Fix #1829: Fix decompilation of optional `in` parameters.

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

@@ -196,6 +196,18 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			readOnlyStruct.Method();
 		}
 
+		public void M(in DateTime a = default(DateTime))
+		{
+		}
+
+		public void M2<T>(in T a = default(T))
+		{
+		}
+
+		public void M3<T>(in T? a = null) where T : struct
+		{
+		}
+
 		public static TReturn Invoker<T1, TReturn>(RefFunc<T1, TReturn> action, T1 value)
 		{
 			return action(value);

ICSharpCode.Decompiler/CSharp/Syntax/TypeSystemAstBuilder.cs

@@ -1268,18 +1268,20 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			if (ShowAttributes) {
 				decl.Attributes.AddRange(ConvertAttributes(parameter.GetAttributes()));
 			}
+			IType parameterType;
 			if (parameter.Type.Kind == TypeKind.ByReference) {
 				// avoid 'out ref'
-				decl.Type = ConvertType(((ByReferenceType)parameter.Type).ElementType);
+				parameterType = ((ByReferenceType)parameter.Type).ElementType;
 			} else {
-				decl.Type = ConvertType(parameter.Type);
+				parameterType = parameter.Type;
 			}
+			decl.Type = ConvertType(parameterType);
 			if (this.ShowParameterNames) {
 				decl.Name = parameter.Name;
 			}
 			if (parameter.IsOptional && parameter.HasConstantValueInSignature && this.ShowConstantValues) {
 				try {
-					decl.DefaultExpression = ConvertConstantValue(parameter.Type, parameter.GetConstantValue(throwOnInvalidMetadata: true));
+					decl.DefaultExpression = ConvertConstantValue(parameterType, parameter.GetConstantValue(throwOnInvalidMetadata: true));
 				} catch (BadImageFormatException ex) {
 					decl.DefaultExpression = new ErrorExpression(ex.Message);
 				}