Use appropriate type for helper variables introduced for named arguments.

ICSharpCode.Decompiler/IL/Instructions/ILFunction.cs

@@ -229,7 +229,18 @@ namespace ICSharpCode.Decompiler.IL
 
 		public ILVariable RegisterVariable(VariableKind kind, IType type, string name = null)
 		{
-			var variable = new ILVariable(kind, type);
+			return RegisterVariable(kind, type, type.GetStackType(), name);
+		}
+
+		public ILVariable RegisterVariable(VariableKind kind, StackType stackType, string name = null)
+		{
+			var type = Method.Compilation.FindType(stackType.ToKnownTypeCode());
+			return RegisterVariable(kind, type, stackType, name);
+		}
+
+		ILVariable RegisterVariable(VariableKind kind, IType type, StackType stackType, string name = null)
+		{
+			var variable = new ILVariable(kind, type, stackType);
 			if (string.IsNullOrWhiteSpace(name)) {
 				name = "I_" + (helperVariableCount++);
 				variable.HasGeneratedName = true;

ICSharpCode.Decompiler/IL/Transforms/NamedArgumentTransform.cs

@@ -79,7 +79,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 		{
 			var call = (CallInstruction)arg.Parent;
 			Debug.Assert(context.Function == call.Ancestors.OfType<ILFunction>().First());
-			var v = context.Function.RegisterVariable(VariableKind.NamedArgument, arg.InferType(context.TypeSystem));
+			var v = context.Function.RegisterVariable(VariableKind.NamedArgument, arg.ResultType);
 			context.Step($"Introduce named argument '{v.Name}'", arg);
 			if (!(call.Parent is Block namedArgBlock) || namedArgBlock.Kind != BlockKind.CallWithNamedArgs) {
 				// create namedArgBlock: