IDE0001: False Positive for default primary constructor

[AliveDevil]

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Steps to Reproduce:
Class, with default constructor:

// A
public abstract class Base<TOther>;
public sealed class Concrete() : Base<Concrete.Impl>() // IDE0001: Simplify Name
{
	public class Impl;
}

// B
public abstract class Base<TOther>();
public sealed class Concrete() : Base<Concrete.Impl>() // IDE0001: Simplify Name
{
	public class Impl;
}

Record class, with argument:

public abstract record class Base<TOther>(object A);
public sealed record class Concrete() : Base<Concrete.Impl>(default!) // IDE0001: Simplify Name
{
	public class Impl;
}

Diagnostic Id: IDE0001: Simplify Name

Expected Behavior: No false-positive, just like this does not produce a false-positive:

public abstract class Base<TOther>();

public sealed class Concrete() : Base<Concrete.Impl>
{
	public class Impl;
}

Actual Behavior: IDE0001 false-positive is emitted for : Base<Concrete.Impl>(), where it shouldn't.

[Orachor]

I'll try fixing this :)

[CyrusNajmabadi]

Awesome. Thanks!

[Orachor]

This one turned out more complicated than I expected. The issue is with the following code:

public abstract class C<T>;

public sealed class D() : C<E>()
{
    public class E;
}

Where the generic parameter E inside the base list is getting incorrectly bound even though members of D are not accessible in this context.

From what I have seen this case should be taken into account (according to the comments here) however, in the end, the binder that is used is one that contains an InContainerBinder (the container being type D) which obviously binds the symbol.

I am still not 100% sure about some of the details as well as not knowing which part of the code needs to be changed (guessing it's the binder lookup).

@CyrusNajmabadi any chance you (or anyone else) might be able to provide some insight?

[CyrusNajmabadi]

I don't have a suggestion for you. It sounds like a binding issue in teh compiler. You might want to talk to @dotnet/roslyn-compiler to see if htey ahve any ideas. If you make a small repro test that demonstrates the problem when calling into the compiler apis, they may be amenable to fixing it.

[333fred]

I'm not sure what you mean by "Where the generic parameter E inside the base list is getting incorrectly bound even though members of D are not accessible in this context." This isn't happening from what I can tell. As an example, this test passes:

        [Fact]
        public void QuickTest()
        {
            var source = """
                public abstract class C<T>;

                public sealed class D() : C<E>()
                {
                    public class E;
                }
                """;

            var comp = CreateCompilation(source);
            comp.VerifyDiagnostics(
                // (3,29): error CS0246: The type or namespace name 'E' could not be found (are you missing a using directive or an assembly reference?)
                // public sealed class D() : C<E>()
                Diagnostic(ErrorCode.ERR_SingleTypeNameNotFound, "E").WithArguments("E").WithLocation(3, 29)
            );

            var tree = comp.SyntaxTrees[0];
            var model = comp.GetSemanticModel(tree);
            var classD = tree.GetRoot().DescendantNodes().OfType<ClassDeclarationSyntax>().Skip(1).First();
            var cBaseList = classD.BaseList.Types[0].Type;
            var eArg = ((GenericNameSyntax)cBaseList).TypeArgumentList.Arguments[0];
            var e = model.GetSymbolInfo(eArg).Symbol;
            Assert.Null(e);
        }

Could you please clarify what you're seeing? If you're calling GetTypeInfo on that location, I indeed suspect that could give you E back, since there's no bound node information and it will likely do a fallback error binding.

[AlekseyTs]

Is this problem really specific to primary constructors?

