Fix deeply nested array offset tracking losing required keys

- Added recursive handling in ArrayType::setExistingOffsetValueType() for
  non-constant array item types, so nested arrays at any depth correctly
  propagate key optionality updates
- New regression test in tests/PHPStan/Analyser/nsrt/bug-13637.php
- Updated bug-7903 expected error count (39→36) as fix resolves 3 false positives
- Root cause: when itemType was a non-constant array (e.g. array<int, array{...}>),
  setExistingOffsetValueType fell through to TypeCombinator::union which re-merged
  the old optional keys with the updated required keys

Closes phpstan/phpstan#13637

Author: github-actions[bot]

src/Type/ArrayType.php

@@ -410,6 +410,23 @@ public function setExistingOffsetValueType(Type $offsetType, Type $valueType): T
 			}
 		}
 
+		if (
+			!$this->itemType->isConstantArray()->yes()
+			&& $this->itemType->isArray()->yes()
+			&& $valueType->isArray()->yes()
+		) {
+			$newItemType = $this->itemType->setExistingOffsetValueType(
+				$valueType->getIterableKeyType(),
+				$valueType->getIterableValueType(),
+			);
+			if (!$newItemType->equals($this->itemType)) {
+				return new self(
+					$this->keyType,
+					$newItemType,
+				);
+			}
+		}
+
 		return new self(
 			$this->keyType,
 			TypeCombinator::union($this->itemType, $valueType),

tests/PHPStan/Analyser/AnalyserIntegrationTest.php

@@ -998,7 +998,7 @@ public function testBug7581(): void
 	public function testBug7903(): void
 	{
 		$errors = $this->runAnalyse(__DIR__ . '/data/bug-7903.php');
-		$this->assertCount(39, $errors);
+		$this->assertCount(36, $errors);
 	}
 
 	public function testBug7901(): void

tests/PHPStan/Analyser/nsrt/bug-13637.php

@@ -0,0 +1,45 @@
+<?php declare(strict_types = 1);
+
+namespace Bug13637;
+
+use function PHPStan\Testing\assertType;
+
+/**
+* @return array<int, array<int, array<int, array{abc: int, def: int, ghi: int}>>>
+*/
+function DoesNotWork() : array {
+	$final = [];
+
+	for ($i = 0; $i < 5; $i++) {
+		$j = $i * 2;
+		$k = $j +1;
+		$l = $i * 3;
+		$final[$i][$j][$k]['abc'] = $i;
+		$final[$i][$j][$k]['def'] = $i;
+		$final[$i][$j][$k]['ghi'] = $i;
+
+		assertType("array{abc: int<0, 4>, def: int<0, 4>, ghi: int<0, 4>}", $final[$i][$j][$k]);
+	}
+
+	return $final;
+}
+
+/**
+* @return array<int, array<int, array{abc: int, def: int, ghi: int}>>
+*/
+function thisWorks() : array {
+	$final = [];
+
+	for ($i = 0; $i < 5; $i++) {
+		$j = $i * 2;
+		$k = $j +1;
+		$l = $i * 3;
+		$final[$i][$j]['abc'] = $i;
+		$final[$i][$j]['def'] = $i;
+		$final[$i][$j]['ghi'] = $i;
+
+		assertType("array{abc: int<0, 4>, def: int<0, 4>, ghi: int<0, 4>}", $final[$i][$j]);
+	}
+
+	return $final;
+}