Support `export * as ns from "mod";`

[demurgos]

In the current spec and proposal, the symmetry between import and export is not complete. In particular, import * as ns from "mod"; does not have an equivalent export * as ns from "mod";.

The following table is presented at the bottom of the README:

Statement Form	[[ModuleRequest]]	[[ImportName]]	[[LocalName]]	[[ExportName]]
...	...	...	...	...
import * as ns from "mod";	"mod"	"*"	"ns"
export * from "mod";	"mod"	"*"	null	null (many)

It seems to imply a symmetry, but there is none.
The export * from "mod" is the only form where the information about the names is non-local and depends on the exported names of another module (you can't know what is actually exported unless you look into "mod"). The equivalent form would be an import * from "mod"; importing many names. This kind of form can cause breaking changes if the dependencies are updated and export new names that collide. Requiring a local name on import (as ns) prevents these collisions. There should be a symmetric safe export allowing you to export the the mod namespace under a single name: export * as ns from "mod".

Reexporting a whole module is common if you want to create a library with a single entry point that reexports the value of its internal modules.

Here is an existing example:

import * as errors from "./errors";
export {errors};

It should be possible to rewrite it without introducing a local name:

export * as errors from "./errors";

Here is a real world example that could be simplified.

[ljharb]

That is already a needs-consensus PR: tc39/ecma262#1174

[demurgos]

Thanks for the link.