Fix unions

Author: emmatown

packages/schema/src/definition.ts

@@ -122,14 +122,8 @@ export class GObjectType<Source, Context> extends GraphQLObjectType<
   }
 }
 
-export type GUnionTypeConfig<Source, Context> = Flatten<
-  Omit<GraphQLUnionTypeConfig<Source, Context>, "types"> & {
-    types: OrFunc<readonly GObjectType<Source, Context>[]>;
-  }
->;
-
 export class GUnionType<Source, Context> extends GraphQLUnionType {
-  constructor(config: Readonly<GUnionTypeConfig<Source, Context>>) {
+  constructor(config: Readonly<GraphQLUnionTypeConfig<Source, Context>>) {
     super(config);
   }
   declare resolveType: Maybe<GraphQLTypeResolver<Source, Context>>;

packages/schema/src/output.ts

@@ -8,6 +8,7 @@ import type {
   GraphQLInterfaceType,
   GraphQLObjectType,
   GraphQLUnionType,
+  GraphQLUnionTypeConfig,
 } from "graphql/type/definition";
 import type { InferValueFromArgs } from "./api-without-context";
 import type {
@@ -28,7 +29,6 @@ import {
   GObjectType,
   GOutputType,
   GUnionType,
-  GUnionTypeConfig,
 } from "./definition";
 
 export type __toMakeTypeScriptEmitImportsForItemsOnlyUsedInJSDoc = [
@@ -293,9 +293,22 @@ export type GraphQLSchemaAPIWithContext<Context> = {
    * });
    * ```
    */
-  union: <Source>(
-    config: GUnionTypeConfig<Source, Context>
-  ) => GUnionType<Source, Context>;
+  union: <Type extends GObjectType<any, Context>>(
+    config: Flatten<
+      Omit<
+        GraphQLUnionTypeConfig<
+          Type extends GObjectType<infer Source, Context> ? Source : never,
+          Context
+        >,
+        "types"
+      > & {
+        types: readonly Type[] | (() => readonly Type[]);
+      }
+    >
+  ) => GUnionType<
+    Type extends GObjectType<infer Source, Context> ? Source : never,
+    Context
+  >;
   /**
    * Creates a GraphQL field.
    *
@@ -596,3 +609,7 @@ export function bindGraphQLSchemaAPIToContext<
     },
   };
 }
+
+type Flatten<T> = {
+  [Key in keyof T]: T[Key];
+} & {};

test-project/index.test-d.ts

@@ -12,6 +12,7 @@ import {
   GInputType,
   GOutputType,
   GScalarType,
+  GUnionType,
 } from "@graphql-ts/schema/definition";
 
 // this isn't really right
@@ -1376,3 +1377,51 @@ const someInputFields = {
     Invariant<typeof Something>
   >();
 }
+
+{
+  const A = g.object<{
+    __typename: "A";
+  }>()({
+    name: "A",
+    fields: {
+      something: g.field({
+        type: g.nonNull(g.String),
+        resolve() {
+          return "A";
+        },
+      }),
+    },
+  });
+  const B = g.object<{
+    __typename: "B";
+  }>()({
+    name: "B",
+    fields: {
+      something: g.field({
+        type: g.nonNull(g.String),
+        resolve() {
+          return "B";
+        },
+      }),
+    },
+  });
+
+  const Something = g.union({
+    name: "Something",
+    types: [A, B],
+  });
+  assertCompatible<
+    Invariant<
+      GUnionType<
+        | {
+            __typename: "A";
+          }
+        | {
+            __typename: "B";
+          },
+        unknown
+      >
+    >,
+    Invariant<typeof Something>
+  >();
+}