fix(query-core): fix context type error in onSuccess (#6355)

* fix(query-core): fix context type error in `onSuccess`

* docs(query-core): update `useMutation` documentations

* fix(query-core): the `context` should not be inferred as `undefined` when `TContext` is defined

---------

Co-authored-by: Dominik Dorfmeister <office@dorfmeister.cc>

docs/react/reference/useMutation.md

@@ -63,7 +63,7 @@ mutate(variables, {
   - This function will fire before the mutation function is fired and is passed the same variables the mutation function would receive
   - Useful to perform optimistic updates to a resource in hopes that the mutation succeeds
   - The value returned from this function will be passed to both the `onError` and `onSettled` functions in the event of a mutation failure and can be useful for rolling back optimistic updates.
-- `onSuccess: (data: TData, variables: TVariables, context?: TContext) => Promise<unknown> | unknown`
+- `onSuccess: (data: TData, variables: TVariables, context: TContext) => Promise<unknown> | unknown`
   - Optional
   - This function will fire when the mutation is successful and will be passed the mutation's result.
   - If a promise is returned, it will be awaited and resolved before proceeding

packages/query-core/src/mutation.ts

@@ -218,7 +218,7 @@ export class Mutation<
         this as Mutation<unknown, unknown, unknown, unknown>,
       )
 
-      await this.options.onSuccess?.(data, variables, this.state.context)
+      await this.options.onSuccess?.(data, variables, this.state.context!)
 
       // Notify cache callback
       await this.#mutationCache.config.onSettled?.(

packages/query-core/src/mutationObserver.ts

@@ -144,29 +144,19 @@ export class MutationObserver<
     notifyManager.batch(() => {
       // First trigger the mutate callbacks
       if (this.#mutateOptions && this.hasListeners()) {
+        const variables = this.#currentResult.variables!
+        const context = this.#currentResult.context
+
         if (action?.type === 'success') {
-          this.#mutateOptions.onSuccess?.(
-            action.data,
-            this.#currentResult.variables!,
-            this.#currentResult.context!,
-          )
-          this.#mutateOptions.onSettled?.(
-            action.data,
-            null,
-            this.#currentResult.variables!,
-            this.#currentResult.context,
-          )
+          this.#mutateOptions.onSuccess?.(action.data, variables, context!)
+          this.#mutateOptions.onSettled?.(action.data, null, variables, context)
         } else if (action?.type === 'error') {
-          this.#mutateOptions.onError?.(
-            action.error,
-            this.#currentResult.variables!,
-            this.#currentResult.context,
-          )
+          this.#mutateOptions.onError?.(action.error, variables, context)
           this.#mutateOptions.onSettled?.(
             undefined,
             action.error,
-            this.#currentResult.variables!,
-            this.#currentResult.context,
+            variables,
+            context,
           )
         }
       }

packages/query-core/src/types.ts

@@ -671,7 +671,7 @@ export interface MutationOptions<
   onSuccess?: (
     data: TData,
     variables: TVariables,
-    context: TContext | undefined,
+    context: TContext,
   ) => Promise<unknown> | unknown
   onError?: (
     error: TError,