Warn when default used with union member target

This NOTE warning makes it clear to modelers that default values on the
target shape of a union member are ignored.

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

@@ -18,7 +18,11 @@
 import java.util.ArrayList;
 import java.util.List;
 import software.amazon.smithy.model.Model;
+import software.amazon.smithy.model.node.Node;
+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.DefaultTrait;
 import software.amazon.smithy.model.validation.AbstractValidator;
 import software.amazon.smithy.model.validation.ValidationEvent;
 import software.amazon.smithy.utils.SmithyInternalApi;
@@ -31,6 +35,17 @@ public List<ValidationEvent> validate(Model model) {
         for (UnionShape union : model.getUnionShapes()) {
             if (union.members().isEmpty()) {
                 events.add(error(union, "Tagged unions must have one or more members"));
+            } 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()))));
+                    }
+                }
             }
         }
         return events;

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

@@ -0,0 +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

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

@@ -0,0 +1,10 @@
+$version: "2.0"
+
+namespace smithy.example
+
+union Example {
+    foo: PrimitiveInteger, // warn
+    foo2: Integer,         // dont' warn
+    bar: PrimitiveBoolean, // warn
+    baz: String            // don't warn
+}