Roslyn: async completion: eats existing code on commit

[DavidKarlas]

Version Used:
VS 2019 Preview 3
Steps to Reproduce:

1. Create standard Console app
2. type override above static void Main.. method
3. create space and press Ctrl+Space to trigger code completion
4. type asdasdasdasdasdasd or any random text
5. select one of code completion options and press Enter

Expected Behavior:
Nicely inserted new code
Actual Behavior:

[KirillOsenkov]

@jinujoseph @ivanbasov @AmadeusW

[DavidKarlas]

Just wanted to clarify one thing, this repro steps work on VS Windows and VS Mac.

[dpoeschl]

Override completion specifically is deleting one preexisting character of code for every character typed during the completion session. So if you invoke override completion and then pick an item from the list manually, it's fine (afaict). If you type to filter/select an overload in the list, then it starts deleting characters on commit.

Min repro + some notes:

1. Paste C# in an empty file:
   ```C#
   class C
   {
   override $$
   void M() { }
   }

2. Remove the $$ and at that location, trigger completion, and type `GetHashCode`
3. Commit completion (e.g. with <kbd>Tab</kbd>)

**Result**: The override is inserted, but the subsequent `"GetHashCode".Length` characters are deleted. So in this case, 11 characters get deleted (the more you type, the more it deletes), leaving this:
```C#
class C
{
    public override int GetHashCode()
    {
        return base.GetHashCode();
    }M() { }
}

@ivanbasov I'm confused about this:
https://github.com/dotnet/roslyn/blob/ec366687c8caead948e58704ad61ed644eb91b5b/src/EditorFeatures/Core/Implementation/IntelliSense/AsyncCompletion/CommitManager.cs#L173

Basically, the triggerSnapshot there contains just override, while the textChange was produced a couple lines earlier using the _current_ document which includes override GetHashCode, so things are mismatched and it falls apart.

However, if the above is the problem, then I'm surprised that it isn't also impacting the other CompletionProvider types with custom GetChangeAsync methods, including the doc comment and IVT providers. I tried them both and couldn't repro the problem.

[jinujoseph]

[DavidKarlas]

@jinujoseph try typing much longer string

[jinujoseph]

@DavidKarlas , @dpoeschl yes , was able to repro when i try to type longer string like GetHashcode and commit

[ivanbasov]

Hmm....I'm on 16.0 preview 4 (28622.103). Also tried a previous preview 4 version. Both work fine for me:

[sharwell]

@ivanbasov This bug is being caught by integration tests

[ivanbasov]

I found how to reproduce it: need to type move letters before Tab. Looking into it.

[jkotalik]

I filed an issue on VS for this: https://developercommunity.visualstudio.com/content/problem/466995/auto-complete-is-incorrectly-formatted-for-method.html. Check the internal ticket for it. The repro steps shows a few issues (which I think both are rooted in the same issue).

• If I try completing a method, the method arguments are placed on the next line rather than the same line.
• If I misspell the method (capitalization, for example), auto complete creates an invalid method.

[jinujoseph]

addressed via #33666