Migration to TypeScript: JavaScript components have types of props set to "undefined" instead of "any" when used from TypeScript

[angryziber]

Describe the bug
I have added svelte-preprocess to a rather large existing project and started converting components one-by-one to lang="ts".
I also have added svelte-check to check for errors.

Unfortunately, converting to ts works only for lowest level components first, because you cannot pass any properties to a js component, which props are all indentified as "undefined". If you convert imported components to ts, even without specifying the type, then props are correctly identified as "any" and everything works.

Is the problem with svelte2tsx or a similar tool? I don't know if it's used behind-the-scenes by svelte-check or not.

To Reproduce
Use a js component with properties from a ts component, then build the project with npm run build

Expected behavior
svelte-check should not emit any errors for existing properties, resulting in successful build.

System (please complete the following information):

• OS: Linux (not relevant)
• IDE: Intellij (not relevant)
• Plugin/Package: svelte-check is used directly from snowpack

Additional context
Gradual migration of svelte components to TypeScript (the rest of code is TypeScript already)

[dummdidumm]

This only seems to be the case when strict mode is turned on in the tsconfig.json and when props have no fallback value.