Improve ConstantArrayType::sliceArray() with non constant integer args

Author: herndlm

Diff for: src/Type/Constant/ConstantArrayType.php

@@ -926,8 +926,23 @@ public function sliceArray(Type $offsetType, Type $lengthType, TrinaryLogic $pre
 			return $this;
 		}
 
-		$offset = $offsetType instanceof ConstantIntegerType ? $offsetType->getValue() : 0;
-		$length = $lengthType instanceof ConstantIntegerType ? $lengthType->getValue() : $keyTypesCount;
+		$offset = $offsetType instanceof ConstantIntegerType ? $offsetType->getValue() : null;
+
+		if ($lengthType instanceof ConstantIntegerType) {
+			$length = $lengthType->getValue();
+		} elseif ($lengthType->isNull()->yes()) {
+			$length = $keyTypesCount;
+		} else {
+			$length = null;
+		}
+
+		if ($offset === null || $length === null) {
+			$builder = ConstantArrayTypeBuilder::createFromConstantArray($this);
+			$builder->degradeToGeneralArray();
+
+			return $builder->getArray()
+				->sliceArray($offsetType, $lengthType, $preserveKeys);
+		}
 
 		if ($length < 0) {
 			// Negative lengths prevent access to the most right n elements

Diff for: tests/PHPStan/Analyser/nsrt/array-slice.php

@@ -43,6 +43,7 @@ public function constantArrays(array $arr): void
 		/** @var array{17: 'foo', b: 'bar', 19: 'baz'} $arr */
 		assertType('array{b: \'bar\', 0: \'baz\'}', array_slice($arr, 1, 2));
 		assertType('array{b: \'bar\', 19: \'baz\'}', array_slice($arr, 1, 2, true));
+		assertType('array<17|19|\'b\', \'bar\'|\'baz\'|\'foo\'>', array_slice($arr, rand(0, 1) ? 0 : 1, rand(0, 1) ? 0 : 1));
 
 		/** @var array{17: 'foo', 19: 'bar', 21: 'baz'}|array{foo: 17, bar: 19, baz: 21} $arr */
 		assertType('array{\'bar\', \'baz\'}|array{bar: 19, baz: 21}', array_slice($arr, 1, 2));