[spec] Grammar does not allow ambients inside an internal module

[mhegazy]

Courtesy of @JsonFreeman

This is just an oversight in the grammar, but the ModuleElement production does not go to AmbientDeclaration on the right side, which implies that an internal module cannot contain ambients. It needs to be added to the RHS.

Actually you need to add AmbientDeclaration, but not including AmbientExternalModuleDeclaration.

[ahejlsberg]

This is by design. Ambient declarations are only allowed at the top level (see sections 11.1 and 12.1 in the spec).

[JsonFreeman]

But ambient declarations inside nonambient modules is allowed by both compilers, and we have been supporting it. According to @RyanCavanaugh and @danquirk there are real world customers who depend on this.

[RyanCavanaugh]

e.g.
https://github.com/BabylonJS/Babylon.js/blob/master/Babylon/Physics/babylon.physicsEngine.ts
https://github.com/BabylonJS/Babylon.js/blob/master/Babylon/Physics/Plugins/babylon.cannonJSPlugin.ts

It's also very useful when you want to directly use node.js's require without having to pollute the global scope with it (this is a very real problem when using node.d.ts with any of the compiler .d.ts files at the same time, for example).

[ahejlsberg]

Ok, got it. Will fix the spec.