Fixed overflow exception and added tests

ICSharpCode.Decompiler.Tests/TestCases/Correctness/DecimalFields.cs

@@ -1,4 +1,4 @@
-// Copyright (c) 2014 AlphaSierraPapa for the SharpDevelop Team
+// Copyright (c) 2014 AlphaSierraPapa for the SharpDevelop Team
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -36,7 +36,27 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Correctness
 			Console.WriteLine(field2);
 			Console.WriteLine(field3);
 			Console.WriteLine(field4);
+			Console.WriteLine(IntToDecimal());
+			Console.WriteLine(UIntToDecimal());
+			Console.WriteLine(LongToDecimal());
+			Console.WriteLine(ULongToDecimal());
 			return 0;
 		}
+
+		public static decimal IntToDecimal() {
+			return (decimal)int.MaxValue;
+		}
+
+		public static decimal UIntToDecimal() {
+			return (decimal)uint.MaxValue;
+		}
+
+		public static decimal LongToDecimal() {
+			return (decimal)long.MaxValue;
+		}
+
+		public static decimal ULongToDecimal() {
+			return (decimal)ulong.MaxValue;
+		}
 	}
 }

ICSharpCode.Decompiler/IL/Transforms/ExpressionTransforms.cs

@@ -433,10 +433,10 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 				{
 					var paramType = inst.Method.Parameters[0].Type.GetDefinition()?.KnownTypeCode;
 					result = paramType switch {
-						KnownTypeCode.Int32 => new LdcDecimal(new decimal((int)val)),
-						KnownTypeCode.UInt32 => new LdcDecimal(new decimal((uint)val)),
+						KnownTypeCode.Int32 => new LdcDecimal(new decimal(unchecked((int)val))),
+						KnownTypeCode.UInt32 => new LdcDecimal(new decimal(unchecked((uint)val))),
 						KnownTypeCode.Int64 => new LdcDecimal(new decimal(val)),
-						KnownTypeCode.UInt64 => new LdcDecimal(new decimal((ulong)val)),
+						KnownTypeCode.UInt64 => new LdcDecimal(new decimal(unchecked((ulong)val))),
 						_ => null
 					};
 					return result is not null;