keep list for $list[1 + $index] assignments

Author: staabm

src/Analyser/NodeScopeResolver.php

@@ -5482,8 +5482,26 @@ private function processAssignVar(
 			}
 
 			if ($varType->isArray()->yes() || !(new ObjectType(ArrayAccess::class))->isSuperTypeOf($varType)->yes()) {
-				if ($varType->isList()->yes() && $scope->hasExpressionType($originalVar)->yes()) {
-					$valueToWrite = TypeCombinator::intersect($valueToWrite, new AccessoryArrayListType());
+				if ($varType->isList()->yes()) {
+					if ($scope->hasExpressionType($originalVar)->yes()) { // keep list for $list[$index] assignments
+						$valueToWrite = TypeCombinator::intersect($valueToWrite, new AccessoryArrayListType());
+					} elseif ($originalVar->dim instanceof BinaryOp\Plus) {
+						if ( // keep list for $list[$index + 1] assignments
+							$originalVar->dim->right instanceof Variable
+							&& $originalVar->dim->left instanceof Node\Scalar\Int_
+							&& $originalVar->dim->left->value === 1
+							&& $scope->hasExpressionType(new ArrayDimFetch($originalVar->var, $originalVar->dim->right))->yes()
+						) {
+							$valueToWrite = TypeCombinator::intersect($valueToWrite, new AccessoryArrayListType());
+						} elseif ( // keep list for $list[1 + $index] assignments
+							$originalVar->dim->left instanceof Variable
+							&& $originalVar->dim->right instanceof Node\Scalar\Int_
+							&& $originalVar->dim->right->value === 1
+							&& $scope->hasExpressionType(new ArrayDimFetch($originalVar->var, $originalVar->dim->left))->yes()
+						) {
+							$valueToWrite = TypeCombinator::intersect($valueToWrite, new AccessoryArrayListType());
+						}
+					}
 				}
 
 				if ($var instanceof Variable && is_string($var->name)) {

tests/PHPStan/Analyser/nsrt/bug-12274.php

@@ -43,6 +43,54 @@ function testKeepListAfterIssetIndex(array $list, int $i): void
 		assertType('list<int>', $list);
 		$list[$i] = 21;
 		assertType('non-empty-list<int>', $list);
+		$list[$i+1] = 21;
+		assertType('non-empty-list<int>', $list);
+	}
+	assertType('list<int>', $list);
+}
+
+/** @param list<int> $list */
+function testKeepListAfterIssetIndexPlusOne(array $list, int $i): void
+{
+	if (isset($list[$i])) {
+		assertType('list<int>', $list);
+		$list[$i+1] = 21;
+		assertType('non-empty-list<int>', $list);
+	}
+	assertType('list<int>', $list);
+}
+
+/** @param list<int> $list */
+function testKeepListAfterIssetIndexOnePlus(array $list, int $i): void
+{
+	if (isset($list[$i])) {
+		assertType('list<int>', $list);
+		$list[1+$i] = 21;
+		assertType('non-empty-list<int>', $list);
 	}
 	assertType('list<int>', $list);
 }
+
+/** @param list<int> $list */
+function testShouldLooseListbyAst(array $list, int $i): void
+{
+	if (isset($list[$i])) {
+		$i++;
+
+		assertType('list<int>', $list);
+		$list[1+$i] = 21;
+		assertType('non-empty-array<int, int>', $list);
+	}
+	assertType('array<int, int>', $list);
+}
+
+/** @param list<int> $list */
+function testShouldLooseListbyAst2(array $list, int $i): void
+{
+	if (isset($list[$i])) {
+		assertType('list<int>', $list);
+		$list[2+$i] = 21;
+		assertType('non-empty-array<int, int>', $list);
+	}
+	assertType('array<int, int>', $list);
+}