Members of re-exported namespace can be used as values but not types

[ultimatelostcause]

TypeScript Version: 3.8.0-dev.20191116

Search Terms:

• export namespace
• export merged namespace
• export namespace type

Code

a.ts:

export namespace Foo {
   export class Bar {}
}

b.ts:

import {Foo as FooImpl} from './a.js'
export namespace Baz {
   export const Foo = FooImpl
}

Expected behavior:

import {Baz} from './b.js'
let x: Baz.Foo.Bar      // should compile without error

Actual behavior:

import {Baz} from './b.js'
let x: Baz.Foo.Bar      // Error: Namespace '".../b".Baz' has no exported member 'Foo'.
let y = new Baz.Foo.Bar // Ok

Playground Link:
I am unsure how to use TypeScript Playground to import custom modules.

Related Issues:

• Merging constants with namespaces seems to be a fundamentally different issue, as there is a symbol conflict. Here that shouldn't be the case.
• Embedding types in namespace declarations also appears to be a different issue, as in this case it is the namespace itself not being recognised as a valid token in the type.
• Stack overflow question I raised in trying to determine if this is a bug or expected behaviour.

[ultimatelostcause]

This is not surprising, since namespaces aren't types themselves, so can't be exported as types and therefore the nested types are lost.

import {Foo as FooImpl} from './a.js'
export namespace Baz {
   export const Foo = FooImpl
   //export type Foo = FooImpl  <-- can't do this, since namespaces aren't types
   //export namespace Foo = FooImpl  <-- can't do this, since namespaces can't be assigned
}

However, since the members can be accessed as values, they should also be accessible as types.

[RyanCavanaugh]

You can use export import Foo = a.FooImpl with a qualified name (so use a top-level module import to a instead of destructring) if this is what you want to happen

[ultimatelostcause]

Apologies, I was unaware of the export import syntax.

This most trivial change to b.ts to resolve this is:

import {Foo as FooImpl} from './a.js'
export namespace Baz {
   export import Foo = FooImpl
}

I note I was not able to find the export import syntax anywhere in the TypeScript documentation. Now I know what to look for, this appears to be a duplicate of issue 10058. I will close this and also update my Stack Overflow question with the answer.