Fix crash, and offer "declare as nullable" for tuple fields

src/EditorFeatures/CSharpTest/Diagnostics/Nullable/CSharpDeclareAsNullableCodeFixTests.cs

@@ -1046,6 +1046,85 @@ await TestMissingInRegularAndScriptAsync(
 class Program
 {
     string [|s|], s2 = ""hello"";
+}", parameters: s_nullableFeature);
+        }
+
+        [Fact]
+        [WorkItem(46354, "https://github.com/dotnet/roslyn/issues/46354")]
+        public async Task FixTupleFieldAssignment()
+        {
+            await TestInRegularAndScript1Async(
+@"#nullable enable
+class Program
+{
+  static void F1((string, string?) t)
+  {
+    if (t.Item2 == null) return;
+    t.Item1 = [|null|];
+  }
+}",
+@"#nullable enable
+class Program
+{
+  static void F1((string?, string?) t)
+  {
+    if (t.Item2 == null) return;
+    t.Item1 = null;
+  }
+}", parameters: s_nullableFeature);
+        }
+
+        [Fact]
+        [WorkItem(46354, "https://github.com/dotnet/roslyn/issues/46354")]
+        public async Task FixTupleNamedFieldAssignment()
+        {
+            await TestInRegularAndScript1Async(
+@"#nullable enable
+class Program
+{
+  static void F1((string Foo, string? Bar) t)
+  {
+    if (t.Bar == null) return;
+    t.Foo = [|null|];
+  }
+}",
+@"#nullable enable
+class Program
+{
+  static void F1((string? Foo, string? Bar) t)
+  {
+    if (t.Bar == null) return;
+    t.Foo = null;
+  }
+}", parameters: s_nullableFeature);
+        }
+
+        [Fact]
+        [WorkItem(46354, "https://github.com/dotnet/roslyn/issues/46354")]
+        public async Task FixTupleGenericFieldAssignment()
+        {
+            await TestInRegularAndScript1Async(
+@"#nullable enable
+class Program
+{
+  static void F1<T>((T, T?) t) where T : class
+  {
+    if (t.Item2 == null) return;
+    t.Item1 = [|null|];
+
+    var (a, b) = t;
+  }
+}",
+@"#nullable enable
+class Program
+{
+  static void F1<T>((T?, T?) t) where T : class
+  {
+    if (t.Item2 == null) return;
+    t.Item1 = null;
+
+    var (a, b) = t;
+  }
 }", parameters: s_nullableFeature);
         }
     }

src/Features/CSharp/Portable/CodeFixes/Nullable/CSharpDeclareAsNullableCodeFixProvider.cs

@@ -219,7 +219,8 @@ declarator.Parent is VariableDeclarationSyntax declaration &&
                 {
                     return TryGetParameterTypeSyntax(parameter);
                 }
-                else if (symbol is IFieldSymbol field)
+                // implicitly declared fields don't have DeclaringSyntaxReferences so filter them out
+                else if (symbol is IFieldSymbol { IsImplicitlyDeclared: false } field)
                 {
                     var syntax = field.DeclaringSyntaxReferences[0].GetSyntax();
                     if (syntax is VariableDeclaratorSyntax declarator &&
@@ -228,6 +229,24 @@ declarator.Parent is VariableDeclarationSyntax declaration &&
                     {
                         return declaration.Type;
                     }
+                    else if (syntax is TupleElementSyntax tupleElement)
+                    {
+                        return tupleElement.Type;
+                    }
+                }
+                // Assigning a tuple field, eg. foo.Item1 = null
+                else if (symbol is IFieldSymbol { CorrespondingTupleField: IFieldSymbol tupleField })
+                {
+                    // the tupleField won't have DeclaringSyntaxReferences because it's implicitly declared, otherwise it
+                    // would have fallen into the branch above. We can use the Locations instead, if there is one and it's in source
+                    if (tupleField.Locations is { Length: 1 } &&
+                        tupleField.Locations[0] is { IsInSource: true } location)
+                    {
+                        if (location.FindNode(default) is TupleElementSyntax tupleElement)
+                        {
+                            return tupleElement.Type;
+                        }
+                    }
                 }
                 else if (symbol is IPropertySymbol property)
                 {