Cannot export { ... } interface when --isolatedModules is passed

[rickbutton]

TypeScript Version: 3.5.3

Search Terms: isolatedModules re-export

Code

interface MyInterface {}
export {
    MyInterface
}

Expected behavior:

This should compile regardless of whether --isolatedModules is passed.

OR the error message could indicate that this is not an accepted way to export an interface in an isolated module. (#28481)

Actual behavior:

This fails to compile with the error message:

Cannot re-export a type when the '--isolatedModules' flag is provided.ts(1205)

when --isolatedModules is passed.

Related Issues: #28481

[RyanCavanaugh]

The error message is slightly off; it should say export not re-export if the symbol originates in a declaration in the same file

[rickbutton]

As in Cannot export a type when the '--isolatedModules' flag is provided.ts(1205)?

I agree, that would make the error message more clear, but is it strictly true? You can safely export the interface inline regardless of the --isolatedModules flag, e.g.:

export interface MyInterface {}

which using export instead of re-export implies is not possible.

[DanielRosenwasser]

Error: A type cannot be exported in an 'export {...}' list under the '--isolatedModules' flag.

Related error message on the original declaration: Consider adding an 'export' keyword here instead..

[mheiber]

@rickbutton , Ryan Cavanaugh pointed out that babel-plugin-typescript chokes on export { Foo } because it can't tell if the type originated in the current module or not.
So we do want an error message here, but likely a different one, like the one Daniel suggested ^.

[rickbutton]

makes sense to me! I will gladly submit a PR to update the error message, when the new error message is chosen.

[vytautas-pranskunas-]

But how then we should export interfaces?

[DanielRosenwasser]

TypeScript 3.8+ will provide export type which is specifically for cases in isolatedModules, Babel, and transpileModule where you need to use an export { ... } statement with a type declaration.

So the original code could be written as

interface MyInterface {}

export type {
    MyInterface
}

[DanielRosenwasser]

The current error message was changed to

Re-exporting a type when the '--isolatedModules' flag is provided requires using 'export type'.

so I think this was fixed in #35200.