Bug Report

🔎 Search Terms

declaration, implementation

🕗 Version & Regression Information

• This changed between versions 2.8.4 and 2.9.1

Note: Prior to 2.9, TypeScript does not permit exported types to reference non-exported types. From 2.9 on, TypeScript permits it, but (as described here) generates invalid declaration files.

⏯ Playground Link

Playground link

However, oddly, the playground does not reproduce this bug. (Which itself is perhaps a secondary issue.)

💻 Code

example.ts

export interface Foo {
    item: Bar;
}

interface Bar {
  baz(): void;
}

namespace Bar {
  export function biz() {
    return 0;
  }
}

Run

tsc --declaration example.ts

🙁 Actual behavior

example.d.ts

export interface Foo {
    item: Bar;
}
interface Bar {
    baz(): void;
}
declare namespace Bar {
    function biz() {
        return 0;
    }
}
export {};

Note that this declaration file is invalid because biz() has an implementation. Attempting to use this declaration file yields:

error TS1183: An implementation cannot be declared in ambient contexts.

🙂 Expected behavior

I expect the produced declaration file to omit all implementations:

example.d.ts

export interface Foo {
    item: Bar;
}
interface Bar {
    baz(): void;
}
declare namespace Bar {
    function biz(): number;
}
export {};

Alternatively, I expect TypeScript to produce an error.

Alternatively, I expect TypeScript to accept implementations in declaration files.

But in no situation do I expect TypeScript to generate a declaration file that it itself does not accept.