1. Open Roslyn.sln at the root of the Roslyn repo in VS2017.
2. Open source file <%ROOT%>srcCompilersCSharpPortableParserLanguageParser.cs.
3. Navigate to private method IsPossibleParameter (line 4060) and add the highlighted || clause to the default case return statement:

default:
  return IsPredefinedType(this.CurrentToken.Kind) || GetModifier(this.CurrentToken) != SyntaxModifier.None;

4. Navigate to private method ParseParameterModifiers (line 4234), and replace the existing while (IsParameterModifier(this.CurrentToken.Kind, allowThisKeyword)) with a while (true) loop, add the following if statement at the start of the while loop:

while (true)
{
  if (!IsParameterModifier(this.CurrentToken.Kind, allowThisKeyword))
  {
    if (GetModifier(this.CurrentToken) != SyntaxModifier.None)
    {
      // Misplaced modifier
      var misplacedModifier = this.EatToken();
      misplacedModifier = this.AddError(misplacedModifier, ErrorCode.ERR_BadMemberFlag, misplacedModifier.Text);
      modifiers.Add(misplacedModifier);
      continue;
    }

    break;
  }
  ...

5. Build the solution.
6. Set VisualStudioVisualStudioSetup.Next.csproj as the startup project and click on Ctrl + F5 to start a new VS instance with the locally built compiler and IDE toolset.
7. Create a new C# class library project, say ClassLibrary, and add the following code:

class Class
{
  static void M(readonly int param)
  {
  }
}

8. Verify that there is a single CS0106 diagnostic in the error list for the invalid readonly modifier and also that the editor has a single squiggle.

9. Build the project and verify that the build output has a single CS0106 diagnostic.