Fix phpstan/phpstan#7280: array_reduce could infer the precise type of the $initial argument within the callable and in the return type

src/Type/Php/ArrayReduceCallbackClosureTypeExtension.php

@@ -0,0 +1,55 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\Type\Php;
+
+use PhpParser\Node\Expr\FuncCall;
+use PHPStan\Analyser\Scope;
+use PHPStan\DependencyInjection\AutowiredService;
+use PHPStan\Reflection\FunctionReflection;
+use PHPStan\Reflection\Native\NativeParameterReflection;
+use PHPStan\Reflection\ParameterReflection;
+use PHPStan\Type\ClosureType;
+use PHPStan\Type\FunctionParameterClosureTypeExtension;
+use PHPStan\Type\GeneralizePrecision;
+use PHPStan\Type\MixedType;
+use PHPStan\Type\NullType;
+use PHPStan\Type\Type;
+
+#[AutowiredService]
+final class ArrayReduceCallbackClosureTypeExtension implements FunctionParameterClosureTypeExtension
+{
+
+	public function isFunctionSupported(FunctionReflection $functionReflection, ParameterReflection $parameter): bool
+	{
+		return $functionReflection->getName() === 'array_reduce' && $parameter->getName() === 'callback';
+	}
+
+	public function getTypeFromFunctionCall(FunctionReflection $functionReflection, FuncCall $functionCall, ParameterReflection $parameter, Scope $scope): ?Type
+	{
+		$args = $functionCall->getArgs();
+
+		if (!isset($args[0])) {
+			return null;
+		}
+
+		$arrayType = $scope->getType($args[0]->value);
+		$valueType = $arrayType->getIterableValueType();
+
+		if (isset($args[2])) {
+			$initialType = $scope->getType($args[2]->value);
+		} else {
+			$initialType = new NullType();
+		}
+
+		$carryType = $initialType->generalize(GeneralizePrecision::templateArgument());
+
+		return new ClosureType(
+			[
+				new NativeParameterReflection('carry', false, $carryType, $parameter->passedByReference(), false, null),
+				new NativeParameterReflection('value', false, $valueType, $parameter->passedByReference(), false, null),
+			],
+			new MixedType(),
+		);
+	}
+
+}

tests/PHPStan/Analyser/nsrt/bug-7280.php

@@ -0,0 +1,59 @@
+<?php // lint >= 8.0
+
+declare(strict_types = 1);
+
+namespace Bug7280;
+
+use function PHPStan\Testing\assertType;
+
+// Test 1: Precise carry type from initial argument with constant array shape
+$result1 = array_reduce(
+	['test1', 'test2'],
+	static function (array $carry, string $value): array {
+		assertType("array{starts: array{}, ends: array{}}", $carry);
+		$carry['starts'][] = $value;
+		$carry['ends'][] = $value;
+
+		return $carry;
+	},
+	initial: ['starts' => [], 'ends' => []],
+);
+
+// Test 2: Arrow function with precise carry type
+$result2 = array_reduce(
+	[1, 2, 3],
+	static fn (int $carry, int $value): int => $carry + $value,
+	0,
+);
+assertType('int', $result2);
+
+// Test 3: Carry type with no initial (defaults to null)
+$result3 = array_reduce(
+	[1, 2, 3],
+	static function (?int $carry, int $value): int {
+		assertType('null', $carry);
+		return ($carry ?? 0) + $value;
+	},
+);
+
+// Test 4: Initial value type narrows carry - literal string initial
+$result4 = array_reduce(
+	['a', 'b', 'c'],
+	static function (string $carry, string $value): string {
+		assertType('string', $carry);
+		return $carry . $value;
+	},
+	'',
+);
+assertType('string', $result4);
+
+// Test 5: Carry type with literal int initial
+$result5 = array_reduce(
+	[1, 2, 3],
+	static function (int $carry, int $value): int {
+		assertType('int', $carry);
+		return $carry + $value;
+	},
+	0,
+);
+assertType('int', $result5);