Add tests for C# 9.0 lambda parameter discards

ICSharpCode.Decompiler.Tests/Helpers/Tester.cs

@@ -251,15 +251,16 @@ namespace ICSharpCode.Decompiler.Tests.Helpers
 				preprocessorSymbols.Add("VB11");
 				preprocessorSymbols.Add("VB14");
 				preprocessorSymbols.Add("VB15");
+
+				if (flags.HasFlag(CompilerOptions.Preview)) {
+					preprocessorSymbols.Add("CS90");
+				}
 			} else if (flags.HasFlag(CompilerOptions.UseMcs)) {
 				preprocessorSymbols.Add("MCS");
 			} else {
 				preprocessorSymbols.Add("LEGACY_CSC");
 				preprocessorSymbols.Add("LEGACY_VBC");
 			}
-			if (flags.HasFlag(CompilerOptions.Preview)) {
-				preprocessorSymbols.Add("CS90");
-			}
 			return preprocessorSymbols;
 		}
 

ICSharpCode.Decompiler.Tests/PrettyTestRunner.cs

@@ -155,7 +155,7 @@ namespace ICSharpCode.Decompiler.Tests
 		[Test]
 		public void DelegateConstruction([ValueSource(nameof(defaultOptionsWithMcs))] CompilerOptions cscOptions)
 		{
-			RunForLibrary(cscOptions: cscOptions);
+			RunForLibrary(cscOptions: cscOptions | CompilerOptions.Preview);
 		}
 
 		[Test]

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

@@ -428,6 +428,18 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			return () => valueTuple.RenamedString;
 		}
 #endif
+
+		public static Func<T, T> Identity<T>()
+		{
+			return (T _) => _;
+		}
+
+#if CS90
+		public static Func<int, int, int, int> LambdaParameterDiscard()
+		{
+			return (int _, int _, int _) => 0;
+		}
+#endif
 	}
 
 	public class Issue1867