[Spec] Duplicate ambient external module name

[mhegazy]

declare module "fs" {
    var x: string;
}
declare module "fs" {
    var y: number;
}

Expected:
!!! Duplicate identifier ''fs''.
!!! Ambient external module declaration cannot be reopened.
Actual: no error

[ahejlsberg]

Changing to spec issue. The new compiler supports merging of all module declarations, including ambient external module declarations. I see no reason why we should only allow merging of internal modules and specifically disallow merging of ambient external modules. In particular, declaration files sometimes extend existing internal modules and may want to do exactly that for ambient external modules.

[sophiajt]

From design meeting: agreed this should merge just like internal modules.

[sophiajt]

Leaving open so we can make the text change to the spec

[iislucas]

When I run this code in typescript I get

ldixon/foo.ts(4,16): error TS2000: Duplicate identifier '"fs"'. Additional locations:
    ldixon/foo.ts(1,16)
ldixon/foo.ts(4,16): error TS2191: Ambient external module declaration cannot be reopened.

Sounds like the correct behaviour is no errors. Would be great to have an npm publish of a version with this fixed :)

[basarat]

agreed this should merge just like internal modules.

DT actually needs this. Thanks guys (/cc @Bartvds @vvakame will possibly make non-instantiated modules less necessary)

[spion]

It seems like this doesn't work if the module is exported by assignment?

declare module "m" {
  function inner(s: string);
  module inner { export interface Test {t: string;} }
  export = inner //error here
}

declare module "m" {
  // even though we're only extending already existing submodule
  module inner { export interface Other { t: number; } }
}

[RyanCavanaugh]

@spion The name inner is not "shared" between declarations here -- when there's an export =, all other declarations inside that module declaration are considered to not be visible at all

[spion]

@RyanCavanaugh I see. The original problem is express.d.ts

https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/express/express.d.ts

Here, express exposes a namespace Express to let us extend Request. Unfortunately those extensions cannot contain types imported from ambient external modules, which means they're of limited use. However, since express-the-module is also a function, the only way to declare the module is to use declaration merging then write export = module at the end, so we can't use declaration merging for ambient external modules either.

I'll open a separate issue.