Fixup add-using to support more modern extension cases

src/EditorFeatures/CSharpTest/CodeActions/AddUsing/AddUsingTests_ExtensionMethods.cs

@@ -1741,8 +1741,7 @@ static class Test
     [InlineData("object")]
     [InlineData("int")]
     public Task TestModernExtension_TypeParameter(string sourceType)
-        // Will work once we get an API from https://github.com/dotnet/roslyn/issues/80273
-        => TestMissingAsync(
+        => TestInRegularAndScriptAsync(
             $$"""
             namespace M
             {
@@ -1756,6 +1755,34 @@ void X()
                 }
             }
 
+            namespace N
+            {
+                static class Test
+                {
+                    extension<T>(T t)
+                    {
+                        public void M1()
+                        {
+                        }
+                    }
+                }
+            }
+            """,
+            $$"""
+            using N;
+
+            namespace M
+            {
+                class C
+                {
+                    void X()
+                    {
+                        {{sourceType}} o = new();
+                        [|o.M1();|]
+                    }
+                }
+            }
+
             namespace N
             {
                 static class Test

src/Features/Core/Portable/AddImport/SymbolReferenceFinder.cs

@@ -8,7 +8,6 @@
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Linq;
-using System.Linq.Expressions;
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.CodeAnalysis.LanguageService;
@@ -616,21 +615,12 @@ private bool IsViableModernExtensionMember(
             if (!member.ContainingType.IsExtension)
                 return false;
 
-            if (member.ContainingType.ExtensionParameter is not { Type: { } extensionParameterType })
-                return false;
-
             if (!member.IsAccessibleWithin(_semanticModel.Compilation.Assembly))
                 return false;
 
-            // TODO: https://github.com/dotnet/roslyn/issues/80273
-            // There is not api yet to know for certain if a modern extension is compatible with a receiver type.
-            // For now, put in a poor man's approach for this.
-
-            receiver = receiver.OriginalDefinition;
-            extensionParameterType = extensionParameterType.OriginalDefinition;
+            var reducedMember = member.ReduceExtensionMember(receiver);
 
-            var conversion = _semanticModel.Compilation.ClassifyCommonConversion(receiver, extensionParameterType);
-            return conversion.Exists && conversion.IsImplicit;
+            return reducedMember != null;
         }
 
         private bool ExpressionBinds(

src/Workspaces/SharedUtilitiesAndExtensions/Workspace/Core/Extensions/ISymbolExtensions.cs

@@ -40,4 +40,19 @@ public static DeclarationModifiers GetSymbolModifiers(this ISymbol symbol)
             .WithIsRequired(symbol.IsRequired())
             .WithPartial(symbol.IsPartial());
     }
+
+#if !ROSLYN_4_12_OR_LOWER
+    public static ISymbol? ReduceExtensionMember(this ISymbol? member, ITypeSymbol? receiverType)
+    {
+        if (member is null || receiverType is null)
+            return null;
+
+        return member switch
+        {
+            IPropertySymbol propertySymbol => propertySymbol.ReduceExtensionMember(receiverType),
+            IMethodSymbol method => method.ReduceExtensionMember(receiverType),
+            _ => null,
+        };
+    }
+#endif
 }