Add unit tests for setExistingOffsetValueType with maybe-array item type

Tests that when the item type is a union of array and non-array (isArray()
returns maybe), the recursive elseif branch is correctly skipped and the
fallback TypeCombinator::union path is used instead. Also tests the definite
array case where the recursive path produces correct merged results.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: phpstan-bot

tests/PHPStan/Type/ArrayTypeTest.php

@@ -301,6 +301,86 @@ public function testHasOffsetValueType(
 		);
 	}
 
+	public static function dataSetExistingOffsetValueType(): array
+	{
+		return [
+			'maybe array item type falls through to union' => [
+				// array<int, int|array<int, array<int, array{abc: int}>>>
+				new ArrayType(
+					new IntegerType(),
+					new UnionType([
+						new IntegerType(),
+						new ArrayType(
+							new IntegerType(),
+							new ArrayType(
+								new IntegerType(),
+								new ConstantArrayType(
+									[new ConstantStringType('abc')],
+									[new IntegerType()],
+								),
+							),
+						),
+					]),
+				),
+				new IntegerType(),
+				new ArrayType(
+					new IntegerType(),
+					new ArrayType(
+						new IntegerType(),
+						new ConstantArrayType(
+							[new ConstantStringType('def')],
+							[new IntegerType()],
+						),
+					),
+				),
+				'array<int, array<int, array<int, array{abc: int}|array{def: int}>>|int>',
+			],
+			'definite array item type uses recursive path' => [
+				// array<int, array<int, array<int, array{abc: int}>>>
+				new ArrayType(
+					new IntegerType(),
+					new ArrayType(
+						new IntegerType(),
+						new ArrayType(
+							new IntegerType(),
+							new ConstantArrayType(
+								[new ConstantStringType('abc')],
+								[new IntegerType()],
+							),
+						),
+					),
+				),
+				new IntegerType(),
+				new ArrayType(
+					new IntegerType(),
+					new ArrayType(
+						new IntegerType(),
+						new ConstantArrayType(
+							[new ConstantStringType('def')],
+							[new IntegerType()],
+						),
+					),
+				),
+				'array<int, array<int, array<int, array{abc: int, def: int}>>>',
+			],
+		];
+	}
+
+	#[DataProvider('dataSetExistingOffsetValueType')]
+	public function testSetExistingOffsetValueType(
+		ArrayType $type,
+		Type $offsetType,
+		Type $valueType,
+		string $expectedType,
+	): void
+	{
+		$actualResult = $type->setExistingOffsetValueType($offsetType, $valueType);
+		$this->assertSame(
+			$expectedType,
+			$actualResult->describe(VerbosityLevel::precise()),
+		);
+	}
+
 	public static function dataSpliceArray(): array
 	{
 		return [