Breaking change and Exception: Collection->groupBy() with callable in the form [$object, 'method']

[LMCom]

• Laravel Version: 7.6.2
• PHP Version: 7.3.17
• Database Driver & Version:

Description:

A breaking change was introduced with version 5.5. It is still there.
To allow multi level grouping, it is checked, if $groupBy is an array.

See
https://github.com/laravel/framework/blob/7.x/src/Illuminate/Support/Collection.php#L431

Introduced:
4fb6206#diff-1f18a78060929ba4f93e96a4d21d022dR657

If $groupBy is an array, the multi level code path is used. However, $groupBy can be a callable, and a callable can be in the form [$object, 'method']. In this case, it should be treated like other callables that are passed to $this->valueRetriever() a few lines later. This was the behaviour before this change.

This is not only a breaking change, but also a bug, because it leads to an exception in data_get() in helpers.php: "explode() expects parameter 2 to be string, object given".

Steps To Reproduce:

Call the method accordingly.

[patriziotomato]

Some of the rare reasons to hate PHP :-(
https://stackoverflow.com/questions/32459408/php-callable-method-not-as-array

[driesvints]

I sent in a PR for this: #32471