From 9d7adbe43aabd9e53c531ad2a300139f3fc05ee1 Mon Sep 17 00:00:00 2001
From: Julien Couvreur <julien.couvreur@gmail.com>
Date: Wed, 9 Jul 2025 14:51:03 -0700
Subject: [PATCH 1/2] Extensions: add test for function type scenario

---
 .../Test/Emit3/Semantics/ExtensionTests.cs    | 81 +++++++++++++++++++
 1 file changed, 81 insertions(+)

diff --git a/src/Compilers/CSharp/Test/Emit3/Semantics/ExtensionTests.cs b/src/Compilers/CSharp/Test/Emit3/Semantics/ExtensionTests.cs
index 12429fe1f8389..3b062a3729425 100644
--- a/src/Compilers/CSharp/Test/Emit3/Semantics/ExtensionTests.cs
+++ b/src/Compilers/CSharp/Test/Emit3/Semantics/ExtensionTests.cs
@@ -35138,6 +35138,87 @@ public void M() { }
         comp.VerifyDiagnostics();
     }
 
+    [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/79309")]
+    public void FunctionType_InstanceReceiver_12()
+    {
+        // static non-extension method vs. extension property
+        var src = """
+var x = new C().M;
+
+public static class E
+{
+    extension(C c)
+    {
+        public int M => 42;
+    }
+}
+
+public class C
+{
+    public static int M() => throw null;
+}
+""";
+        var comp = CreateCompilation(src);
+        comp.VerifyEmitDiagnostics(
+            // (1,9): error CS8917: The delegate type could not be inferred.
+            // var x = new C().M;
+            Diagnostic(ErrorCode.ERR_CannotInferDelegateType, "new C().M").WithLocation(1, 9));
+
+        var tree = comp.SyntaxTrees.First();
+        var model = comp.GetSemanticModel(tree);
+        var localDeclaration = GetSyntax<VariableDeclarationSyntax>(tree, "var x = new C().M");
+        Assert.Equal("?", model.GetTypeInfo(localDeclaration.Type).Type.ToTestDisplayString());
+
+        // without non-extension method
+        src = """
+var x = new C().M;
+System.Console.Write(x);
+
+public static class E
+{
+    extension(C c)
+    {
+        public int M => 42;
+    }
+}
+
+public class C
+{
+}
+""";
+        comp = CreateCompilation(src);
+        CompileAndVerify(comp, expectedOutput: "42").VerifyDiagnostics();
+
+        tree = comp.SyntaxTrees.First();
+        model = comp.GetSemanticModel(tree);
+        localDeclaration = GetSyntax<VariableDeclarationSyntax>(tree, "var x = new C().M");
+        Assert.Equal("System.Int32", model.GetTypeInfo(localDeclaration.Type).Type.ToTestDisplayString());
+
+        // analogous non-extension scenario
+        src = """
+var x = new C().M;
+System.Console.Write(x);
+
+public class Base
+{
+    public int M => 42;
+}
+
+public class C : Base
+{
+    public static int M() => throw null;
+}
+""";
+        comp = CreateCompilation(src);
+        comp.VerifyEmitDiagnostics(
+            // (1,9): error CS8917: The delegate type could not be inferred.
+            // var x = new C().M;
+            Diagnostic(ErrorCode.ERR_CannotInferDelegateType, "new C().M").WithLocation(1, 9),
+            // (11,23): warning CS0108: 'C.M()' hides inherited member 'Base.M'. Use the new keyword if hiding was intended.
+            //     public static int M() => throw null;
+            Diagnostic(ErrorCode.WRN_NewRequired, "M").WithArguments("C.M()", "Base.M").WithLocation(11, 23));
+    }
+
     [Fact]
     public void FunctionType_ColorColorReceiver_01()
     {

From b64b7781e77ddb6b0951e851fe2038eeae6e7615 Mon Sep 17 00:00:00 2001
From: Julien Couvreur <julien.couvreur@gmail.com>
Date: Mon, 21 Jul 2025 13:10:21 -0700
Subject: [PATCH 2/2] Add 'new'

---
 .../CSharp/Test/Emit3/Semantics/ExtensionTests.cs          | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/src/Compilers/CSharp/Test/Emit3/Semantics/ExtensionTests.cs b/src/Compilers/CSharp/Test/Emit3/Semantics/ExtensionTests.cs
index 3b062a3729425..736d4ea587e2c 100644
--- a/src/Compilers/CSharp/Test/Emit3/Semantics/ExtensionTests.cs
+++ b/src/Compilers/CSharp/Test/Emit3/Semantics/ExtensionTests.cs
@@ -35206,17 +35206,14 @@ public class Base
 
 public class C : Base
 {
-    public static int M() => throw null;
+    public static new int M() => throw null;
 }
 """;
         comp = CreateCompilation(src);
         comp.VerifyEmitDiagnostics(
             // (1,9): error CS8917: The delegate type could not be inferred.
             // var x = new C().M;
-            Diagnostic(ErrorCode.ERR_CannotInferDelegateType, "new C().M").WithLocation(1, 9),
-            // (11,23): warning CS0108: 'C.M()' hides inherited member 'Base.M'. Use the new keyword if hiding was intended.
-            //     public static int M() => throw null;
-            Diagnostic(ErrorCode.WRN_NewRequired, "M").WithArguments("C.M()", "Base.M").WithLocation(11, 23));
+            Diagnostic(ErrorCode.ERR_CannotInferDelegateType, "new C().M").WithLocation(1, 9));
     }
 
     [Fact]