From 2a0675fb4a9672f991efbd330df1c4197bc29d17 Mon Sep 17 00:00:00 2001
From: ds5678 <49847914+ds5678@users.noreply.github.com>
Date: Mon, 5 May 2025 12:47:03 -0700
Subject: [PATCH] Handle explicit optional parameter after default parameter

---
 .../TestCases/Pretty/OptionalArguments.cs     |  4 +++
 .../CSharp/Syntax/TypeSystemAstBuilder.cs     |  3 +-
 .../TypeSystem/IParameter.cs                  | 30 +++++++++++++++++++
 .../Implementation/MetadataParameter.cs       |  6 ++--
 4 files changed, 38 insertions(+), 5 deletions(-)

diff --git a/ICSharpCode.Decompiler.Tests/TestCases/Pretty/OptionalArguments.cs b/ICSharpCode.Decompiler.Tests/TestCases/Pretty/OptionalArguments.cs
index d69c8f35a..f76f75183 100644
--- a/ICSharpCode.Decompiler.Tests/TestCases/Pretty/OptionalArguments.cs
+++ b/ICSharpCode.Decompiler.Tests/TestCases/Pretty/OptionalArguments.cs
@@ -16,6 +16,7 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+using System;
 using System.Collections.Generic;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
@@ -307,6 +308,9 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 		{
 		}
 #endif
+		public static void Issue3469([Optional][DefaultParameterValue(0)] int i, [Optional] DateTime d)
+		{
+		}
 
 #if CS120
 		public static D LambdaWithOptionalParameter()
diff --git a/ICSharpCode.Decompiler/CSharp/Syntax/TypeSystemAstBuilder.cs b/ICSharpCode.Decompiler/CSharp/Syntax/TypeSystemAstBuilder.cs
index 3583c6a44..b95655ba5 100644
--- a/ICSharpCode.Decompiler/CSharp/Syntax/TypeSystemAstBuilder.cs
+++ b/ICSharpCode.Decompiler/CSharp/Syntax/TypeSystemAstBuilder.cs
@@ -1735,8 +1735,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			{
 				decl.Name = parameter.Name;
 			}
-			if (parameter.IsOptional && decl.ParameterModifier is ReferenceKind.None or ReferenceKind.In or ReferenceKind.RefReadOnly
-				&& parameter.HasConstantValueInSignature && this.ShowConstantValues)
+			if (parameter.GetDefaultValueAssignmentAllowed() && this.ShowConstantValues)
 			{
 				try
 				{
diff --git a/ICSharpCode.Decompiler/TypeSystem/IParameter.cs b/ICSharpCode.Decompiler/TypeSystem/IParameter.cs
index 481613318..923ec06e3 100644
--- a/ICSharpCode.Decompiler/TypeSystem/IParameter.cs
+++ b/ICSharpCode.Decompiler/TypeSystem/IParameter.cs
@@ -105,4 +105,34 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// </summary>
 		IParameterizedMember? Owner { get; }
 	}
+
+	internal static class IParameterExtensions
+	{
+		public static bool GetDefaultValueAssignmentAllowed(this IParameter parameter)
+		{
+			if (!DefaultValueAssignmentAllowedIndividual(parameter))
+				return false;
+
+			if (parameter.Owner == null)
+				return true;
+
+			for (int i = parameter.Owner.Parameters.Count - 1; i >= 0; i--)
+			{
+				IParameter otherParameter = parameter.Owner.Parameters[i];
+				if (otherParameter == parameter)
+					break;
+
+				if (DefaultValueAssignmentAllowedIndividual(otherParameter) || otherParameter.IsParams)
+					continue;
+
+				return false;
+			}
+			return true;
+
+			static bool DefaultValueAssignmentAllowedIndividual(IParameter parameter)
+			{
+				return parameter.IsOptional && parameter.HasConstantValueInSignature && parameter.ReferenceKind is ReferenceKind.None or ReferenceKind.In or ReferenceKind.RefReadOnly;
+			}
+		}
+	}
 }
diff --git a/ICSharpCode.Decompiler/TypeSystem/Implementation/MetadataParameter.cs b/ICSharpCode.Decompiler/TypeSystem/Implementation/MetadataParameter.cs
index 14bee08dc..3067f135e 100644
--- a/ICSharpCode.Decompiler/TypeSystem/Implementation/MetadataParameter.cs
+++ b/ICSharpCode.Decompiler/TypeSystem/Implementation/MetadataParameter.cs
@@ -63,14 +63,14 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 			var metadata = module.metadata;
 			var parameter = metadata.GetParameter(handle);
 
-			bool defaultValueAssignmentAllowed = ReferenceKind is ReferenceKind.None or ReferenceKind.In or ReferenceKind.RefReadOnly;
+			bool defaultValueAssignmentAllowed = this.GetDefaultValueAssignmentAllowed();
 
-			if (IsOptional && (!defaultValueAssignmentAllowed || !HasConstantValueInSignature))
+			if (IsOptional && !defaultValueAssignmentAllowed)
 			{
 				b.Add(KnownAttribute.Optional);
 			}
 
-			if (!(IsDecimalConstant || !HasConstantValueInSignature) && (!defaultValueAssignmentAllowed || !IsOptional))
+			if (!IsDecimalConstant && HasConstantValueInSignature && !defaultValueAssignmentAllowed)
 			{
 				b.Add(KnownAttribute.DefaultParameterValue, KnownTypeCode.Object, GetConstantValue(throwOnInvalidMetadata: false));
 			}