Fix #2920: Implement support for DefaultParameterValueAttribute.

ICSharpCode.Decompiler.Tests/TestCases/Pretty/OptionalArguments.cs

@@ -18,6 +18,7 @@
 
 using System.Collections.Generic;
 using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
 
 namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 {
@@ -225,5 +226,62 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 
 		}
 #endif
+
+		public static void Issue2920a(int x)
+		{
+		}
+		public static void Issue2920b([DefaultParameterValue(3)] int x)
+		{
+		}
+		public static void Issue2920c(ref int x)
+		{
+		}
+		public static void Issue2920d([DefaultParameterValue(3)] ref int x)
+		{
+		}
+		public static void Issue2920e(out int x)
+		{
+			x = 0;
+		}
+		public static void Issue2920f([DefaultParameterValue(3)] out int x)
+		{
+			x = 0;
+		}
+#if CS70
+		public static void Issue2920g(in int x)
+		{
+		}
+		public static void Issue2920h([DefaultParameterValue(3)] in int x)
+		{
+		}
+#endif
+		public static void Issue2920i([Optional] int x)
+		{
+		}
+		public static void Issue2920j(int x = 3)
+		{
+		}
+		public static void Issue2920k([Optional] ref int x)
+		{
+		}
+		public static void Issue2920l([Optional][DefaultParameterValue(3)] ref int x)
+		{
+		}
+		public static void Issue2920m([Optional] out int x)
+		{
+			x = 0;
+		}
+		public static void Issue2920n([Optional][DefaultParameterValue(3)] out int x)
+		{
+			x = 0;
+		}
+#if CS70
+		public static void Issue2920o([Optional] in int x)
+		{
+		}
+		public static void Issue2920p(in int x = 3)
+		{
+		}
+#endif
 	}
 }

ICSharpCode.Decompiler/CSharp/Syntax/TypeSystemAstBuilder.cs

@@ -1675,7 +1675,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			{
 				decl.Name = parameter.Name;
 			}
-			if (parameter.IsOptional && parameter.HasConstantValueInSignature && this.ShowConstantValues)
+			if (parameter.IsOptional && decl.ParameterModifier is ParameterModifier.None or ParameterModifier.In && parameter.HasConstantValueInSignature && this.ShowConstantValues)
 			{
 				try
 				{

ICSharpCode.Decompiler/TypeSystem/Implementation/KnownAttributes.cs

@@ -89,6 +89,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		In,
 		Out,
 		Optional,
+		DefaultParameterValue,
 		CallerMemberName,
 		CallerFilePath,
 		CallerLineNumber,
@@ -165,6 +166,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 			new TopLevelTypeName("System.Runtime.InteropServices", nameof(InAttribute)),
 			new TopLevelTypeName("System.Runtime.InteropServices", nameof(OutAttribute)),
 			new TopLevelTypeName("System.Runtime.InteropServices", nameof(OptionalAttribute)),
+			new TopLevelTypeName("System.Runtime.InteropServices", nameof(DefaultParameterValueAttribute)),
 			new TopLevelTypeName("System.Runtime.CompilerServices", nameof(CallerMemberNameAttribute)),
 			new TopLevelTypeName("System.Runtime.CompilerServices", nameof(CallerFilePathAttribute)),
 			new TopLevelTypeName("System.Runtime.CompilerServices", nameof(CallerLineNumberAttribute)),
@@ -220,6 +222,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 				case KnownAttribute.MarshalAs:
 				case KnownAttribute.PermissionSet:
 				case KnownAttribute.Optional:
+				case KnownAttribute.DefaultParameterValue:
 				case KnownAttribute.In:
 				case KnownAttribute.Out:
 				case KnownAttribute.IndexerName:

ICSharpCode.Decompiler/TypeSystem/Implementation/MetadataParameter.cs

@@ -64,8 +64,17 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 			var metadata = module.metadata;
 			var parameter = metadata.GetParameter(handle);
 
-			if (IsOptional && !HasConstantValueInSignature)
+			bool defaultValueAssignmentAllowed = ReferenceKind is ReferenceKind.None or ReferenceKind.In;
+
+			if (IsOptional && (!defaultValueAssignmentAllowed || !HasConstantValueInSignature))
+			{
 				b.Add(KnownAttribute.Optional);
+			}
+
+			if (!(IsDecimalConstant || !HasConstantValueInSignature) && (!defaultValueAssignmentAllowed || !IsOptional))
+			{
+				b.Add(KnownAttribute.DefaultParameterValue, KnownTypeCode.Object, GetConstantValue(throwOnInvalidMetadata: false));
+			}
 
 			if (!IsOut && !IsIn)
 			{