Fix TypeWithAnnotations.ToTypeWithState() for (untyped) null literal

src/Compilers/CSharp/Portable/Binder/Semantics/OverloadResolution/MethodTypeInference.cs

@@ -1542,7 +1542,7 @@ private bool ExactOrBoundsNullableInference(ExactOrBoundsKind kind, TypeWithAnno
             return false;
 
             // True if the type is nullable.
-            bool isNullableOnly(TypeWithAnnotations type)
+            static bool isNullableOnly(TypeWithAnnotations type)
                 => type.NullableAnnotation.IsAnnotated();
         }
 

src/Compilers/CSharp/Portable/Symbols/TypeWithAnnotations.cs

@@ -772,11 +772,6 @@ internal bool IsSameAs(TypeWithAnnotations other)
         /// </summary>
         internal TypeWithState ToTypeWithState()
         {
-            if (Type is null)
-            {
-                return default;
-            }
-
             // This operation reflects reading from an lvalue, which produces an rvalue.
             // Reading from a variable of a type parameter (that could be substituted with a nullable type), but which
             // cannot itself be annotated (because it isn't known to be a reference type), may yield a null value
@@ -785,6 +780,10 @@ internal TypeWithState ToTypeWithState()
 
             static NullableFlowState getFlowState(TypeSymbol type, NullableAnnotation annotation)
             {
+                if (type is null)
+                {
+                    return annotation.IsAnnotated() ? NullableFlowState.MaybeNull : NullableFlowState.NotNull;
+                }
                 if (type.IsPossiblyNullableReferenceTypeTypeParameter())
                 {
                     return annotation switch { NullableAnnotation.Annotated => NullableFlowState.MaybeDefault, NullableAnnotation.NotAnnotated => NullableFlowState.MaybeNull, _ => NullableFlowState.NotNull };

src/Compilers/CSharp/Test/Semantic/Semantics/NullableReferenceTypesTests.cs

@@ -51082,12 +51082,12 @@ static void G(string x)
                 new[] { source }, options: WithNonNullTypesTrue(),
                 parseOptions: TestOptions.Regular8);
             comp.VerifyDiagnostics(
-                // (9,54): warning CS8603: Possible null reference return.
+                // (9,9): warning CS8602: Dereference of a possibly null reference.
                 //         F(() => { if (x.Length > 0) return x; return null; }).ToString();
-                Diagnostic(ErrorCode.WRN_NullReferenceReturn, "null").WithLocation(9, 54),
-                // (10,45): warning CS8603: Possible null reference return.
+                Diagnostic(ErrorCode.WRN_NullReferenceReceiver, "F(() => { if (x.Length > 0) return x; return null; })").WithLocation(9, 9),
+                // (10,9): warning CS8602: Dereference of a possibly null reference.
                 //         F(() => { if (x.Length == 0) return null; return x; }).ToString();
-                Diagnostic(ErrorCode.WRN_NullReferenceReturn, "null").WithLocation(10, 45));
+                Diagnostic(ErrorCode.WRN_NullReferenceReceiver, "F(() => { if (x.Length == 0) return null; return x; })").WithLocation(10, 9));
         }
 
         [Fact]
@@ -133331,6 +133331,160 @@ internal interface IB : IA { }
             comp.VerifyEmitDiagnostics();
         }
 
+        [Fact]
+        [WorkItem(46342, "https://github.com/dotnet/roslyn/issues/46342")]
+        public void LambdaNullReturn_01()
+        {
+            var source =
+@"#nullable enable
+using System;
+class Program
+{
+    static void F<T>(Func<T> f)
+    {
+    }
+    static void M<T>(bool b, T t) where T : class
+    {
+        F(() =>
+        {
+            if (b) return null;
+            return t;
+        });
+    }
+}";
+            var comp = CreateCompilation(source);
+            comp.VerifyEmitDiagnostics();
+        }
+
+        [Fact]
+        [WorkItem(46342, "https://github.com/dotnet/roslyn/issues/46342")]
+        public void LambdaNullReturn_02()
+        {
+            var source =
+@"#nullable enable
+using System;
+class Program
+{
+    static void F<T>(Func<T> f)
+    {
+    }
+    static void M<T>(bool b, T t) where T : class?
+    {
+        F(() =>
+        {
+            if (b) return null;
+            return t;
+        });
+    }
+}";
+            var comp = CreateCompilation(source, parseOptions: TestOptions.RegularPreview);
+            comp.VerifyEmitDiagnostics();
+        }
+
+        [Fact]
+        [WorkItem(46342, "https://github.com/dotnet/roslyn/issues/46342")]
+        public void LambdaNullReturn_03()
+        {
+            var source =
+@"#nullable enable
+using System;
+class Program
+{
+    static void F<T>(Func<T> f)
+    {
+    }
+    static void M<T>(T? t) where T : class
+    {
+        F(() =>
+        {
+            if (t is null) return null;
+            return t;
+        });
+    }
+}";
+            var comp = CreateCompilation(source);
+            comp.VerifyEmitDiagnostics();
+        }
+
+        [Fact]
+        [WorkItem(46342, "https://github.com/dotnet/roslyn/issues/46342")]
+        public void LambdaNullReturn_04()
+        {
+            var source =
+@"#nullable enable
+using System;
+class Program
+{
+    static void F<T>(Func<T> f)
+    {
+    }
+    static void M<T>(T? t) where T : class?
+    {
+        F(() =>
+        {
+            if (t is null) return null;
+            return t;
+        });
+    }
+}";
+            var comp = CreateCompilation(source, parseOptions: TestOptions.RegularPreview);
+            comp.VerifyEmitDiagnostics();
+        }
+
+        [Fact]
+        [WorkItem(46342, "https://github.com/dotnet/roslyn/issues/46342")]
+        public void LambdaNullReturn_05()
+        {
+            var source =
+@"#nullable enable
+using System;
+using System.Threading.Tasks;
+class Program
+{
+    static void F<T>(Func<Task<T>> f)
+    {
+    }
+    static void M<T>(T? t) where T : class
+    {
+        F(async () =>
+        {
+            await Task.Yield();
+            if (t is null) return null;
+            return t;
+        });
+    }
+}";
+            var comp = CreateCompilation(source);
+            comp.VerifyEmitDiagnostics();
+        }
+
+        [Fact]
+        [WorkItem(46342, "https://github.com/dotnet/roslyn/issues/46342")]
+        public void LambdaNullReturn_06()
+        {
+            var source =
+@"#nullable enable
+using System;
+using System.Threading.Tasks;
+class Program
+{
+    static void F<T>(Func<Task<T>> f)
+    {
+    }
+    static void M<T>(T? t) where T : class?
+    {
+        F(async () =>
+        {
+            await Task.Yield();
+            if (t is null) return null;
+            return t;
+        });
+    }
+}";
+            var comp = CreateCompilation(source, parseOptions: TestOptions.RegularPreview);
+            comp.VerifyEmitDiagnostics();
+        }
+
         [Fact]
         [WorkItem(45862, "https://github.com/dotnet/roslyn/issues/45862")]
         public void Issue_45862()