In D10235#297751, @ashoat wrote:

I'd like to try my hand at parameterizing getRelativeMemberInfos. What's the best way to do it? Won't I need at least one usage of the minimally-encoded variant to be able to test that the types work out? Should I check out a later diff?

You could patch in D10285 which is later in the stack and has no flow issues. In D10272 I introduced a new getRelativeMemberInfos that basically calls getMinimallyEncodedRelativeMemberInfos or getLegacyRelativeMemberInfos based on threadInfo that's passed in. This getRelativeMemberInfos that handles both LegacyRawThreadInfo and MinimallyEncodedRawThreadInfo is consumed in baseRelativeMemberInfoSelectorForMembersOfThread.

Both getLegacyRelativeMemberInfos and getMinimallyEncodedRelativeMemberInfos are effectively identical. You could try

1. Pasting the contents of either getLegacyRelativeMemberInfos or getMinimallyEncodedRelativeMemberInfos into getRelativeMemberInfos
2. Try to parametrize getRelativeMemberInfos such that flow issue in baseRelativeMemberInfoSelectorForMembersOfThread is resolved.

Let me know if there's anything else I can clarify or if I'm misunderstanding anything

Realizing my explanation above is probably difficult to follow.

I just created a diff (https://phab.comm.dev/D10291) that you can patch in and should be a more useful place to start from.

I basically replaced the contents of getRelativeMemberInfos with the contents of get[Legacy/MinimallyEncoded]RelativeMemberInfos (which are effectively identical) and replace usages of those with the newly updated getRelativeMemberInfos.

That should narrow things down to just the 12 relevant flow issues.

Got it working with this patch: https://gist.github.com/Ashoat/38315c14a5a16974b840aff796a814eb

type ExtractArrayParam = <T>(arr: $ReadOnlyArray<T>) => T;

function getRelativeMemberInfos<
  TI: MinimallyEncodedRawThreadInfo | LegacyRawThreadInfo,
>(
  threadInfo: ?TI,
  currentUserID: ?string,
  userInfos: UserInfos,
): $ReadOnlyArray<$ReadOnly<{
  ...$Call<
    ExtractArrayParam,
    $PropertyType<TI, 'members'>,
  >,
  +username: ?string,
  +isViewer: boolean,
}>> {
  const relativeMemberInfos: Array<$ReadOnly<{
    ...$Call<
      ExtractArrayParam,
      $PropertyType<TI, 'members'>,
    >,
    +username: ?string,
    +isViewer: boolean,
  }>> = [];
  if (!threadInfo) {
    return relativeMemberInfos;
  }
  const memberInfos = threadInfo.members;
  for (const memberInfoInput of memberInfos) {
    const memberInfo: $Call<
      ExtractArrayParam,
      $PropertyType<TI, 'members'>,
    > = memberInfoInput;
    if (!memberInfo.role) {
      continue;
    }
    const username: ?string = userInfos[memberInfo.id]
      ? userInfos[memberInfo.id].username
      : null;
    const isViewer: boolean = memberInfo.id === currentUserID;
    const relativeMemberInfo: $ReadOnly<{
      ...$Call<
        ExtractArrayParam,
        $PropertyType<TI, 'members'>,
      >,
      +username: ?string,
      +isViewer: boolean,
    }> = {
      ...memberInfo,
      username,
      isViewer,
    };
    relativeMemberInfos.unshift(relativeMemberInfo);
  }
  return relativeMemberInfos;
}

(Did that before reading your most recent comment – not sure if it matters, let me know if it does)