Fixing VS Crash on invalid "MemberNotNull"

src/Compilers/CSharp/Portable/FlowAnalysis/LocalDataFlowPass.cs

@@ -113,12 +113,14 @@ protected virtual bool IsEmptyStructType(TypeSymbol type)
             return _emptyStructTypeCache.IsEmptyStructType(type);
         }
 
+#nullable enable
         /// <summary>
         /// Force a variable to have a slot.  Returns -1 if the variable has an empty struct type.
         /// </summary>
         protected virtual int GetOrCreateSlot(Symbol symbol, int containingSlot = 0, bool forceSlotEvenIfEmpty = false)
         {
             Debug.Assert(containingSlot >= 0);
+            Debug.Assert(symbol != null);
 
             if (symbol.Kind == SymbolKind.RangeVariable) return -1;
 
@@ -169,6 +171,7 @@ protected virtual int GetOrCreateSlot(Symbol symbol, int containingSlot = 0, boo
 
             return slot;
         }
+#nullable restore
 
         private int GetSlotDepth(int slot)
         {

src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs

@@ -715,6 +715,10 @@ int getSlotForFieldOrProperty(Symbol member)
 
                 if (!isStatic)
                 {
+                    if (MethodThisParameter is null)
+                    {
+                        return -1;
+                    }
                     thisSlot = GetOrCreateSlot(MethodThisParameter);
                     if (thisSlot < 0)
                     {
@@ -1268,6 +1272,7 @@ static MethodSymbol getTopLevelMethod(MethodSymbol method)
             }
         }
 
+#nullable enable
         protected override int GetOrCreateSlot(Symbol symbol, int containingSlot = 0, bool forceSlotEvenIfEmpty = false)
         {
 
@@ -1276,6 +1281,7 @@ protected override int GetOrCreateSlot(Symbol symbol, int containingSlot = 0, bo
 
             return base.GetOrCreateSlot(symbol, containingSlot, forceSlotEvenIfEmpty);
         }
+#nullable restore
 
         private void VisitAndUnsplitAll<T>(ImmutableArray<T> nodes) where T : BoundNode
         {

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

@@ -129603,6 +129603,46 @@ public async Task Handle(Request request)
                 );
         }
 
+
+        [Fact]
+        [WorkItem(44049, "https://github.com/dotnet/roslyn/issues/44049")]
+        public void MemberNotNull_InstanceMemberOnStaticMethod()
+        {
+            var source =
+@"using System.Diagnostics.CodeAnalysis;
+
+class C
+{
+    public string? field;
+
+    [MemberNotNull(""field"")]
+    public static void M()
+    {
+    }
+
+    public void Test()
+    {
+        M();
+        field.ToString();
+    }
+}";
+            var comp = CreateCompilation(
+                new[]
+                {
+                    source, MemberNotNullAttributeDefinition
+                },
+                options: WithNonNullTypesTrue(),
+                targetFramework: TargetFramework.NetCoreApp30,
+                parseOptions: TestOptions.RegularPreview);
+            comp.VerifyDiagnostics(
+                // (5,20): warning CS0649: Field 'C.field' is never assigned to, and will always have its default value null
+                //     public string? field;
+                Diagnostic(ErrorCode.WRN_UnassignedInternalField, "field").WithArguments("C.field", "null").WithLocation(5, 20),
+                // (15,9): warning CS8602: Dereference of a possibly null reference.
+                //         field.ToString();
+                Diagnostic(ErrorCode.WRN_NullReferenceReceiver, "field").WithLocation(15, 9));
+        }
+
         [Fact, WorkItem(39417, "https://github.com/dotnet/roslyn/issues/39417")]
         public void CustomAwaitable_DetectSettingNullableToNonNullableType()
         {