Bug report

Description / Observed Behavior

When wrapping useSWRMutation in a generic helper function, the trigger return type becomes an uncallable union of TriggerWithoutArgs | TriggerWithOptionsArgs | TriggerWithArgs.

TypeScript error:

TS2349: This expression is not callable.
  Each member of the union type
  'TriggerWithoutArgs<Data, any, Key, Args> | TriggerWithOptionsArgs<Data, any, Key, Args> | TriggerWithArgs<...>'
  has signatures, but none of those signatures are compatible with each other.

The conditional type that determines which trigger interface to use:

trigger: [ExtraArg] extends [never]
  ? TriggerWithoutArgs<...>
  : IsUndefinedIncluded<ExtraArg> extends true
    ? TriggerWithOptionsArgs<...>
    : TriggerWithArgs<...>

cannot be evaluated when ExtraArg is a generic type parameter (not yet resolved). TypeScript defers the conditional type rather than resolving it to a single branch. The deferred type is opaque and uncallable because TS cannot determine which of the three incompatible call signatures applies.

Note: the [ExtraArg] extends [never] wrapping correctly prevents union distribution — this is a deferred conditional type issue, not a distributive conditional type issue.

Expected Behavior

trigger should be callable when useSWRMutation is used inside a generic wrapper function.