Fix analyzer tracking of nullable enums (#110331)

Co-authored-by: Sven Boemer <[email protected]>
Co-authored-by: Jeff Schwartz <[email protected]>

Author: 3 people

src/tools/illink/src/ILLink.RoslynAnalyzer/TrimAnalysis/SingleValueExtensions.cs

@@ -21,7 +21,7 @@ public static class SingleValueExtensions
 							new GenericParameterValue ((ITypeParameterSymbol) underlyingType)),
 					// typeof(Nullable<>)
 					TypeKind.Error => new SystemTypeValue (new TypeProxy (type)),
-					TypeKind.Class or TypeKind.Struct or TypeKind.Interface =>
+					TypeKind.Class or TypeKind.Enum or TypeKind.Interface or TypeKind.Struct =>
 						new NullableSystemTypeValue (new TypeProxy (type), new SystemTypeValue (new TypeProxy (underlyingType))),
 					_ => UnknownValue.Instance
 				};

src/tools/illink/test/Mono.Linker.Tests.Cases/Reflection/RunClassConstructor.cs

@@ -22,6 +22,7 @@ public static void Main ()
 			TestIfElseUsingRuntimeTypeHandle (1);
 			TestIfElseUsingType (1);
 			TestNullableValueType ();
+			TestNullableEnum ();
 		}
 
 		[Kept]
@@ -132,6 +133,16 @@ static void TestNullableValueType ()
 			RuntimeHelpers.RunClassConstructor (typeof (int?).TypeHandle);
 		}
 
+		[Kept]
+		[KeptMember ( "value__")]
+		enum MyEnum {}
+
+		[Kept]
+		static void TestNullableEnum ()
+		{
+			RuntimeHelpers.RunClassConstructor (typeof (Nullable<MyEnum>).TypeHandle);
+		}
+
 		[Kept]
 		[KeptMember (".cctor()")]
 		class OnlyUsedViaReflection