Fix #3181: missing type information on NullReferenceExpression used with nullable value types.

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

@@ -316,5 +316,15 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 		{
 			return a?.b.c(1)?.d[10];
 		}
+
+		private static string Issue3181()
+		{
+#if EXPECTED_OUTPUT
+			return ((int?)null)?.ToString();
+#else
+			int? x = null;
+			return x?.ToString();
+#endif
+		}
 	}
 }

ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs

@@ -626,20 +626,30 @@ namespace ICSharpCode.Decompiler.CSharp
 			Expression expr;
 			IType constantType;
 			object constantValue;
-			if (type.IsReferenceType == true || type.IsKnownType(KnownTypeCode.NullableOfT))
+			if (type.IsReferenceType == true)
 			{
 				expr = new NullReferenceExpression();
 				constantType = SpecialType.NullType;
 				constantValue = null;
+				return expr.WithRR(new ConstantResolveResult(constantType, constantValue));
+			}
+			else if (type.IsKnownType(KnownTypeCode.NullableOfT))
+			{
+				expr = new NullReferenceExpression();
+				constantType = SpecialType.NullType;
+				constantValue = null;
+				var crr = new ConstantResolveResult(constantType, constantValue);
+				return new CastExpression(ConvertType(type), expr.WithRR(crr))
+					.WithRR(new ConversionResolveResult(type, crr, Conversion.NullLiteralConversion));
 			}
 			else
 			{
 				expr = new DefaultValueExpression(ConvertType(type));
 				constantType = type;
 				constantValue = CSharpResolver.GetDefaultValue(type);
-			}
 				return expr.WithRR(new ConstantResolveResult(constantType, constantValue));
 			}
+		}
 
 		protected internal override TranslatedExpression VisitSizeOf(SizeOf inst, TranslationContext context)
 		{