TS with .js files and JSDoc: type is found, displayed and used - despite "TS2304: Cannot find name XYZ"

TS 3.4.5, Webstorm 2019.1.1, node v11.14.0, Linux

Project: .ts files in src/, transpiled using Babel to lib/, but files under test/ are .js files

tsconfig.json has allowJs and checkJs set to true in order to check the .js files under test/ — which (CommonJS) require(...) transpiled .js files from lib/ (there also are .d.ts and .d.ts.map files for all files).

Every single TS type declared in src/ has an accompanying JSDoc @typedef (don't know if that is important or if TS does not even use it since there is the original TS type too).

The problem I see is a weird one, because what Typescript says and what it actually does is completely opposite!

In the IDE I see every single type declared in src/ files as read, and on mouseover I get TS2304 Cannot find name "XYZ".:

However, and this is the weird part (1 of 2): It all works!

Showing the type (in WebStorm, CTRL plus mouse-hover over a symbol, shows the correct information. Even on those red types, I get the type name and the file name of where it is declared. Variables show the correct autocompletion suggestions and type information, jumping to the type definition (the TS one) works too.

The other weird part (2 of 2) is that when I follow the suggestions to change e.g.

 * @param {SHA256Hash} hash

to

 * @param {import('../lib/core-types.js').SHA256Hash} hash

the symbols are no longer red — but now nothing works. No more type information or anything. (Tried different import paths, also directly to the src/ files and without extension, for example.)

This is the reason why I file this as a bug. It actually works, so it is not a feature request.

So all it needs is for TS to stop showing the "cannot find name" errors, I don't need a change in behavior, since it does find the names just fine.

PS: I realize this is probably deeper, the plugin does not specifically remove the comments, I think it is the path.remove() command from the underlying AST. That doesn't make it any better or even right though. Others had similar issues in the past.

Start with the reported tsconfig.json settings, especially allowJs and checkJs, and reproduce the diagnostic in the test/ JavaScript files that require transpiled files from lib/. Compare JSDoc references such as @param {SHA256Hash} with the import(...) form and inspect the related behavior from issue #17606. Done means valid JSDoc type names no longer produce TS2304 while preserving the reported type information and navigation.