Treat unsealed from-encoding arrays as multi-candidate in mb_convert_encoding()

The PHP 8 "can this return false?" check counted only the explicit
`getValueTypes()` of the from-encoding array, so a one-explicit-key
unsealed array — `mb_convert_encoding($s, 'UTF-8', array{'FOO',
...<int, string>})` — was treated as a single guaranteed encoding and
the `false` branch was dropped. The unsealed extras can supply further
candidates, making it an auto-detect list that may fail.

Extend the `count(...) > 1` gate with `|| isUnsealed()->yes()` so the
result keeps `false` as a possible outcome.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: ondrejmirtes

src/Type/Php/MbConvertEncodingFunctionReturnTypeExtension.php

@@ -71,7 +71,11 @@ public function getTypeFromFunctionCall(
 				$constantArrays = $fromEncodingArgType->getConstantArrays();
 				if (count($constantArrays) > 0) {
 					foreach ($constantArrays as $constantArray) {
-						if (count($constantArray->getValueTypes()) > 1) {
+						// Unsealed extras can add further encoding candidates
+						// on top of the explicit ones, so the list may hold
+						// 2+ entries (auto-detect → may return false) even when
+						// only one explicit value is present.
+						if (count($constantArray->getValueTypes()) > 1 || $constantArray->isUnsealed()->yes()) {
 							$returnFalseIfCannotDetectEncoding = true;
 							break;
 						}

tests/PHPStan/Analyser/nsrt/mb-convert-encoding-php8.php

@@ -8,6 +8,7 @@
  * @param list<string> $stringList
  * @param list<int> $intList
  * @param 'foo'|'bar'|array{foo: string, bar: int, baz: 'foo'}|bool $union
+ * @param array{'FOO', ...<int, string>} $unsealedEncodings
  */
 function test_mb_convert_encoding(
 	mixed $mixed,
@@ -19,6 +20,7 @@ function test_mb_convert_encoding(
 	array $intList,
 	string|array|bool $union,
 	int $int,
+	array $unsealedEncodings,
 ): void {
 	\PHPStan\Testing\assertType('array|string', mb_convert_encoding($mixed, 'UTF-8'));
 	\PHPStan\Testing\assertType('string', mb_convert_encoding($constantString, 'UTF-8'));
@@ -45,4 +47,9 @@ function test_mb_convert_encoding(
 
 	\PHPStan\Testing\assertType('string|false', mb_convert_encoding($string, 'UTF-8', 'auto'));
 	\PHPStan\Testing\assertType('string|false', mb_convert_encoding($string, 'UTF-8', ' AUTO '));
+
+	// One explicit encoding, but the unsealed extras can add more, so the
+	// from-encoding list may hold 2+ candidates → auto-detect → false is
+	// possible.
+	\PHPStan\Testing\assertType('string|false', mb_convert_encoding($string, 'UTF-8', $unsealedEncodings));
 };