Support field access on variables of type Enum (#105524)

Follow-up to #105351

This adds support for the case when a variable is of type Enum and
there are no generics involved, so the following no longer produces
trim warnings:

```csharp
static void M(Enum v) {
    v.GetType().GetFields();
}
```

Fixes #105506

Author: sbomer

src/coreclr/tools/aot/ILCompiler.Compiler/Compiler/Dataflow/HandleCallAction.cs

@@ -390,6 +390,10 @@ private partial bool TryHandleIntrinsic (
                             // currently it won't do.
 
                             TypeDesc? staticType = (valueNode as IValueWithStaticType)?.StaticType?.Type;
+                            if (staticType?.IsByRef == true)
+                            {
+                                staticType = ((ByRefType)staticType).ParameterType;
+                            }
                             if (staticType is null || (!staticType.IsDefType && !staticType.IsArray))
                             {
                                 DynamicallyAccessedMemberTypes annotation = default;
@@ -432,6 +436,10 @@ private partial bool TryHandleIntrinsic (
                                 // we will also make it just work, even if the annotation doesn't match the usage.
                                 AddReturnValue(new SystemTypeValue(staticType));
                             }
+                            else if (staticType.IsTypeOf("System", "Enum"))
+                            {
+                                AddReturnValue(_reflectionMarker.Annotations.GetMethodReturnValue(calledMethod, _isNewObj, DynamicallyAccessedMemberTypes.PublicFields));
+                            }
                             else
                             {
                                 Debug.Assert(staticType is MetadataType || staticType.IsArray);

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

@@ -113,6 +113,8 @@ private partial bool TryHandleIntrinsic (
 							// where a parameter is annotated and if something in the method sets a specific known type to it
 							// we will also make it just work, even if the annotation doesn't match the usage.
 							AddReturnValue (new SystemTypeValue (new (staticType)));
+						} else if (staticType.IsTypeOf ("System", "Enum")) {
+							AddReturnValue (FlowAnnotations.Instance.GetMethodReturnValue (calledMethod, _isNewObj, DynamicallyAccessedMemberTypes.PublicFields));
 						} else {
 							var annotation = FlowAnnotations.GetTypeAnnotation (staticType);
 							AddReturnValue (FlowAnnotations.Instance.GetMethodReturnValue (calledMethod, _isNewObj, annotation));

src/tools/illink/src/linker/Linker.Dataflow/HandleCallAction.cs

@@ -120,6 +120,8 @@ private partial bool TryHandleIntrinsic (
 						// currently it won't do.
 
 						TypeReference? staticType = (valueNode as IValueWithStaticType)?.StaticType?.Type;
+						if (staticType?.IsByReference == true)
+							staticType = ((ByReferenceType) staticType).ElementType;
 						TypeDefinition? staticTypeDef = staticType?.ResolveToTypeDefinition (_context);
 						if (staticType is null || staticTypeDef is null) {
 							DynamicallyAccessedMemberTypes annotation = default;
@@ -153,6 +155,8 @@ private partial bool TryHandleIntrinsic (
 							// where a parameter is annotated and if something in the method sets a specific known type to it
 							// we will also make it just work, even if the annotation doesn't match the usage.
 							AddReturnValue (new SystemTypeValue (staticType));
+						} else if (staticTypeDef.IsTypeOf ("System", "Enum")) {
+							AddReturnValue (_context.Annotations.FlowAnnotations.GetMethodReturnValue (calledMethod, _isNewObj, DynamicallyAccessedMemberTypes.PublicFields));
 						} else {
 							// Make sure the type is marked (this will mark it as used via reflection, which is sort of true)
 							// This should already be true for most cases (method params, fields, ...), but just in case

src/tools/illink/test/Mono.Linker.Tests.Cases/DataFlow/ObjectGetTypeDataflow.cs

@@ -97,7 +97,6 @@ public static void Test ()
 		class EnumTypeSatisfiesPublicFields
 		{
 			[Kept]
-			[ExpectedWarning ("IL2072")]
 			static void ParameterType (Enum instance)
 			{
 				instance.GetType ().RequiresPublicFields ();
@@ -113,7 +112,6 @@ class FieldType
 				public FieldType (Enum instance) => field = instance;
 
 				[Kept]
-				[ExpectedWarning ("IL2072")]
 				public void Test ()
 				{
 					field.GetType ().RequiresPublicFields ();
@@ -134,18 +132,36 @@ static Enum ReturnType ()
 			}
 
