fix: Fix readFragment() not deriving complex return types

.changeset/neat-numbers-shake.md

@@ -0,0 +1,5 @@
+---
+'gql.tada': patch
+---
+
+Fix `readFragment()` not inferring the types of complex fragments, i.e. fragments that derive with a union type.

src/__tests__/api.test-d.ts

@@ -1,6 +1,11 @@
 import { describe, it, expectTypeOf } from 'vitest';
-import type { mirrorFragmentTypeRec } from '../api';
 
+import type { simpleSchema } from './fixtures/simpleSchema';
+import type { parseDocument } from '../parser';
+import type { ResultOf, FragmentOf, mirrorFragmentTypeRec, getDocumentNode } from '../api';
+import { readFragment } from '../api';
+
+type schema = simpleSchema;
 type value = { __value: true };
 type data = { __data: true };
 
@@ -24,4 +29,70 @@ describe('mirrorFragmentTypeRec', () => {
     >();
     expectTypeOf<mirrorFragmentTypeRec<readonly value[], data>>().toEqualTypeOf<readonly data[]>();
   });
+
+  it('mirrors complex types', () => {
+    type complex = { a: true } | { b: true };
+    type actual = mirrorFragmentTypeRec<value, complex>;
+    expectTypeOf<actual>().toEqualTypeOf<complex>();
+  });
+});
+
+describe('readFragment', () => {
+  it('unmasks regular fragments', () => {
+    type fragment = parseDocument<`
+      fragment Fields on Todo {
+        id
+      }
+    `>;
+
+    type document = getDocumentNode<fragment, schema>;
+    const result = readFragment({} as document, {} as FragmentOf<document>);
+    expectTypeOf<typeof result>().toEqualTypeOf<ResultOf<document>>();
+  });
+
+  it('unmasks fragments with optional spreads', () => {
+    type fragment = parseDocument<`
+      fragment Fields on Todo {
+        ... @defer {
+          id
+        }
+      }
+    `>;
+
+    type document = getDocumentNode<fragment, schema>;
+    const result = readFragment({} as document, {} as FragmentOf<document>);
+    expectTypeOf<typeof result>().toEqualTypeOf<ResultOf<document>>();
+  });
+
+  it('unmasks fragments of interfaces', () => {
+    type fragment = parseDocument<`
+      fragment Fields on ITodo {
+        id
+        ... on BigTodo {
+          wallOfText
+        }
+        ... on SmallTodo {
+          maxLength
+        }
+      }
+    `>;
+
+    type document = getDocumentNode<fragment, schema>;
+    const result = readFragment({} as document, {} as FragmentOf<document>);
+    expectTypeOf<typeof result>().toEqualTypeOf<ResultOf<document>>();
+  });
+
+  it('unmasks fragments of interfaces with optional spreads', () => {
+    type fragment = parseDocument<`
+      fragment Fields on ITodo {
+        ... on ITodo @defer {
+          id
+        }
+      }
+    `>;
+
+    type document = getDocumentNode<fragment, schema>;
+    const result = readFragment({} as document, {} as FragmentOf<document>);
+    expectTypeOf<typeof result>().toEqualTypeOf<ResultOf<document>>();
+  });
 });

src/api.ts

@@ -188,7 +188,7 @@ function parse<const In extends stringLiteral<In>>(input: In): parseDocument<In>
   return _parse(input) as any;
 }
 
-type getDocumentNode<
+export type getDocumentNode<
   Document extends DocumentNodeLike,
   Introspection extends IntrospectionLikeType,
   Fragments extends { [name: string]: any } = {},
@@ -347,13 +347,9 @@ type fragmentOfTypeRec<Document extends DocumentDefDecorationLike> =
  * @see {@link readFragment} for how to read from fragment masks.
  */
 function readFragment<
-  const Document extends DocumentDefDecorationLike,
+  const Document extends DocumentDefDecorationLike & DocumentDecoration<any, any>,
   const Fragment extends fragmentOfTypeRec<Document>,
-  const Data,
->(
-  _document: DocumentDecoration<Data, any> & Document,
-  fragment: Fragment
-): fragmentOfTypeRec<Document> extends Fragment ? unknown : mirrorFragmentTypeRec<Fragment, Data> {
+>(_document: Document, fragment: Fragment): mirrorFragmentTypeRec<Fragment, ResultOf<Document>> {
   return fragment as any;
 }