psalm/docs/running_psalm/issues/PossiblyInvalidPropertyAssignment.md

PossiblyInvalidPropertyAssignment

Emitted when trying to assign a property on a value that may not be an object or may be an object that doesn’t have the desired property.

class A {
    /** @var ?string */
    public $bar;
}

/** @return A|int */
function foo() {
    return rand(0, 1) ? new A : 5;
}

$a = foo();
$a->bar = "5";