importsNotUsedAsValues = error should apply to export statements

TypeScript Version: 3.9.2

Search Terms: importsNotUsedAsValues, error, export

Code

import { Interface } from "./module";
export { Interface } from "./module";

With a "importsNotUsedAsValues" set to "error".

Expected behavior:

tsc would error on the export statement.

Actual behavior:

tsc does not error on the export statement, it only errors on the import statement. TypeScript, by default, would elide both statements in the compile and should error in the same way.

Playground Link: N/A

[andrewbranch]

https://www.typescriptlang.org/tsconfig

In first look it seems that --importsNotUsedAsValues should yell for all pure type, export & import…

Why? It conspicuously says “import” and not “export” right in the name of the option. If there are docs that gave you this idea, let me know and I’ll update them. But the TSConfig reference reads pretty clearly to me that it’s about imports.

[andrewbranch]

Setting --importsNotUsedAsValues to anything other than the default allows you to be more explicit about your module dependency graph. In either preserve or error mode, you can count on the fact that import type declarations will always be elided and normal import declarations will never be elided (though this guarantee does not extend to the names imported, only to the statement on the whole—i.e., import { Interface } from './foo' becomes import './foo'). preserve is important because it allows users to maintain a predictable module order and side effects when an import otherwise would have been elided. error is frankly not that useful in my opinion, but it forces you to explicitly annotate imports that would be elided under the default remove mode. This means that when you have an error-free compilation, the emit should be exactly the same as it would be if you had not specified --importsNotUsedAsValues and not used import type at all. So by definition, it offers you literally nothing over the default in terms of emit behavior; it’s purely for people who want the human-readable signal of “this import will be elided” and “this import will not,” but are happy with the module graph they get under remove.

This is why I think the original suggestion here does make sense—because an export ... from does affect the module graph and could be used to trigger side effects in the same way that imports do. Notably, not only does export { PureType } from './module' not get an error with --importsNotUsedAsValues=error, but it also gets fully erased in the emit in error/preserve—we should probably be keeping it as export {} from './module' if we’re treating it as an effective import.