`genType` emits relative `./S.gen` for workspace (file:) dependencies instead of package-qualified sury/src/S.gen

Summary

When a consumer package depends on another package via a pnpm/npm workspace file: link, genType emits the import path for cross-package types as a relative path (./S.gen) instead of the package-qualified path (sury/src/S.gen). The relative path does not resolve — there's no S.gen.{ts,d.ts} next to the generated file — so downstream tsc fails with TS2307: Cannot find module './S.gen'.

Versions

• rescript: 12.2.0 (also reproduces on 12.0.0-beta.5)
• Package manager: pnpm@9.0.5
• Node: 22.6.0
• OS: Ubuntu 24.04

Minimal repro

Two-package pnpm workspace. lib publishes hand-written S.gen.d.ts; app imports its type via @genType.

repro/
├─ pnpm-workspace.yaml         # packages: ["packages/*"]
├─ packages/
│  ├─ lib/
│  │  ├─ package.json          # { "name": "lib" }
│  │  ├─ rescript.json         # below
│  │  └─ src/
│  │     ├─ S.res              # let string: t<string> ; type error = {…}
│  │     └─ S.gen.d.ts         # hand-written: export type t<T> = …; export type error = …
│  └─ app/
│     ├─ package.json          # { "dependencies": { "lib": "file:../lib" } }
│     ├─ rescript.json         # below
│     └─ src/
│        └─ Demo.res           # @genType let x: S.error = …

packages/lib/rescript.json:

{
  "name": "lib",
  "namespace": false,
  "suffix": ".res.mjs",
  "package-specs": { "module": "esmodule", "in-source": true },
  "sources": [{ "dir": "src", "public": ["S"] }]
}

packages/app/rescript.json:

{
  "name": "app",
  "sources": [{ "dir": "src", "subdirs": true }],
  "package-specs": { "module": "esmodule", "in-source": true },
  "suffix": ".res.mjs",
  "dependencies": ["lib"],
  "gentypeconfig": { "generatedFileExtension": ".gen.ts" }
}

packages/app/src/Demo.res:

@genType
let err: S.error = S.Error.make(…)

Run pnpm install && pnpm --filter=app rescript build.

Actual output

packages/app/src/Demo.gen.ts:

import type {error as S_error} from './S.gen';   // ❌ relative — no such file next to Demo.gen.ts

tsc --noEmit reports:

Demo.gen.ts(…,…): error TS2307: Cannot find module './S.gen' or its corresponding type declarations.

Expected output

import type {error as S_error} from 'lib/src/S.gen';

(This was the behavior in earlier versions — the same repo has a committed Demo.gen.ts from a prior generation that used from 'lib/src/S.gen'.)

Context / investigation

• app/lib/bs/.sourcedirs.json correctly lists ["lib", "/abs/path/packages/lib"] under "pkgs", so the dependency is known to the build.
• Poking at the bsc.exe strings, the relevant logic lives in compiler/gentype/ModuleResolver.ml (resolve_module, import_path_for_reason_module_name, read_bs_dependencies_dirs) and compiler/gentype/ImportPath.ml.
• It looks like when the dep's source directory is reachable via a workspace path, the resolver adds S to the consumer's local module_name_map and that takes precedence over the dep lookup, so the import is emitted relative.
• No gentypeconfig option currently works around this: shims treats its value as a shim-file path (emits ./lib/src/S.gen.shim), and there's no packageName / importPath / modulesMap option documented in the bsc binary.

Workaround

Ship a shim next to the generated file that re-exports from the dep:

// packages/app/src/S.gen.ts
export type { error, t } from "lib/src/S.gen";

This keeps the relative ./S.gen import resolving, but is obviously a workaround.

Does it reproduce outside file: deps?

Not when lib is a real npm dependency (resolved under node_modules/lib) — that case emits 'lib/src/S.gen' correctly. Only workspace / file: installs hit this.

Start in compiler/gentype/ModuleResolver.ml, especially resolve_module, import_path_for_reason_module_name, and read_bs_dependencies_dirs, then inspect compiler/gentype/ImportPath.ml. Reproduce with the pnpm workspace described and run tsc --noEmit. Done means the generated import uses the package-qualified lib/src/S.gen path for file: dependencies and the type check passes without a shim.