1. error CS0648: '' is a type not supported by the language
   [master] Update dependencies from dotnet/coreclr corefx#37141 (comment)

2. error CS4001: Cannot await ''
   https://dnceng.visualstudio.com/internal/_build/results?buildId=170149&view=logs&jobId=7b614801-ed38-51b7-97d8-b2f3e799aa24&taskId=82d75caa-41ea-59a8-35f3-0486fa4f1cf4&lineStart=1551&lineEnd=1552&colStart=1&colEnd=1

3. System\Collections\Immutable\ImmutableDictionary_2.cs(785,52): error CS1503: Argument 1: cannot convert from '' to 'object' [F:\workspace_work\1\s\src\System.Collections.Immutable\src\System.Collections.Immutable.csproj]
   https://dnceng.visualstudio.com/internal/_build/results?buildId=163632

4. BasicEventSourceTest/Harness/Listeners.cs(285,43): error CS0023: Operator '?' cannot be applied to operand of type ''
   https://dnceng.visualstudio.com/internal/_build/results?buildId=166101

Talked with @stephentoub and his guess is that this could be a threading related compiler bug.

Happens with Microsoft.Net.Compilers.Toolset 3.1.0-beta3-19217-05.