[AliveDevil]

Yes:

namespace Roslyn75026;

public abstract class Base<TOther>;

public sealed class PrimaryToBasePrimary() : Base<PrimaryToBasePrimary.Impl>()
{
    public class Impl;
}

public sealed class PrimaryToBaseDefault() : Base<PrimaryToBaseDefault.Impl>
{
    public class Impl;
}

public sealed class DefaultToBaseDefault : Base<DefaultToBaseDefault.Impl>
{
    public DefaultToBaseDefault()
    {
    }

    public class Impl;
}

public sealed class DefaultToBase : Base<DefaultToBase.Impl>
{
    public DefaultToBase() : base()
    {
    }

    public class Impl;
}

public sealed class ImplicitToBaseDefault : Base<ImplicitToBaseDefault.Impl>
{
    public class Impl;
}

Only the PrimaryToBasePrimary creats the IDE0001 suggestion.

[AliveDevil]

This btw doesn't affect default-primaries only, but all primaries, i.e.:

public abstract record class Base<TOther>(string A);
public sealed record class Concrete() : Base<Concrete.Impl>("A")
{
  public class Impl;
}

also has the IDE0001 annotation, but can only be worked around by discarding the primary constructor altogether.
Edit: Updated entry-post to reflect this case as well.

[AlekseyTs]

It is not obvious why IDE is getting confused. If it gets an unexpected result from Compiler's API, we (Compiler team) need more details:

• what API
• under what conditions
• etc.

[Orachor]

The issue can be reproduced with this test:

  [Fact]
  public void Test()
  {
      var source = """
              public abstract class C<T>;

              public sealed class D() : C<D.E>() // Starting with working code
              {
                  public class E;
              }
              """;

      var comp = CSharpTestBase.CreateCompilation(source);

      var tree = comp.SyntaxTrees[0];
      var model = comp.GetSemanticModel(tree);
      var classD = tree.GetRoot().DescendantNodes().OfType<ClassDeclarationSyntax>().Skip(1).First();
      var cBaseList = classD.BaseList!.Types[0].Type;
      var deArg = ((GenericNameSyntax)cBaseList).TypeArgumentList.Arguments[0];

      var ceArg = SyntaxFactory.GenericName(SyntaxFactory.Identifier("C"), SyntaxFactory.TypeArgumentList([SyntaxFactory.IdentifierName("E")]));
      var simplified = SyntaxFactory.PrimaryConstructorBaseType(ceArg);

      model.TryGetSpeculativeSemanticModel(deArg.SpanStart, simplified, out var speculativeModel); // Replace C<D.E>() with C<E>()

      var eArg = ((GenericNameSyntax)simplified.Type).TypeArgumentList.Arguments[0];
      var e = speculativeModel!.GetSymbolInfo(eArg).Symbol;
      Assert.Null(e); // throws, e is D.E here
  }

I am not 100% sure but I'm assuming that the speculative semantic model is on the IDE side of things, so perhaps there are no issues with GetSymbolInfo and it's just a matter of incorrectly using the API (since the original example works correctly).

[AlekseyTs]

The used overload of TryGetSpeculativeSemanticModel is documented as "Get a SemanticModel object that is associated with a constructor initializer ..."
What this meant to say - it is about the executable part of the node, i.e. the argument list. The type part is not even part of the binding process, any changes to the base type portion of the node are completely ignored. No matter what that part say, the initializer part is bound in context of the same constructor declared by the original syntax. For the purpose of that speculative semantic model, the base type doesn't change and a possibility of it changing isn't even considered as something to pay attention to.
When the symbol info is requested, we determine that the node is not bound as part of the initializer. It is bound in an error recovery mode, as a stand-alone node in context of the initializer, where the nested type is in scope. I guess, we could tighten behavior of the speculative semantic model so that it doesn't return any semantic information for nodes outside of the argument list. However, that probably won't help the goal that IDE is trying to achieve in this scenario. I think IDE should use a "regular" way of speculating on base types within base list. Something like:

model.GetSpeculativeSymbolInfo(deArg.SpanStart, SyntaxFactory.IdentifierName("E"), SpeculativeBindingOption.BindAsTypeOrNamespace)

Or whatever mechanism is used when argument list is not present.

[Orachor]

Thanks for the in depth explanation, now I understand 🙂