Fix up default value for parameters with the in modifier (#26785)

src/System.Management.Automation/engine/runtime/Binding/Binders.cs

@@ -7201,6 +7201,13 @@ namespace System.Management.Automation.Language
                     var argValue = parameters[i].DefaultValue;
                     if (argValue == null)
                     {
+                        if (parameterType.IsByRef)
+                        {
+                            // When the default value is null for a ByRef parameter (e.g. an optional `in` parameter
+                            // using `default`), expression trees cannot create Expression.Default for the T& type.
+                            // In that case we switch to the element type and use Default(TElement) instead.
+                            parameterType = parameterType.GetElementType();
+                        }
                         argExprs[i] = Expression.Default(parameterType);
                     }
                     else if (!parameters[i].HasDefaultValue && parameterType != typeof(object) && argValue == Type.Missing)

test/powershell/engine/Basic/CLRBinding.Tests.ps1

@@ -27,6 +27,12 @@ public class TestClass
     public static string StaticWithOptionalExpected() => StaticWithOptional();
     public static string StaticWithOptional([Optional] string value) => value;
 
+    public static int PrimitiveTypeWithInDefault(in int value = default) => value;
+
+    public static Guid ValueTypeWithInDefault(in Guid value = default) => value;
+
+    public static string RefTypeWithInDefault(in string value = default) => value;
+
     public object InstanceWithDefaultExpected() => InstanceWithDefault();
     public object InstanceWithDefault(object value = null) => value;
 
@@ -101,6 +107,21 @@ public class TestClassCstorWithOptional
         $actual | Should -Be $expected
     }
 
+    It "Binds to static method with primitive type with in modifier and default argument" {
+        $actual = [CLRBindingTests.TestClass]::PrimitiveTypeWithInDefault()
+        $actual | Should -Be 0
+    }
+
+    It "Binds to static method with value type with in modifier and default argument" {
+        $actual = [CLRBindingTests.TestClass]::ValueTypeWithInDefault()
+        $actual | Should -Be ([Guid]::Empty)
+    }
+
+    It "Binds to static method with ref type with in modifier and default argument" {
+        $actual = [CLRBindingTests.TestClass]::RefTypeWithInDefault()
+        $null -eq $actual | Should -BeTrue
+    }
+
     It "Binds to instance method with default argument" {
         $c = [CLRBindingTests.TestClass]::new()