Fix false detection of backing field mutation

Author: jnm2

PropertyChangedAnalyzers.Test/INPC002MutablePublicPropertyShouldNotify/Valid.cs

@@ -1018,5 +1018,56 @@ protected virtual void OnPropertyChanged(string propertyName)
 
             RoslynAssert.Valid(Analyzer, Descriptor, code);
         }
+
+        [Test]
+        public static void SettingPropertyOfAnUnrelatedInstance()
+        {
+            var code = @"
+namespace ValidCode
+{
+    using System.ComponentModel;
+    using System.Runtime.CompilerServices;
+
+    internal class Settings
+    {
+        public static Settings Default { get; }
+
+        public string Address { get; set; }
+    }
+
+    public class SomeViewModel : INotifyPropertyChanged
+    {
+        private string address;
+
+        public event PropertyChangedEventHandler? PropertyChanged;
+
+        public string Address
+        {
+            get => this.address;
+            set
+            {
+                if (!Set(ref this.address, value)) return;
+
+                Settings.Default.Address = value;
+            }
+        }
+
+        protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
+        {
+            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
+        }
+
+        protected bool Set<T>(ref T location, T value, [CallerMemberName] string propertyName = null)
+        {
+            if (RuntimeHelpers.Equals(location, value)) return false;
+            location = value;
+            OnPropertyChanged(propertyName);
+            return true;
+        }
+    }
+}";
+
+            RoslynAssert.Valid(Analyzer, code);
+        }
     }
 }

PropertyChangedAnalyzers/Helpers/Setter.cs

@@ -110,20 +110,19 @@ IdentifierNameSyntax identifierName
 
         internal static AssignmentExpressionSyntax? AssignsValueToBackingField(AccessorDeclarationSyntax setter)
         {
+            if (setter.FirstAncestor<PropertyDeclarationSyntax>() is not { } property || !property.TrySingleReturned(out var backingExpression))
+            {
+                return null;
+            }
+
             using var walker = AssignmentWalker.Borrow(setter);
+
             foreach (var assignment in walker.Assignments)
             {
-                if (assignment is { Right: IdentifierNameSyntax { Identifier: { ValueText: "value" } } })
+                if (assignment is { Right: IdentifierNameSyntax { Identifier: { ValueText: "value" } } } &&
+                    MemberPath.Equals(backingExpression, assignment.Left))
                 {
-                    switch (assignment.Left)
-                    {
-                        case IdentifierNameSyntax _:
-                        case MemberAccessExpressionSyntax { Expression: ThisExpressionSyntax _ }:
-                        case MemberAccessExpressionSyntax { Expression: IdentifierNameSyntax _ }:
-                        case MemberAccessExpressionSyntax { Expression: MemberAccessExpressionSyntax { Expression: IdentifierNameSyntax _ } }:
-                        case MemberAccessExpressionSyntax { Expression: MemberAccessExpressionSyntax { Expression: ThisExpressionSyntax _ } }:
-                            return assignment;
-                    }
+                    return assignment;
                 }
             }
 

ValidCode/NotMutations.cs

@@ -0,0 +1,44 @@
+namespace ValidCode
+{
+    using System.ComponentModel;
+    using System.Runtime.CompilerServices;
+
+    public class NotMutations : INotifyPropertyChanged
+    {
+        public event PropertyChangedEventHandler? PropertyChanged;
+
+        protected virtual void OnPropertyChanged([CallerMemberName] string? propertyName = null)
+        {
+            this.PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
+        }
+
+        protected bool Set<T>(ref T location, T value, [CallerMemberName] string? propertyName = null)
+        {
+            if (RuntimeHelpers.Equals(location, value)) return false;
+            location = value;
+            OnPropertyChanged(propertyName);
+            return true;
+        }
+
+#pragma warning disable INPC001
+        internal class Settings
+#pragma warning restore INPC001
+        {
+            public static Settings Default { get; } = new();
+
+            public int SamePropertyNameOnUnrelatedInstance { get; set; }
+        }
+
+        private int samePropertyNameOnUnrelatedInstance;
+        public int SamePropertyNameOnUnrelatedInstance
+        {
+            get => this.samePropertyNameOnUnrelatedInstance;
+            set
+            {
+                if (!Set(ref samePropertyNameOnUnrelatedInstance, value)) return;
+
+                Settings.Default.SamePropertyNameOnUnrelatedInstance = value;
+            }
+        }
+    }
+}