Try improving handling of unions

The goal is to handle "complex" unions (unions that aren't merely subsets of a single primitive type) without interfering with simple enumerated-constant-style unions (such as `'development' | 'production' | 'test'`) or booleans.

Fixes #2848

lib/index.d.ts

@@ -740,11 +740,30 @@ declare namespace Joi {
 
     type NullableType<T> = undefined | null | T
 
+    type IsPrimitiveSubset<T> =
+        [T] extends [string]
+        ? true
+        : [T] extends [number]
+        ? true
+        : [T] extends [bigint]
+        ? true
+        : [T] extends [boolean]
+        ? true
+        : [T] extends [symbol]
+        ? true
+        : [T] extends [null]
+        ? true
+        : [T] extends [undefined]
+        ? true
+        : false;
+
     type IsUnion<T, U extends T = T> =
       T extends unknown ? [U] extends [T] ? false : true : false;
 
+    type IsNonPrimitiveSubsetUnion<T> = true extends IsUnion<T> ? true extends IsPrimitiveSubset<T> ? false : true : false;
+
     type ObjectPropertiesSchema<T = any> =
-        true extends IsUnion<Exclude<T, undefined | null>>
+        true extends IsNonPrimitiveSubsetUnion<Exclude<T, undefined | null>>
         ? Joi.AlternativesSchema
         : T extends NullableType<string>
         ? Joi.StringSchema

test/index.ts

@@ -1263,11 +1263,13 @@ const userSchema2 = Joi.object<User>().keys({
 interface Comment {
     text: string;
     user: User;
+    isNew: boolean;
 }
 
 const commentSchemaObject = Joi.object<Comment, true>({
     text: Joi.string().required(),
-    user: userSchemaObject
+    user: userSchemaObject,
+    isNew: Joi.boolean().required(),
 });
 
 interface Comment2 {
@@ -1287,11 +1289,15 @@ const commentSchemaObject2 = Joi.object<Comment2, true>({
 interface CommentWithAlternatives {
     text: string;
     user: string | User;
+    type: 'topLevel' | 'pingback' | 'reply';
+    reported: boolean | number;
 }
 
 const commentWithAlternativesSchemaObject = Joi.object<CommentWithAlternatives, true>({
     text: Joi.string().required(),
     user: Joi.alternatives(Joi.string(), userSchemaObject),
+    type: Joi.string().required().valid('topLevel', 'pingback', 'reply'),
+    reported: Joi.alternatives(Joi.boolean(), Joi.number()),
 });
 
 expect.error(userSchema2.keys({ height: Joi.number() }));