Incremental parsing producing an invalid tree

[sharwell]

Version Used: 8adbb93

Steps to Reproduce:

Run the test CSharpCompletionCommandHandlerTests.LocalFunctionAttributeNamedPropertyCompletionCommitWithTab and wait for an assertion failure in SyntaxTreeExtensions.VerifySource.

The test attempts to apply this change and incrementally parse the result:

 
 class MyAttribute : System.Attribute
 {
     public string Name { get; set; }
 }
 
 public class Goo
 {
     void M()
     {
-        [MyAttribute(Nam
+        [MyAttribute(Name =
         void local1() { }
     }
 }
                             

Expected Behavior:

The full text of the CompilationUnitSyntax should match this original SyntaxTree text:

class MyAttribute : System.Attribute
{
    public string Name { get; set; }
}

public class Goo
{
    void M()
    {
        [MyAttribute(Name =
        void local1() { }
    }
}
                            

Actual Behavior:

The full text of the CompilationUnitSyntax is this:

class MyAttribute : System.Attribute
{
    public string Name { get; set; }
}

public class Goo
{
    void M()
    {
{ }
    }
}
                            

[sharwell]

I was able to narrow down the location where the invalid tree is produced to this:

roslyn/src/Compilers/CSharp/Portable/Syntax/CSharpSyntaxTree.cs

Lines 545 to 548 in a4efc4a

	using var lexer = new InternalSyntax.Lexer(newText, this.Options);
	using var parser = new InternalSyntax.LanguageParser(lexer, oldTree?.GetRoot(), workingChanges);
	var compilationUnit = (CompilationUnitSyntax)parser.ParseCompilationUnit().CreateRed();