Fix #1865: At least one of the first two operands in string concatenation needs to be of type `string`, so we can't always drop `ToString()` calls.

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

@@ -107,11 +107,19 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Correctness
 			Console.WriteLine(s);
 		}
 
+		static void TestCharPlusChar(string a)
+		{
+			Console.WriteLine("TestCharPlusChar:");
+			Console.WriteLine(a[0] + a[1]);
+			Console.WriteLine(a[0].ToString() + a[1].ToString());
+		}
+
 		static void Main()
 		{
 			TestClass();
 			TestStruct();
 			TestStructMutation();
+			TestCharPlusChar("ab");
 		}
 	}
 }

ICSharpCode.Decompiler/CSharp/Transforms/ReplaceMethodCallsWithOperators.cs

@@ -61,11 +61,16 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			if (IsStringConcat(method) && CheckArgumentsForStringConcat(arguments)) {
 				bool isInExpressionTree = invocationExpression.Ancestors.OfType<LambdaExpression>().Any(
 					lambda => lambda.Annotation<IL.ILFunction>()?.Kind == IL.ILFunctionKind.ExpressionTree);
-				Expression expr = arguments[0].Detach();
+				Expression arg0 = arguments[0].Detach();
+				Expression arg1 = arguments[1].Detach();
 				if (!isInExpressionTree) {
-					expr = RemoveRedundantToStringInConcat(expr, method, isLastArgument: false).Detach();
+					arg1 = RemoveRedundantToStringInConcat(arg1, method, isLastArgument: arguments.Length == 2).Detach();
+					if (arg1.GetResolveResult().Type.IsKnownType(KnownTypeCode.String)) {
+						arg0 = RemoveRedundantToStringInConcat(arg0, method, isLastArgument: false).Detach();
+					}
 				}
-				for (int i = 1; i < arguments.Length; i++) {
+				var expr = new BinaryOperatorExpression(arg0, BinaryOperatorType.Add, arg1);
+				for (int i = 2; i < arguments.Length; i++) {
 					var arg = arguments[i].Detach();
 					if (!isInExpressionTree) {
 						arg = RemoveRedundantToStringInConcat(arg, method, isLastArgument: i == arguments.Length - 1).Detach();