Fix empty/null handling for Type.BaseType intrinsic (#2694)

Author: sbomer

src/ILLink.Shared/TrimAnalysis/HandleCallAction.cs

@@ -614,6 +614,11 @@ GenericParameterValue genericParam
 			// Type.BaseType
 			//
 			case IntrinsicId.Type_get_BaseType: {
+					if (instanceValue.IsEmpty ()) {
+						returnValue = MultiValueLattice.Top;
+						break;
+					}
+
 					foreach (var value in instanceValue) {
 						if (value is ValueWithDynamicallyAccessedMembers valueWithDynamicallyAccessedMembers) {
 							DynamicallyAccessedMemberTypes propagatedMemberTypes = DynamicallyAccessedMemberTypes.None;
@@ -650,6 +655,7 @@ GenericParameterValue genericParam
 								AddReturnValue (GetMethodReturnValue (calledMethod, returnValueDynamicallyAccessedMemberTypes));
 						} else if (value == NullValue.Instance) {
 							// Ignore nulls - null.BaseType will fail at runtime, but it has no effect on static analysis
+							returnValue ??= MultiValueLattice.Top;
 							continue;
 						} else {
 							// Unknown input - propagate a return value without any annotation - we know it's a Type but we know nothing about it

test/Mono.Linker.Tests.Cases/DataFlow/TypeBaseTypeDataFlow.cs

@@ -229,19 +229,17 @@ static void TestAnnotatedAndUnannotated (
 			type.BaseType.RequiresPublicMethods ();
 		}
 
-		[ExpectedWarning ("IL2072", nameof (DataFlowTypeExtensions) + "." + nameof (DataFlowTypeExtensions.RequiresPublicMethods))]
 		static void TestNull ()
 		{
 			Type type = null;
 			type.BaseType.RequiresPublicMethods ();
 		}
 
-		[ExpectedWarning ("IL2072", nameof (DataFlowTypeExtensions) + "." + nameof (DataFlowTypeExtensions.RequiresPublicMethods))]
 		static void TestNoValue ()
 		{
 			Type t = null;
 			Type noValue = Type.GetTypeFromHandle (t.TypeHandle);
-			// Warns about the base type even though the above throws an exception at runtime.
+			// No warning because the above throws an exception at runtime.
 			noValue.BaseType.RequiresPublicMethods ();
 		}