Fix #2222: switch-expression does not support implicit conversions.

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

@@ -22,6 +22,14 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 {
 	public static class SwitchExpressions
 	{
+		public class ImplicitlyCastToString
+		{
+			public static implicit operator string(ImplicitlyCastToString val)
+			{
+				return "foo";
+			}
+		}
+
 		public enum State
 		{
 			False,
@@ -146,5 +154,16 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 					return "Default";
 			}
 		}
+
+		public static string Issue2222()
+		{
+			return (string)new ImplicitlyCastToString() switch
+			{
+				"foo" => "foo",
+				"bar" => "bar",
+				"quux" => "quux",
+				_ => "default",
+			};
+		}
 	}
 }

ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs

@@ -3627,7 +3627,7 @@ namespace ICSharpCode.Decompiler.CSharp
 					.ConvertTo(
 						typeSystem.FindType(KnownTypeCode.String),
 						this,
-						allowImplicitConversion: true
+						allowImplicitConversion: false // switch-expression does not support implicit conversions
 					);
 			}
 			else