Document behavior of unions containing unions

Fixes #131

src/tests/fixtures/unions/UnionAsMemberOfItself.invalid.ts

@@ -0,0 +1,22 @@
+/** @gqlType */
+export default class SomeType {
+  /** @gqlField */
+  me: Actor;
+}
+
+/** @gqlType */
+class User {
+  __typename = "User";
+  /** @gqlField */
+  name: string;
+}
+
+/** @gqlType */
+class Entity {
+  __typename = "Entity";
+  /** @gqlField */
+  description: string;
+}
+
+/** @gqlUnion */
+type Actor = User | Entity | Actor;

src/tests/fixtures/unions/UnionAsMemberOfItself.invalid.ts.expected

@@ -0,0 +1,33 @@
+-----------------
+INPUT
+----------------- 
+/** @gqlType */
+export default class SomeType {
+  /** @gqlField */
+  me: Actor;
+}
+
+/** @gqlType */
+class User {
+  __typename = "User";
+  /** @gqlField */
+  name: string;
+}
+
+/** @gqlType */
+class Entity {
+  __typename = "Entity";
+  /** @gqlField */
+  description: string;
+}
+
+/** @gqlUnion */
+type Actor = User | Entity | Actor;
+
+-----------------
+OUTPUT
+-----------------
+src/tests/fixtures/unions/UnionAsMemberOfItself.invalid.ts:22:30 - error: Union type Actor can only include Object types, it cannot include Actor.
+
+22 type Actor = User | Entity | Actor;
+                                ~~~~~

src/tests/fixtures/unions/UnionAsMemberOfOtherUnion.invalid.ts

@@ -0,0 +1,32 @@
+/** @gqlType */
+export default class SomeType {
+  /** @gqlField */
+  me: Actor;
+}
+
+/** @gqlType */
+class User {
+  __typename = "User";
+  /** @gqlField */
+  name: string;
+}
+
+/** @gqlType */
+class Entity {
+  __typename = "Entity";
+  /** @gqlField */
+  description: string;
+}
+
+/** @gqlType */
+class Admin {
+  __typename = "Admin";
+  /** @gqlField */
+  description: string;
+}
+
+/** @gqlUnion */
+type Foo = User | Entity;
+
+/** @gqlUnion */
+type Actor = Admin | Foo;

src/tests/fixtures/unions/UnionAsMemberOfOtherUnion.invalid.ts.expected

@@ -0,0 +1,43 @@
+-----------------
+INPUT
+----------------- 
+/** @gqlType */
+export default class SomeType {
+  /** @gqlField */
+  me: Actor;
+}
+
+/** @gqlType */
+class User {
+  __typename = "User";
+  /** @gqlField */
+  name: string;
+}
+
+/** @gqlType */
+class Entity {
+  __typename = "Entity";
+  /** @gqlField */
+  description: string;
+}
+
+/** @gqlType */
+class Admin {
+  __typename = "Admin";
+  /** @gqlField */
+  description: string;
+}
+
+/** @gqlUnion */
+type Foo = User | Entity;
+
+/** @gqlUnion */
+type Actor = Admin | Foo;
+
+-----------------
+OUTPUT
+-----------------
+src/tests/fixtures/unions/UnionAsMemberOfOtherUnion.invalid.ts:32:22 - error: Union type Actor can only include Object types, it cannot include Foo.
+
+32 type Actor = Admin | Foo;
+                        ~~~