illink: Fix TypeMap instantiation not propagating to TypeMapAssociation entries

src/tools/illink/src/linker/Linker.Steps/MarkStep.cs

@@ -3515,7 +3515,7 @@ static bool TypeIsInlineArrayType(TypeDefinition type)
             return false;
         }
 
-        protected virtual void MarkRequirementsForInstantiatedTypes(TypeDefinition type)
+        protected internal virtual void MarkRequirementsForInstantiatedTypes(TypeDefinition type)
         {
             if (Annotations.IsInstantiated(type))
                 return;

src/tools/illink/src/linker/Linker/TypeMapHandler.cs

@@ -109,7 +109,7 @@ void MarkTypeMapAttribute(CustomAttributeWithOrigin entry, DependencyInfo info)
             // Mark the target type as instantiated
             if (entry.TargetType is { } targetType
                 && _context.Resolve(UnwrapToResolvableType(targetType)) is TypeDefinition targetTypeDef)
-                _context.Annotations.MarkInstantiated(targetTypeDef);
+                _markStep.MarkRequirementsForInstantiatedTypes(targetTypeDef);
         }
 
         public void ProcessType(TypeDefinition definition)

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

@@ -85,6 +85,15 @@
 [assembly: TypeMapAssemblyTarget<UsedTypeMapUniverse>("library")]
 [assembly: TypeMapAssemblyTarget<UnusedTypeMap2>("library")] // Should be removed
 
+// Verify that when a TypeMapAttribute's target type is instantiated by TypeMap processing,
+// the associated TypeMapAssociation entry is also kept. Without the fix, only MarkInstantiated
+// was called (not MarkRequirementsForInstantiatedTypes), preventing ProcessInstantiated from
+// being called and leaving TypeMapAssociation entries unmarked.
+[assembly: TypeMap<UsedTypeMap>("BothInExternalAndProxy", typeof(BothInExternalAndProxy), typeof(BothInExternalAndProxyTrimTarget))] // Kept
+[assembly: TypeMapAssociation<UsedTypeMap>(typeof(BothInExternalAndProxy), typeof(BothInExternalAndProxyTarget))] // Kept
+[assembly: KeptAttributeAttribute(typeof(TypeMapAttribute<UsedTypeMap>), "BothInExternalAndProxy", typeof(BothInExternalAndProxy), typeof(BothInExternalAndProxyTrimTarget))]
+[assembly: KeptAttributeAttribute(typeof(TypeMapAssociationAttribute<UsedTypeMap>), typeof(BothInExternalAndProxy), typeof(BothInExternalAndProxyTarget))]
+
 namespace Mono.Linker.Tests.Cases.Reflection
 {
     [SetupLinkerAction("link", "System.Private.CoreLib")] // Needed to get the RemoveAttributeInstances in embedded xml
@@ -234,6 +243,11 @@ static void ConstrainedStaticCall<T>(T t) where T : IStaticInterface
             _ = new int();
             _ = TypeMapping.GetOrCreateExternalTypeMapping<string>();
             _ = TypeMapping.GetOrCreateProxyTypeMapping<string>();
+
+            // Instantiate BothInExternalAndProxyTrimTarget to trigger the TypeMap entry for BothInExternalAndProxy.
+            // BothInExternalAndProxy is only instantiated via TypeMap processing (not directly), which verifies
+            // that MarkRequirementsForInstantiatedTypes is called to propagate instantiation to TypeMapAssociation.
+            _ = new BothInExternalAndProxyTrimTarget();
         }
 
         [ExpectBodyModified]
@@ -572,4 +586,12 @@ class ArrayTypeTrimTargetClass;
     class ArrayTypeTrimTargetUnusedTarget;
 
     class ArrayTypeTrimTargetUnusedClass;
+
+    [Kept]
+    [KeptMember(".ctor()")]
+    class BothInExternalAndProxyTrimTarget;
+    [Kept]
+    class BothInExternalAndProxy;
+    [Kept]
+    class BothInExternalAndProxyTarget;
 }