.alternate should always be null for true mounts. Something may get upgraded but in that case it should disconnect the .alternate. So I think something else is broken if that's not the case.

Plain function:

First attempt
- mount indeterminate
  - throws
  - (stays indeterminate)

Second attempt
- mount indeterminate
  - current != null so resets alternate // <-- this is why there's no false positive for memo()-less components
  - gets tagged as functional

Memo:

First attempt
- update memo
  - gets tagged as simple
  - updates simple
    - throws

Second attempt
- updates simple
  - false positive <--- alternate wasn't reset

It seems like we use something else as an indicator whether we were suspended and should disconnect the alternate. For classes, we check the instance. For functions, we know we suspended because we're still Indeterminate. But for memo, we don't seem to use any indicator.

Actually looks like classes have a special IncompleteClassComponent tag for this.

We should be able to remove this warning entirely, now that we have this invariant:

https://github.com/facebook/react/blob/f24a0da6e0f59484e5aafd0825bb1a6ed27d7182/packages/react-reconciler/src/ReactFiberHooks.js#L488-L493

I think I meant to do this in my PR but I must have forgotten.