fix(core): Fix legacy GCG generated Variables types

.changeset/dry-islands-double.md

@@ -0,0 +1,12 @@
+---
+'@urql/core': patch
+---
+
+Fix generated empty `Variables` type as passed to generics, that outputs a type of `{ [var: string]: never; }`.
+A legacy/unsupported version of `typescript-urql`, which wraps `urql`'s React hooks, generates
+empty variables types as the following code snippet, which is not detected:
+
+```ts
+type Exact<T extends { [key: string]: unknown }> = { [K in keyof T]: T[K] };
+type Variables = Exact<{ [key: string]: never }>;
+```

packages/core/src/types.ts

@@ -264,9 +264,15 @@ export type GraphQLRequestParams<
       : Variables extends {
           [P in keyof Variables]: Exclude<Variables[P], null | void>;
         }
-      ? {
-          variables: Variables;
+      ? Variables extends {
+          [P in keyof Variables]: never;
         }
+        ? {
+            variables?: Variables;
+          }
+        : {
+            variables: Variables;
+          }
       : {
           variables?: Variables;
         }))