Incorrectly asserting non-empty-list after combining with empty list

[robchett]

Fixes #9594

[robchett]

Tracked this down to here:

psalm/src/Psalm/Internal/Type/TypeCombiner.php

Lines 1364 to 1448 in 5370492

	if (!$combination->array_type_params || $combination->array_type_params[1]->isNever()) {
	if (!$overwrite_empty_array
	&& $combination->array_type_params
	) {
	foreach ($combination->objectlike_entries as &$objectlike_entry) {
	$objectlike_entry = $objectlike_entry->setPossiblyUndefined(true);
	}
	unset($objectlike_entry);
	}
	if ($combination->objectlike_value_type
	&& $combination->objectlike_value_type->isMixed()
	&& $combination->array_type_params
	&& !$combination->array_type_params[1]->isNever()
	) {
	$combination->objectlike_entries = array_filter(
	$combination->objectlike_entries,
	static fn(Union $type): bool => !$type->possibly_undefined,
	);
	}
	if ($combination->objectlike_entries) {
	$fallback_key_type = null;
	if ($combination->objectlike_key_type) {
	$fallback_key_type = $combination->objectlike_key_type;
	} elseif ($combination->array_type_params
	&& $combination->array_type_params[0]->isArrayKey()
	) {
	$fallback_key_type = $combination->array_type_params[0];
	}
	$fallback_value_type = null;
	if ($combination->objectlike_value_type) {
	$fallback_value_type = $combination->objectlike_value_type;
	} elseif ($combination->array_type_params
	&& $combination->array_type_params[1]->isMixed()
	) {
	$fallback_value_type = $combination->array_type_params[1];
	}
	$sealed = $combination->objectlike_sealed && (
	!$combination->array_type_params
	|| (isset($combination->array_type_params[1])
	&& $combination->array_type_params[1]->isNever()
	)
	);
	if ($combination->all_arrays_callable) {
	$objectlike = new TCallableKeyedArray(
	$combination->objectlike_entries,
	null,
	$sealed || $fallback_key_type === null || $fallback_value_type === null
	? null
	: [$fallback_key_type, $fallback_value_type],
	(bool)$combination->all_arrays_lists,
	$from_docblock,
	);
	} else {
	$objectlike = new TKeyedArray(
	$combination->objectlike_entries,
	null,
	$sealed || $fallback_key_type === null || $fallback_value_type === null
	? null
	: [$fallback_key_type, $fallback_value_type],
	(bool)$combination->all_arrays_lists,
	$from_docblock,
	);
	}
	$new_types[] = $objectlike;
	} else {
	$key_type = $combination->objectlike_key_type ?? Type::getArrayKey();
	$value_type = $combination->objectlike_value_type ?? Type::getMixed();
	if ($combination->array_always_filled) {
	$array_type = new TNonEmptyArray([$key_type, $value_type]);
	} else {
	$array_type = new TArray([$key_type, $value_type]);
	}
	$new_types[] = $array_type->setFromDocblock($from_docblock);
	}
	// if we're merging an empty array with an object-like, clobber empty array
	$combination->array_type_params = [];
	}

Need to get my head around the interactions between $overwrite_empty_array, $objectlike_entry->setPossiblyUndefined and $combination->array_type_params = [];

Current best effort is switching $overwrite_empty_array at the callsite

psalm/src/Psalm/Internal/Analyzer/Statements/Expression/Assignment/ArrayAssignmentAnalyzer.php

Lines 692 to 704 in 5370492

	if (!$new_child_type) {
	if ($templated_assignment) {
	$new_child_type = $root_type;
	} else {
	$new_child_type = Type::combineUnionTypes(
	$root_type,
	$array_assignment_type,
	$codebase,
	true,
	true,
	);
	}
	}

but it needs to be smarter than that as then we loose [] + T' = non-empty-list<T>

[orklah]

Yeah, this breaks things pretty dramatically :D

Not sure if I'll be able to find time to dig into it to help in the foreseeable future

[robchett]

Finally found the time to crack this, nested array type combiners were inheriting overwrite_empty_array so I've made it only apply to root level array type modifications and not any value types.

[orklah]

Thanks!