Fix: Prevent generic_param_token corruption in KeyValuePair nullability detection

Author: medhatiwari

src/libraries/System.Private.CoreLib/src/System/Reflection/NullabilityInfoContext.cs

@@ -501,6 +501,14 @@ private bool TryUpdateGenericParameterNullability(NullabilityInfo nullability, T
             }
 
             var state = NullabilityState.Unknown;
+            // Special handling for KeyValuePair generic parameters that may be missing nullable attributes
+            if (IsKeyValuePairGenericParameter(genericParameter))
+            {
+                // KeyValuePair generic parameters should be treated as nullable by default
+                nullability.ReadState = NullabilityState.Nullable;
+                nullability.WriteState = NullabilityState.Nullable;
+                return true;
+            }
             if (CreateParser(genericParameter.GetCustomAttributesData()).ParseNullableState(0, ref state))
             {
                 nullability.ReadState = state;
@@ -518,6 +526,17 @@ private bool TryUpdateGenericParameterNullability(NullabilityInfo nullability, T
             return false;
         }
 
+        private static bool IsKeyValuePairGenericParameter(Type genericParameter)
+        {
+            if (!genericParameter.IsGenericParameter)
+                return false;
+            var declaringType = genericParameter.DeclaringType;
+            return declaringType != null &&
+                   declaringType.IsGenericTypeDefinition &&
+                   declaringType.FullName == "System.Collections.Generic.KeyValuePair`2" &&
+                   (genericParameter.Name == "TKey" || genericParameter.Name == "TValue");
+        }
+
         private bool TryUpdateGenericTypeParameterNullabilityFromReflectedType(NullabilityInfo nullability, Type genericParameter, Type context, Type reflectedType)
         {
             Debug.Assert(genericParameter.IsGenericParameter &&

src/mono/mono/metadata/class-init.c

@@ -2768,7 +2768,8 @@ mono_class_layout_fields (MonoClass *klass, int base_instance_size, int packing_
 	/* Publish the data */
 	mono_loader_lock ();
 	if (!klass->rank)
-		klass->sizes.class_size = class_size;
+		if (klass_byval_arg->type != MONO_TYPE_VAR && klass_byval_arg->type != MONO_TYPE_MVAR)
+			klass->sizes.class_size = class_size;
 	klass->has_static_refs = has_static_refs;
 	klass->has_weak_fields = has_weak_fields;
 	for (i = 0; i < top; ++i) {