'const enum' conflict with 'importsNotUsedAsValue''

[ajafff]

TypeScript Version: 3.8.0-dev.20200104

Search Terms:

Code

// @importsNotUsedAsValue: true

// @filename: a.ts
export const enum E {
  One,
  Two,
}

// @filename: b.ts
import type {E} from './a';

export let v = E.One;

Expected behavior:

No error.

Actual behavior:

Type-only import causes Enum 'E' cannot be used as a value because only its type has been imported.ts(1362)
Since importing const enums at runtime is not necessary due to inlining, they should be allowed to be used with type-only import.

If I use a regular import, I get This import is never used as a value and must use 'import type' because the 'importsNotUsedAsValue' is set to 'error'.ts(1371)

Related Issues: Introduced by @andrewbranch in #35200

[ajafff]

Also note that the type-only import of the const enum is reported as unused.

[andrewbranch]

I see the motivation, but import type means more than just “don’t emit this import declaration.” It filters the symbol meanings down to just type and type-containing namespace meanings. We’re currently discussing a similar scenario:

import type { BaseClass } from './a'
declare class C extends BaseClass {} // Should this be treated as having a value?

and the instincts on the team are mixed. But using a type-only import in the right-hand side of an assignment looks really unexpected to me.

[andrewbranch]

If I use a regular import, I get This import is never used as a value and must use 'import type' because the 'importsNotUsedAsValue' is set to 'error'.ts(1371)

This part sounds like a bug

[falsandtru]

I believe the error Enum 'E' cannot be used as a value because only its type has been imported.ts(1362) is correct. You must not use a type as a value. And I believe base classes of ambient classes should be treated as having no value because ambient declarations just declare types of existing values.