probably why this bug went unnoticed for such a long time - it mostly affects imports of "extra files" that are not in source already.

I traced this behaviour to the following code:

TypeScript/src/services/stringCompletions.ts

Line 633 in b87e301

containsPath(rootDirectory, scriptDirectory, basePath, ignoreCase) ? scriptDirectory.substr(rootDirectory.length) : undefined)!; // TODO: GH#18217

If scriptDirectory is the same as rootDirectory, relativeDirectory is computed to "", and relativeDirectory is correctly appended to the root directories. This results in TSServer correctly searching both src/ and build/dst.

However, if scriptDirectory (in our example /Volumes/git/app/src/tmp) is a child of rootDirectory (/Volumes/git/app/src), this line will result in relativeDirectory being /tmp instead of /tmp, causing combinePaths a few lines down to output /tmp. This results in TSServer incorrectly searching src/tmp and /tmp (instead of build/dts/tmp). This is effectively an off-by-one error because we're accidentally grabbing a leading slash.

🔎 Search Terms

tsserver, getPathCompletionsAtPosition, import path, rootDirs

🕗 Version & Regression Information

As far as I can tell from the code this bug has always existed.