Fix #2283: Disallow all implicit conversions in switch on string.

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

@@ -411,7 +411,9 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 
 		public static string SwitchOverImplicitString(ImplicitString s)
 		{
-			switch (s)
+			// we emit an explicit cast, because the rules used by the C# compiler are counter-intuitive:
+			// The C# compiler does *not* take the type of the switch labels into account at all.
+			switch ((string)s)
 			{
 				case "First case":
 					return "Text1";

ICSharpCode.Decompiler/CSharp/StatementBuilder.cs

@@ -205,7 +205,9 @@ namespace ICSharpCode.Decompiler.CSharp
 					.ConvertTo(
 						typeSystem.FindType(KnownTypeCode.String),
 						exprBuilder,
-						allowImplicitConversion: true
+						// switch statement does support implicit conversions in general, however, the rules are
+						// not very intuitive and in order to prevent bugs, we emit an explicit cast.
+						allowImplicitConversion: false
 					);
 				type = exprBuilder.compilation.FindType(KnownTypeCode.String);
 			}