Add DocumentTypeDecoration interface with __apiType key

.changeset/lovely-ghosts-suffer.md

@@ -0,0 +1,5 @@
+---
+"@graphql-typed-document-node/core": minor
+---
+
+Export `DocumentTypeDecoration` interface with `__apiType` key so that it can be used to extend types other than `DocumentNode`

packages/core/src/index.ts

@@ -1,37 +1,40 @@
 import type { DocumentNode } from "graphql";
 
-export interface TypedDocumentNode<
-  Result = { [key: string]: any },
-  Variables = { [key: string]: any }
-> extends DocumentNode {
+export interface DocumentTypeDecoration<TResult, TVariables> {
   /**
    * This type is used to ensure that the variables you pass in to the query are assignable to Variables
    * and that the Result is assignable to whatever you pass your result to. The method is never actually
    * implemented, but the type is valid because we list it as optional
    */
-  __apiType?: (variables: Variables) => Result;
+  __apiType?: (variables: TVariables) => TResult;
 }
 
+export interface TypedDocumentNode<
+  TResult = { [key: string]: any },
+  TVariables = { [key: string]: any }
+> extends DocumentNode,
+    DocumentTypeDecoration<TResult, TVariables> {}
+
 /**
- * Helper for extracting a TypeScript type for operation result from a TypedDocumentNode.
+ * Helper for extracting a TypeScript type for operation result from a TypedDocumentNode and TypedDocumentString.
  * @example
  * const myQuery = { ... }; // TypedDocumentNode<R, V>
  * type ResultType = ResultOf<typeof myQuery>; // Now it's R
  */
-export type ResultOf<T> = T extends TypedDocumentNode<
+export type ResultOf<T> = T extends DocumentTypeDecoration<
   infer ResultType,
   infer VariablesType
 >
   ? ResultType
   : never;
 
 /**
- * Helper for extracting a TypeScript type for operation variables from a TypedDocumentNode.
+ * Helper for extracting a TypeScript type for operation variables from a TypedDocumentNode and TypedDocumentString.
  * @example
  * const myQuery = { ... }; // TypedDocumentNode<R, V>
  * type VariablesType = VariablesOf<typeof myQuery>; // Now it's V
  */
-export type VariablesOf<T> = T extends TypedDocumentNode<
+export type VariablesOf<T> = T extends DocumentTypeDecoration<
   infer ResultType,
   infer VariablesType
 >