Skip implode() constant-fold for unsealed constant arrays

`inferConstantType` built the exact result string from the explicit
`getValueTypes()` only — `implode(',', array{'a', 'b', ...<int,
string>})` inferred `'a,b'` even though the unsealed extras can append
further segments. That's an unsound constant fold.

Bail out of the constant fold when the input `isUnsealed()->yes()` and
fall through to the accessory-based result, which only keeps what's
provable (e.g. `non-falsy-string` when the separator is non-falsy and
the explicit prefix guarantees at least one separator).

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

Author: ondrejmirtes

src/Type/Php/ImplodeFunctionReturnTypeExtension.php

@@ -113,6 +113,13 @@ private function implode(Type $arrayType, Type $separatorType): Type
 
 	private function inferConstantType(ConstantArrayType $arrayType, ConstantStringType $separatorType, bool $isNonEmpty): ?Type
 	{
+		// Unsealed extras can append further segments the constant fold
+		// can't see, so the exact string result would be unsound. Fall
+		// back to the accessory-based result.
+		if ($arrayType->isUnsealed()->yes()) {
+			return null;
+		}
+
 		$sep = $separatorType->getValue();
 		$valueTypes = $arrayType->getValueTypes();
 		$limit = InitializerExprTypeResolver::CALCULATE_SCALARS_LIMIT;

tests/PHPStan/Analyser/nsrt/implode.php

@@ -68,4 +68,21 @@ public function constArrays6($constArr) {
 	public function constArrays7($constArr) {
 		assertType("'1a'|'1b'|'1c'|'2a'|'2b'|'2c'|'a'|'b'|'c'", implode('', $constArr));
 	}
+
+	/** @param array{'a', 'b', ...<int, string>} $unsealed */
+	public function unsealedConstArr($unsealed) {
+		// The unsealed `<int, string>` extras can append more segments, so
+		// the exact constant fold `'a,b'` is unsound. The result keeps only
+		// what's guaranteed: with a non-falsy separator and at least two
+		// explicit elements, the output always contains a comma.
+		assertType('non-falsy-string', implode(',', $unsealed));
+	}
+
+	/** @param array{'a', 'b', ...<int, string>} $unsealed */
+	public function unsealedConstArrEmptySeparator($unsealed) {
+		// Empty separator + a possibly-empty unsealed value type leaves no
+		// accessory PHPStan can prove, so the result widens to `string`
+		// (still sound — no bogus constant fold).
+		assertType('string', implode('', $unsealed));
+	}
 }