 			[Kept]
-			[ExpectedWarning ("IL2072")]
 			static void TestReturnType ()
 			{
 				ReturnType ().GetType ().RequiresPublicFields ();
 			}
 
+			[Kept]
+			static void OutParameter (out Enum value)
+			{
+				value = EnumType.Value;
+			}
+
+			[Kept]
+			// Analyzer doesn't assign a value to the out parameter after calling the OutParameter method,
+			// so when it looks up the value of the local 'value', it returns an empty value, and the
+			// GetType intrinsic handling can't see that the out param satisfies the public fields requirement.
+			// Similar for the other cases below.
+			[ExpectedWarning ("IL2072", Tool.Analyzer, "https://github.com/dotnet/runtime/issues/101734")]
+			static void TestOutParameter ()
+			{
+				OutParameter (out var value);
+				value.GetType ().RequiresPublicFields ();
+			}
+
 			[Kept]
 			public static void Test ()
 			{
 				ParameterType (EnumType.Value);
 				new FieldType (EnumType.Value).Test ();
 				TestReturnType ();
+				TestOutParameter ();
 			}
 		}
 
@@ -173,10 +189,10 @@ public static void TestAccessFromType ()
 
 			// Note: this doesn't warn for ILLink as a consequence of https://github.com/dotnet/runtime/issues/105345.
 			// ILLink sees the field type as a generic parameter, whereas the other tools see it as System.Enum.
-			// The special handling that treats Enum as satisfying PublicFields only applies to generic parameter constraints,
-			// so ILLink doesn't warn here. Once this 105345 is fixed, ILLink should match the warning behavior of ILC
-			// here and in the similar cases below.
-			[ExpectedWarning ("IL2072", Tool.NativeAot | Tool.Analyzer, "https://github.com/dotnet/runtime/issues/105345")]
+			// The other tools don't warn because of the built-in handling for variables of type System.Enum,
+			// and ILLink doesn't warn because this goes through the case that handles generic parameters constrained to be Enum.
+			// When https://github.com/dotnet/runtime/issues/105345 is fixed this should go through the built-in handling for
+			// System.Enum, like it does for ILC and the analyzer.
 			static void TestAccessTypeGenericParameterAsField ()
 			{
 				TypeGenericParameterAsField<Enum>.field.GetType ().RequiresPublicFields ();
@@ -214,7 +230,7 @@ public static void TestAccessFromType ()
 				}
 			}
 
-			[ExpectedWarning ("IL2072", Tool.NativeAot | Tool.Analyzer, "https://github.com/dotnet/runtime/issues/105345")]
+			// Note: this happens to work for ILLink due to https://github.com/dotnet/runtime/issues/105345.
 			static void TestAccessTypeGenericParameterAsReturnType ()
 			{
 				TypeGenericParameterAsReturnType<Enum>.Method ().GetType ().RequiresPublicFields ();
@@ -230,15 +246,15 @@ public static void Method (out T instance)
 				}
 
 				[Kept]
-				[ExpectedWarning ("IL2072")]
+				[ExpectedWarning ("IL2072", Tool.Analyzer, "https://github.com/dotnet/runtime/issues/101734")]
 				public static void TestAccessFromType ()
 				{
 					Method (out var instance);
 					instance.GetType ().RequiresPublicFields ();
 				}
 			}
 
-			[ExpectedWarning ("IL2072")]
+			[ExpectedWarning ("IL2072", Tool.Analyzer, "https://github.com/dotnet/runtime/issues/101734")]
 			static void TestAccessTypeGenericParameterAsOutParam ()
 			{
 				TypeGenericParameterAsOutParam<Enum>.Method (out var instance);
@@ -259,7 +275,7 @@ static T MethodGenericParameterAsReturnType<T> () where T : Enum
 			}
 
 			[Kept]
-			[ExpectedWarning ("IL2072", Tool.NativeAot | Tool.Analyzer, "https://github.com/dotnet/runtime/issues/105345")]
+			// Note: this happens to work for ILLink due to https://github.com/dotnet/runtime/issues/105345.
 			static void TestMethodGenericParameterAsReturnType ()
 			{
 				MethodGenericParameterAsReturnType<Enum> ().GetType ().RequiresPublicFields ();