Fix #1567: Do not transform string.Concat method calls to + operators if named arguments are used.

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

@@ -97,6 +97,16 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			};
 		}
 
+		private static string GetStr(int unused)
+		{
+			return " ";
+		}
+
+		public static string Issue1567(string str1, string str2)
+		{
+			return string.Concat(str1.Replace('"', '\''), str2: str2.Replace('"', '\''), str1: GetStr(42));
+		}
+
 		private void CallerMemberName([CallerMemberName] string memberName = null)
 		{
 

ICSharpCode.Decompiler/CSharp/Transforms/ReplaceMethodCallsWithOperators.cs

@@ -154,8 +154,16 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			if (arguments.Length < 2)
 				return false;
 
-			return arguments[0].GetResolveResult().Type.IsKnownType(KnownTypeCode.String) ||
-				arguments[1].GetResolveResult().Type.IsKnownType(KnownTypeCode.String);
+			bool valid = false;
+
+			foreach (var argument in arguments) {
+				if (argument is NamedArgumentExpression)
+					return false;
+				if (argument.GetResolveResult().Type.IsKnownType(KnownTypeCode.String))
+					valid = true;
+			}
+
+			return valid;
 		}
 
 		static BinaryOperatorType? GetBinaryOperatorTypeFromMetadataName(string name)