Handle explicit optional parameter after default parameter (#3470)

* Handle explicit optional parameter after default parameter

* Rename and add documentation

* Move extension method

* Add unit test for lamba function

* Apply suggestions from code review

Co-authored-by: Siegfried Pammer <siegfried@pammer.io>

---------

Co-authored-by: Siegfried Pammer <siegfried@pammer.io>

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;
@@ -307,8 +308,17 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 		{
 		}
 #endif
-
+		public static void Issue3469a([Optional][DefaultParameterValue(0)] int i, [Optional] DateTime d)
+		{
+		}
 #if CS120
+		public static Action<int, DateTime> Issue3469b()
+		{
+#pragma warning disable CS9099 // Parameter 1 has default value 'default(int)' in lambda but '<missing>' in the target delegate type
+			return ([Optional][DefaultParameterValue(0)] int i, [Optional] DateTime d) => {
+			};
+#pragma warning restore CS9099
+		}
 		public static D LambdaWithOptionalParameter()
 		{
 			return (int x = 10) => x;

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.IsDefaultValueAssignmentAllowed() && this.ShowConstantValues)
 			{
 				try
 				{

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.IsDefaultValueAssignmentAllowed();
 
-			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));
 			}

ICSharpCode.Decompiler/TypeSystem/TypeSystemExtensions.cs

@@ -612,6 +612,45 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		}
 		#endregion
 
+		#region IParameter.IsDefaultValueAssignmentAllowed
+		/// <summary>
+		/// Checks if the parameter is allowed to be assigned a default value.
+		/// </summary>
+		/// <remarks>
+		/// This checks <see cref="IParameter.IsOptional"/>, <see cref="IParameter.HasConstantValueInSignature"/>, <see cref="IParameter.ReferenceKind"/>,
+		/// and <see cref="IParameter.IsParams"/> on this parameter and all subsequent parameters.
+		/// If the parameter has no <see cref="IParameter.Owner"/>, it does not check subsequent parameters.
+		/// </remarks>
+		/// <param name="parameter">The parameter</param>
+		/// <returns>True if the <paramref name="parameter"/> has a default value and is allowed to be assigned a default value.</returns>
+		public static bool IsDefaultValueAssignmentAllowed(this IParameter parameter)
+		{
+			if (!DefaultValueAssignmentAllowedIndividual(parameter))
+				return false;
+
+			if (parameter.Owner == null)
+				return true; // Shouldn't happen, but we need to check for it.
+
+			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;
+			}
+		}
+		#endregion
+
 		#region IAssembly.GetTypeDefinition(string,string,int)
 		/// <summary>
 		/// Gets the type definition for a top-level type.