Fix plus operator with constant arrays (#4266)

Author: SplotyCode

src/Reflection/InitializerExprTypeResolver.php

@@ -34,6 +34,7 @@
 use PHPStan\Type\Accessory\AccessoryNonFalsyStringType;
 use PHPStan\Type\Accessory\AccessoryNumericStringType;
 use PHPStan\Type\Accessory\AccessoryUppercaseStringType;
+use PHPStan\Type\Accessory\HasOffsetValueType;
 use PHPStan\Type\Accessory\NonEmptyArrayType;
 use PHPStan\Type\ArrayType;
 use PHPStan\Type\BenevolentUnionType;
@@ -1078,6 +1079,16 @@ public function getPlusType(Expr $left, Expr $right, callable $getTypeCallback):
 				TypeCombinator::union($leftType->getIterableValueType(), $rightType->getIterableValueType()),
 			);
 
+			foreach ($leftType->getConstantArrays() as $type) {
+				foreach ($type->getKeyTypes() as $i => $offsetType) {
+					if ($type->isOptionalKey($i)) {
+						continue;
+					}
+					$valueType = $type->getValueTypes()[$i];
+					$arrayType = TypeCombinator::intersect($arrayType, new HasOffsetValueType($offsetType, $valueType));
+				}
+			}
+
 			if ($leftType->isIterableAtLeastOnce()->yes() || $rightType->isIterableAtLeastOnce()->yes()) {
 				$arrayType = TypeCombinator::intersect($arrayType, new NonEmptyArrayType());
 			}

tests/PHPStan/Analyser/nsrt/bug-11912.php

@@ -0,0 +1,14 @@
+<?php
+
+namespace Bug11912;
+
+/**
+ * @param array<string, mixed> $results
+ * @param list<string> $names
+ */
+function appendResults(array $results, array $names): bool {
+	// Make sure 'names' comes first in array
+	$results = ['names' => $names] + $results;
+	\PHPStan\Testing\assertType("list<string>", $results['names']);
+	return true;
+}