export import of types is not allowed in namespaces when using erasableSyntaxOnly

[phryneas]

🔎 Search Terms

export import alias type-only namespaces erasableSyntaxOnly

🕗 Version & Regression Information

• I was unable to test this on prior versions because erasableSyntaxOnly was just released

⏯ Playground Link

https://www.typescriptlang.org/play/?erasableSyntaxOnly=true&ts=5.8.2#code/PTAEBcAsEsGdQO4HsBOBrWBYAUAOwIYC2AprAA74DGxEAnmaaAN46ijEAeZq4oAJsUoAbfChrh6NAGJIkbALygAjDgC+OHCFAAjAK68ocfklK4A5L2ToANKHxCoSXQHNIoaODPwJDALRJcIVpbcFkcAhJyKnFJWAB1D0gAJWJObhReFmw2NJ53QnTeABVQRR9GNQ1sLUN4IyQ0fFpwolIKalAAN3tdRiycrjzKANheADNZUuVK7Bxyuyny2AA6GSQ5yVAORdiEqBTcjOWi1dkgA

💻 Code

// this works
namespace types {
  export declare type Foo  = 1
}

// but this doesn't work, although it's type-only, too
namespace typesWithReexport {
  export import T = types 
}

// ignoring the error, generally these should be both accessible
type a = types.Foo
type x = typesWithReexport.T.Foo

🙁 Actual behavior

export import re-exporting a namespace (or namespace member) of a type-only namespace from another type-only namespace results in This syntax is not allowed when 'erasableSyntaxOnly' is enabled.

🙂 Expected behavior

It should not error.

Additional information about the issue

@jakebailey asked me to open an issue to track this on bsky: https://bsky.app/profile/jakebailey.dev/post/3ljdc3ou7oc2w

[jakebailey]

Hm, maybe I misunderstood what you were reporting. Take a look at the js output for that second namespace; it has js emit and therefore cannot be allowed.

[phryneas]

@jakebailey is there a way to make this possible?
Adding a statement like export import type or something like explicit type-only namespaces that will not emit maybe?

Right now, the only way around this is to re-export every type by hand by redeclaring it, and if the original type has generics, that gets very long, like

export type X<A,B,C> = types.X<A,B,C>

as it requires repeating all generics of the original type, and essentially erases any JSDoc unless I repeat it.

It's also very prone to run "out of sync" - if I add another generic to the original type, I might miss it in a manual "re-export" declaration.

[jakebailey]

In its current state, no; since this uses types, it means you need cross-file analysis to do correctly.

export import type seems like a good idea, though...