fix: fallback to positional match for back compat renames

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Utilities/BackCompatHelper.cs

@@ -184,13 +184,8 @@ public static void RestorePreviousParameterNames(
                     string? preservedName = null;
 
                     var inputParameter = parameter.InputParameter;
-                    if (inputParameter is not null)
+                    if (inputParameter is not null && string.Equals(parameter.Name, inputParameter.Name, StringComparison.Ordinal))
                     {
-                        if (!string.Equals(parameter.Name, inputParameter.Name, StringComparison.Ordinal))
-                        {
-                            continue;
-                        }
-
                         var originalName = inputParameter.OriginalName;
                         if (!string.IsNullOrEmpty(originalName))
                         {

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/TestData/TypeProviderTests/BuildMethodsForBackCompatibilityRestoresSynthesizedParameterNameBySignatureMatch.cs

@@ -0,0 +1,15 @@
+// <auto-generated/>
+
+#nullable disable
+
+namespace Test
+{
+    public partial class TestClient
+    {
+        public string Foo(string defaultName)
+        {
+            this.Validate(defaultName);
+            return defaultName;
+        }
+    }
+}

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/TestData/TypeProviderTests/BuildMethodsForBackCompatibilityRestoresSynthesizedParameterNameBySignatureMatch/TestClient.cs

@@ -0,0 +1,7 @@
+namespace Test
+{
+    public class TestClient
+    {
+        public string Foo(string defaultName) { return null; }
+    }
+}

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/TypeProviderTests.cs

@@ -970,6 +970,36 @@ public async Task BuildMethodsForBackCompatibilityRestoresRenamedParameterBySign
             Assert.AreEqual(Helpers.GetExpectedFromFile(), actual);
         }
 
+        // Validates that synthesized parameters retain the positional fallback even when their public name
+        // differs from the InputParameter they originated from.
+        [Test]
+        public async Task BuildMethodsForBackCompatibilityRestoresSynthesizedParameterNameBySignatureMatch()
+        {
+            await MockHelpers.LoadMockGeneratorAsync(lastContractCompilation: async () => await Helpers.GetCompilationFromDirectoryAsync());
+
+            var inputParameter = InputFactory.QueryParameter("wireName", InputPrimitiveType.String, isRequired: true);
+            var parameter = new ParameterProvider(
+                "default",
+                $"",
+                new CSharpType(typeof(string)),
+                inputParameter: inputParameter);
+            var fooMethod = new MethodProvider(
+                new MethodSignature("Foo", $"", MethodSignatureModifiers.Public, new CSharpType(typeof(string)), $"", [parameter]),
+                new MethodBodyStatement[]
+                {
+                    Snippet.This.Invoke("Validate", parameter).Terminate(),
+                    Snippet.Return(parameter),
+                },
+                new TestTypeProvider());
+
+            var typeProvider = new TestTypeProvider(name: "TestClient", methods: [fooMethod]);
+
+            typeProvider.ProcessTypeForBackCompatibility();
+
+            var actual = new TypeProviderWriter(typeProvider).Write().Content;
+            Assert.AreEqual(Helpers.GetExpectedFromFile(), actual);
+        }
+
         // When the restored name is used both as an argument (AsArgument -> _asArgument) and
         // as a variable (-> _asVariable), materializing both cached expressions before the rename, the
         // rename must keep them sharing one declaration. Otherwise the writer renames the two