fix: ensure variance of types matches how values are used (#2786)

Co-authored-by: Ivan Goncharov <ivan.goncharov.ua@gmail.com>
Co-authored-by: Arda TANRIKULU <ardatanrikulu@gmail.com>
Co-authored-by: Arda Tanrıkulu <20847995+ardatan@users.noreply.github.com>

integrationTests/ts/index.ts

@@ -95,3 +95,44 @@ const { data } = wrappedExecute(typedQueryDocument);
 if (data != null) {
   const typedData: ResponseData = data;
 }
+
+declare function runQueryA(
+  q: TypedQueryDocumentNode<{ output: string }, { input: string | null }>,
+): void;
+
+// valid
+declare const optionalInputRequiredOutput: TypedQueryDocumentNode<
+  { output: string },
+  { input: string | null }
+>;
+runQueryA(optionalInputRequiredOutput);
+
+declare function runQueryB(
+  q: TypedQueryDocumentNode<{ output: string | null }, { input: string }>,
+): void;
+
+// still valid: We still accept {output: string} as a valid result.
+// We're now passing in {input: string} which is still assignable to {input: string | null}
+runQueryB(optionalInputRequiredOutput);
+
+// valid: we now accept {output: null} as a valid Result
+declare const optionalInputOptionalOutput: TypedQueryDocumentNode<
+  { output: string | null },
+  { input: string | null }
+>;
+runQueryB(optionalInputOptionalOutput);
+
+// valid: we now only pass {input: string} to the query
+declare const requiredInputRequiredOutput: TypedQueryDocumentNode<
+  { output: string },
+  { input: string }
+>;
+runQueryB(requiredInputRequiredOutput);
+
+// valid: we now accept {output: null} as a valid Result AND
+//        we now only pass {input: string} to the query
+declare const requiredInputOptionalOutput: TypedQueryDocumentNode<
+  { output: null },
+  { input: string }
+>;
+runQueryB(requiredInputOptionalOutput);

src/utilities/typedQueryDocumentNode.d.ts

@@ -9,6 +9,12 @@ export interface TypedQueryDocumentNode<
 > extends DocumentNode {
   readonly definitions: ReadonlyArray<ExecutableDefinitionNode>;
   // FIXME: remove once TS implements proper way to enforce nominal typing
-  readonly __enforceStructuralTypingOnResponseDataType?: TResponseData;
-  readonly __enforceStructuralTypingOnRequestVariablesType?: TRequestVariables;
+  /**
+   * 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
+   */
+  __ensureTypesOfVariablesAndResultMatching?: (
+    variables: TRequestVariables,
+  ) => TResponseData;
 }