Duplicated type leads TSC to pick the wrong one in the *.d.ts file (for implicit return type)

[lorem--ipsum]

TypeScript Version: ^2.9.2 (also tested with @next with no difference)

Search Terms:
duplicated type, implicit return, type definition file

Code

// component-a/component-a.tsx
export type ComponentAPosition = 'top' | 'bottom' | 'left' | 'right';

export class ComponentA {
  doAThing(value: ComponentAPosition) {}
}

// component-b/component-b.tsx
export type ComponentBPosition = 'top' | 'right' | 'bottom' | 'left';

export class ComponentB {
  doSomething(values: ComponentBPosition) {
    return values;
  }
}

Expected behavior:

// component-b.d.ts
export declare type ComponentBPosition = 'top' | 'right' | 'bottom' | 'left';
export declare class ComponentB {
    doSomething(values: ComponentBPosition): ComponentBPosition;
}

Actual behavior:

// component-b.d.ts
export declare type ComponentBPosition = 'top' | 'right' | 'bottom' | 'left';
export declare class ComponentB {
    doSomething(values: ComponentBPosition): import("../component-a/component-a").ComponentAPosition;
}

Playground Link: Not a playground, but a repo (run npm i && tsc, although the output is in the repo anyway under the build/ directory): https://github.com/lorem--ipsum/tsc-test

Related Issues: -

Thoughts:
The file that gets its type definition is component-b.tsx. It needs to be compiled after component)-a.tsx (I assume the order is alphabetical). The bug does happen when running tsc --watch, but only on the first run, not when component-b.tsx is changed. The returned type is correct when stated explicitly in the source file. It doesn't occur either when the two types (ComponentAPosition and ComponentBPosition aren't strictly similar).

[mhegazy]

Type aliases are just that, aliases. the compiler inlines the type alias value for every use of the alias. for visualization purposes (e.g. hover in an IDE, error messages, and declaration file emit), the compiler tries to find a "name" for a union type, and for that it uses the first type alias declaration it saw for this union type. in this case, it is ComponentAPosition. Since the two types are equivalent, there should not be any functional impact to the compiler picking one or the other.

A workaround to avoid this is to give explict type annotations to your functions. e.g.":

export class ComponentB {
    doSomething(values: ComponentBPosition): ComponentBPosition {
        return values;
    }
}

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.

[lorem--ipsum]

Good bot (also, thank you guys for the comments :-)