Fix `Add import` is not suggested for extension methods defined in projects targeting a different framework than the project using the extension method

[just-ero]

Version Used

Compiler: 4.13.0-2.24565.3 (c267dc98)
.NET: 9.0.100
Visual Studio Code: 1.95.3, f1a4fb101478ce6ec82fe9627c43efbf9e98c813, x64
C# Extension: v2.57.28
C# Dev Kit Extension: v1.14.8

Steps to Reproduce

1. Create or open a C# solution in Visual Studio Code (Visual Studio untested).
2. Create two projects:
   • Project 1:

     <Project Sdk="Microsoft.NET.Sdk">
     <PropertyGroup>
      <TargetFramework>net8.0</TargetFramework>
     </PropertyGroup>
     </Project>

   • Project 2:

     <Project Sdk="Microsoft.NET.Sdk">
     <PropertyGroup>
      <TargetFramework>net9.0</TargetFramework>
     </PropertyGroup>
     <ItemGroup>
      <ProjectReference Include="path/to/Project 1" />
     </ItemGroup>
     </Project>

3. Add the following code wherever syntactically applicable in Project 1:

   namespace Project1;
   
   public class C;

   namespace Project1.Extensions;
   
   public static class CExtensions {
    public static void M(this C c) { }
   }

4. Add the following code wherever syntactically applicable in Project 2:

   using Project1;
   
   namespace Project2;
   
   class Repro {
    void M(C c) {
      c.M();
    }
   }

5. Attempt to apply Add import on c.M().

Expected Behavior

The fix is suggested.

Actual Behavior

It is not.

[just-ero]

Note: The extension method is picked up while typing (typing c.M will contain M from the extension namespace) and does add the required namespace upon accepting the suggestion.

However, this is also not always the case. I have not been able to figure out the conditions of this behavior.

[CyrusNajmabadi]

@genlu can you ptal.