Spread in an array literal, and the string keys it refuses

Merging arrays was array_merge, which is a function call in the middle of what is otherwise a literal, and the argument order is not obvious to anyone reading it quickly.

$all = [...$defaults, ...$overrides, 'extra' => 1];

// but only integer keys, in 7.4:
$a = ['x' => 1];
$b = [...$a];
// Fatal error: Cannot unpack array with string keys

// so for associative arrays, still:
$merged = array_merge($defaults, $overrides);

String keys are supported from 8.1, which makes this less useful than it first appears — most PHP arrays being merged are associative. Where it does apply, it accepts any Traversable, so a generator can be unpacked into a literal without iterator_to_array. Integer keys are renumbered rather than preserved, exactly as array_merge does, which is the behaviour to check before swapping one for the other.