Union type imported from external library collapsed in emitted type declarations

[rmunch]

Bug Report

I've got an application that consumes two libraries like so:

The problem I'm facing is that the TypeScript build for LibB is outputting a collapsed "snapshot" of a union type that is imported from LibA rather than preserving a reference to the externally-defined union type. Because of this, when the application is using a newer version of LibA, the application cannot use types from the union that do not exist in the older version referenced by LibB.

As a workaround, I can update LibB to use the newer version of LibA, publish a new version, and update App to point to it, but since the implementation of LibB does not need to be updated, this step should be unnecessary.

I believe there have been various past proposals to solve this (such as in #35654), as well as a workarounds for specific use cases, but I haven't found a workaround that solves this particular case.

Thank you!

🔎 Search Terms

preserve type alias
prevent union type collapse

🕗 Version & Regression Information

v4.5.4, and other recent versions.

⏯ Playground Link

Because this issue only occurs when a project uses the emitted declarations from another project, I may not be able to provide a playground link.

💻 Code

# LibA (as of v1.1.0)
export type TypeFromLibA = "a" | "b";

# LibA (as of v1.5.0)
export type TypeFromLibA = "a" | "b" | "c";

# LibB
import type TypeFromLibA from "lib-a"; // ex. as npm library

export const functionFromLibB = (val: TypeFromLibA) => {
   ...
}

🙁 Actual behavior

# App

import functionFromLibB from "lib-b";

functionFromLibB("a"); // OK
functionFromLibB("c"); // Argument of type '"c"' is not assignable to parameter of type '"a" | "b"'.ts(2345)

🙂 Expected behavior

# App

import myFunction from "lib-b";

myFunction("a"); // OK
myFunction("c"); // OK

[RyanCavanaugh]

What is MyUnionType ?

[rmunch]

@RyanCavanaugh I got that typo fixed (MyUnionType should've been TypeFromLibA). Thank you!

[RyanCavanaugh]

This should work fine in any package manager that sets up node_modules correctly (with 1.5 in the parent and 1.1 as a subfolder of B), which is now "all of them". If not, please open a new issue with a concrete repo we can clone and investigate. Thanks!