Typescript setState accepts a partial state, even if the replace option is enabled

[Yonom]

The setState function of Store accepts a Partial<T> type, even if the replace option is enabled. This has caused me to accidentally ship bugs in production in the past. My suggestion would be to update the setState types to conditionally require a non-partial T if the second argument of the function is set to true.

Example:

type MyStore = { foo: string; bar: string };

const useMyStore = create<MyStore>()(() => ({ foo: "hello", bar: "world" }));

useMyStore.setState({}, true); // <- this does not produce an error, even though the new value is invalid

Suggested change in vanilla.ts

 type SetStateInternal<T> = {
   _(
     partial: T | Partial<T> | { _(state: T): T | Partial<T> }['_'],
-    replace?: boolean | undefined,
+    replace?: false | undefined,
   ): void
+  _(
+    partial: T | { _(state: T): T }['_'],
+    replace: true,
+  ): void
 }['_']

[dai-shi]

I agree it's a reasonable fix. Would you like to open a PR please?
I don't know if it causes some troubles in some edge cases.

[Yonom]

Of course, will open a PR :)

I adopted it in my (small) repository and it caught a bug I had missed.
So, this breaks existing (incorrect) code; perhaps best to only ship it with the next major release

[dai-shi]

Then, send the PR to v5 branch please.

[dai-shi]

#2580