Illustrate caching issue

[eps1lon]

This is more of an issue report but with a full example to better illustrate the issue.

Have you given some thought to how we force jest to run the transformer again on every file that is affected by a change?

Imagine this case:

• Given Compound.svelte which imports Sample.svelte
• If I change something in Sample.svelte that affects how Compound.svelte behaves and therefore changes actual values in Compound.spec.js jest will only re-transform Sample.svelte. This means Compound.svelte will still return the same test result.
• You can try this by checking out this branch and
  1. run jest (fills transform cache, passes)
  2. change the initial value of clicks in Sample.svelte to 1
  3. run jest again and now only Sample.svelte will fail while Compound.svelte still passes
  4. run yarn jest --clearCache
  5. run yarn test and now both cases will fail

jest --no-cache does only work if you run it on all tests. Once you add a pattern to it or let it run only on changed files we observe the same behavior as with cache.

It is not an issue with jest not re-running. If I only run Compound.spec.js it will still re-run if I make changes to Sample.svelte.