Warn when box trait found on union member

smithy-model/src/main/java/software/amazon/smithy/model/validation/validators/UnionValidator.java

@@ -22,6 +22,7 @@
 import software.amazon.smithy.model.shapes.MemberShape;
 import software.amazon.smithy.model.shapes.Shape;
 import software.amazon.smithy.model.shapes.UnionShape;
+import software.amazon.smithy.model.traits.BoxTrait;
 import software.amazon.smithy.model.traits.DefaultTrait;
 import software.amazon.smithy.model.validation.AbstractValidator;
 import software.amazon.smithy.model.validation.ValidationEvent;
@@ -38,16 +39,30 @@ public List<ValidationEvent> validate(Model model) {
             } else {
                 for (MemberShape member : union.getAllMembers().values()) {
                     Shape target = model.expectShape(member.getTarget());
-                    if (target.hasTrait(DefaultTrait.class)) {
-                        events.add(note(member, String.format(
-                                "This union member targets `%s`, a shape with a default value of `%s`. Note that "
-                                + "default values are only applicable to structures and ignored in tagged unions. It "
-                                + "is a best practice for union members to target shapes with no default value.",
-                                target.getId(), Node.printJson(target.expectTrait(DefaultTrait.class).toNode()))));
-                    }
+                    validateUnionMemberTarget(member, target, events);
+                    validateUnionMember(member, events);
                 }
             }
         }
         return events;
     }
+
+    private void validateUnionMemberTarget(MemberShape member, Shape target, List<ValidationEvent> events) {
+        if (target.hasTrait(DefaultTrait.class)) {
+            events.add(note(member, String.format(
+                    "This union member targets `%s`, a shape with a default value of `%s`. Note that "
+                    + "default values are only applicable to structures and ignored in tagged unions. It "
+                    + "is a best practice for union members to target shapes with no default value (for example, "
+                    + "instead of targeting PrimitiveInteger, target Integer).",
+                    target.getId(), Node.printJson(target.expectTrait(DefaultTrait.class).toNode()))));
+        }
+    }
+
+    private void validateUnionMember(MemberShape member, List<ValidationEvent> events) {
+        if (member.hasTrait(BoxTrait.class)) {
+            events.add(warning(member, member.expectTrait(BoxTrait.class),
+                               "Invalid box trait found on a union member. The box trait on union members "
+                               + "has no effect because union members are implicitly nullable."));
+        }
+    }
 }

smithy-model/src/test/resources/software/amazon/smithy/model/errorfiles/validators/defaults/warn-when-union-targets-default.errors

@@ -1,2 +1,2 @@
-[NOTE] smithy.example#Example$foo: This union member targets `smithy.api#PrimitiveInteger`, a shape with a default value of `0`. Note that default values are only applicable to structures and ignored in tagged unions. It is a best practice for union members to target shapes with no default value. | Union
-[NOTE] smithy.example#Example$bar: This union member targets `smithy.api#PrimitiveBoolean`, a shape with a default value of `false`. Note that default values are only applicable to structures and ignored in tagged unions. It is a best practice for union members to target shapes with no default value. | Union
+[NOTE] smithy.example#Example$foo: This union member targets `smithy.api#PrimitiveInteger`, a shape with a default value of `0`. Note that default values are only applicable to structures and ignored in tagged unions. It is a best practice for union members to target shapes with no default value (for example, instead of targeting PrimitiveInteger, target Integer). | Union
+[NOTE] smithy.example#Example$bar: This union member targets `smithy.api#PrimitiveBoolean`, a shape with a default value of `false`. Note that default values are only applicable to structures and ignored in tagged unions. It is a best practice for union members to target shapes with no default value (for example, instead of targeting PrimitiveInteger, target Integer). | Union

smithy-model/src/test/resources/software/amazon/smithy/model/errorfiles/validators/unions/warn-box-on-union-member.errors

@@ -0,0 +1,2 @@
+[WARNING] smithy.example#Example$a: Invalid box trait found on a union member. The box trait on union members has no effect because union members are implicitly nullable. | Union
+[NOTE] smithy.example#Example$a: This union member targets `smithy.api#PrimitiveInteger`, a shape with a default value of `0`. Note that default values are only applicable to structures and ignored in tagged unions. It is a best practice for union members to target shapes with no default value (for example, instead of targeting PrimitiveInteger, target Integer). | Union

smithy-model/src/test/resources/software/amazon/smithy/model/errorfiles/validators/unions/warn-box-on-union-member.smithy

@@ -0,0 +1,10 @@
+$version: "1.0"
+
+namespace smithy.example
+
+union Example {
+    @box
+    a: PrimitiveInteger,
+
+    b: Integer
+}