Declaration emit should not inline type definitions

TypeScript Version: 3.8.3, 3.8.1, probably others

Search Terms:

declaration inlining, dts inlining, declaration inline, inline literal, declaration literal

Code

// parent.ts
import { num, obj } from "./child"
export const reExportNum = num;
export const reExportObj = obj;

// child.d.ts
export declare const num: number;
export declare const obj: { a: 1 };

tsc index.ts --declaration

Expected behavior:

Declaration emit for parent.ts should not inline types.

// parent.d.ts
import { num, obj } from "./child"
export declare const reExportNum: typeof num;
export declare const reExportObj: typeof obj;

Actual behavior:

Today, declaration emit for parent.ts inlines the types and eliminates the import of the child.d.ts type definition.

// parent.d.ts
export declare const reExportNum: number;
export declare const reExportObj: {
    a: 1;
};

This is a correctness issue, because consumers of parent.d.ts will not get the correct types if the types in child.d.ts change.

In practice, this is most likely to happen when parent and child are in separate packages, because they are published independently, i.e. an application uses parent-package which uses types from child-package. This is exacerbated by the current practice on npm of parent-package depending on an unpinned version, using package.json dependency syntax "child-package": "*".

This issue was co-authored with @robpalme