Handle explicit optional parameter after default parameter

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()

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
 				{

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;
+			}
+		}
+	}
 }

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));
 			}