Fix MA0150 false positive on user-defined operators returning a string

src/Meziantou.Analyzer/Rules/DoNotUseToStringIfObjectAnalyzer.cs

@@ -87,6 +87,11 @@ public void AnalyzeInvocation(OperationAnalysisContext context)
         internal static void AnalyzeAdd(OperationAnalysisContext context)
         {
             var operation = (IBinaryOperation)context.Operation;
+
+            // Only the built-in string concatenation calls ToString on the operands
+            if (operation.OperatorKind is not BinaryOperatorKind.Add || operation.OperatorMethod is not null)
+                return;
+
             if (!operation.Type.IsString())
                 return;
 

tests/Meziantou.Analyzer.Test/Rules/DoNotUseToStringIfObjectAnalyzerTests.cs

@@ -678,6 +678,25 @@ public struct A{ }
         return test.RunAsync();
     }
 
+    [Fact]
+    public Task UserDefinedOperatorReturningString()
+    {
+        var test = CreateTest();
+        test.TestCode = """
+            var a = new Foo();
+            _ = a - a;
+            _ = a + a;
+
+            sealed class Foo
+            {
+                public static string operator -(Foo x, Foo y) => "";
+                public static string operator +(Foo x, Foo y) => "";
+            }
+            """;
+
+        return test.RunAsync();
+    }
+
     [Fact]
     public Task SealedRecord_Concat()
     {