Fix phpstan/phpstan#14170: list lost in iterable type after iteration

- Override getOffsetValueType() on IterableType to return the item type instead of mixed
- Skip generalOffsetAccessibleType intersection for iterable types in produceArrayDimFetchAssignValueToWrite to prevent decomposing iterable into array|ArrayAccess
- Update existing iterable test to reflect correct offset value type
- New regression test in tests/PHPStan/Analyser/nsrt/bug-14170.php

Author: staabm

src/Analyser/ExprHandler/AssignHandler.php

@@ -946,6 +946,7 @@ private function produceArrayDimFetchAssignValueToWrite(array $dimFetchStack, ar
 			if (
 				!$offsetValueType instanceof MixedType
 				&& !$offsetValueType->isArray()->yes()
+				&& !($offsetValueType->isIterable()->yes() && !$offsetValueType->isObject()->yes())
 			) {
 				if ($offsetType !== null && $offsetType->isInteger()->yes()) {
 					$offsetValueType = TypeCombinator::intersect($offsetValueType, StaticTypeFactory::intOffsetAccessibleType());

src/Type/IterableType.php

@@ -195,6 +195,15 @@ public function hasOffsetValueType(Type $offsetType): TrinaryLogic
 		return TrinaryLogic::createMaybe();
 	}
 
+	public function getOffsetValueType(Type $offsetType): Type
+	{
+		if ($this->getIterableKeyType()->isSuperTypeOf($offsetType)->no()) {
+			return new ErrorType();
+		}
+
+		return $this->itemType;
+	}
+
 	public function toNumber(): Type
 	{
 		return new ErrorType();

tests/PHPStan/Analyser/nsrt/bug-14170.php

@@ -0,0 +1,35 @@
+<?php
+
+declare(strict_types = 1);
+
+namespace Bug14170;
+
+use function PHPStan\Testing\assertType;
+
+/**
+ * @param iterable<string, list<string>> $convert
+ * @param-out iterable<string, list<string>> $convert
+ */
+function example3a(iterable &$convert): void
+{
+	foreach ($convert as &$inner) {
+		foreach ($inner as &$val) {
+			$val = strtoupper($val);
+		}
+	}
+	assertType('iterable<string, list<string>>', $convert);
+}
+
+/**
+ * @param iterable<string, list<string>> $convert
+ * @param-out iterable<string, list<string>> $convert
+ */
+function example3b(iterable &$convert): void
+{
+	foreach ($convert as $outerKey => $inner) {
+		foreach ($inner as $key => $val) {
+			$convert[$outerKey][$key] = strtoupper($val);
+		}
+	}
+	assertType('iterable<string, list<string>>', $convert);
+}

tests/PHPStan/Analyser/nsrt/iterable.php

@@ -95,7 +95,7 @@ public function doFoo(
 											assertType('mixed', $iterableWithIterableTypehint[0]);
 											assertType('mixed', $mixed);
 											assertType('iterable<Iterables\Bar>', $iterableWithConcreteTypehint);
-											assertType('mixed', $iterableWithConcreteTypehint[0]);
+											assertType('Iterables\Bar', $iterableWithConcreteTypehint[0]);
 											assertType('Iterables\Bar', $bar);
 											assertType('iterable', $this->doBar());
 											assertType('iterable<Iterables\Baz>', $this->doBaz());