Fix #3895: keep byref-like ToString in string concatenation

String concatenation cannot box a byref-like value, so removing its explicit
ToString call produces invalid C#. Preserve that call while retaining the
surrounding string-concatenation reconstruction.

Assisted-by: Copilot:gpt-5.6-sol:GitHub Copilot CLI
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 86d2918e-5a24-48b4-9a86-41d331ec3720

Author: sailro

ICSharpCode.Decompiler.Tests/PrettyTestRunner.cs

@@ -338,6 +338,12 @@ public async Task AnonymousTypes([ValueSource(nameof(defaultOptionsWithMcs))] Co
 			await RunForLibrary(cscOptions: cscOptions);
 		}
 
+		[Test]
+		public async Task StringConcatenation([ValueSource(nameof(roslyn3OrNewerOptions))] CompilerOptions cscOptions)
+		{
+			await RunForLibrary(cscOptions: cscOptions);
+		}
+
 		[Test]
 		public async Task Async([ValueSource(nameof(defaultOptions))] CompilerOptions cscOptions)
 		{

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

@@ -0,0 +1,35 @@
+// Copyright (c) 2026 Sebastien Lebreton
+// 
+// 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
+// without restriction, including without limitation the rights to use, copy, modify, merge,
+// publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
+// to whom the Software is furnished to do so, subject to the following conditions:
+// 
+// The above copyright notice and this permission notice shall be included in all copies or
+// substantial portions of the Software.
+// 
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
+// INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
+// PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
+// FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+using System;
+
+namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
+{
+	public static class StringConcatenation
+	{
+		public static string WithSpan(Span<char> value)
+		{
+			return "prefix" + value.ToString();
+		}
+
+		public static string WithReadOnlySpan(ReadOnlySpan<char> value)
+		{
+			return "prefix" + value.ToString();
+		}
+	}
+}

ICSharpCode.Decompiler/CSharp/Transforms/ReplaceMethodCallsWithOperators.cs

@@ -366,6 +366,11 @@ internal static Expression RemoveRedundantToStringInConcat(Expression expr, IMet
 			var toStringMethod = m.Get<Expression>("call").Single().GetSymbol() as IMethod;
 			var target = m.Get<Expression>("target").Single();
 			var type = target.GetResolveResult().Type;
+			if (type.IsByRefLike)
+			{
+				// ref structs cannot be converted to object for use with +
+				return expr;
+			}
 			if (!(isLastArgument || ToStringIsKnownEffectFree(type)))
 			{
 				// ToString() order of evaluation matters, see CheckArgumentsForStringConcat().