Arrow functions - allow more precise return type even when native return type is present

Author: ondrejmirtes

src/Analyser/MutatingScope.php

@@ -80,6 +80,7 @@
 use PHPStan\Type\ThisType;
 use PHPStan\Type\Type;
 use PHPStan\Type\TypeCombinator;
+use PHPStan\Type\TypehintHelper;
 use PHPStan\Type\TypeTraverser;
 use PHPStan\Type\TypeUtils;
 use PHPStan\Type\TypeWithClassName;
@@ -1306,8 +1307,11 @@ private function resolveType(Expr $node): Type
 				);
 			}
 
-			if ($node->returnType === null && $node instanceof Expr\ArrowFunction) {
+			if ($node instanceof Expr\ArrowFunction) {
 				$returnType = $this->getType($node->expr);
+				if ($node->returnType !== null) {
+					$returnType = TypehintHelper::decideType($this->getFunctionType($node->returnType, false, false), $returnType);
+				}
 			} else {
 				$returnType = $this->getFunctionType($node->returnType, $node->returnType === null, false);
 			}

tests/PHPStan/Analyser/NodeScopeResolverTest.php

@@ -10915,13 +10915,17 @@ public function dataArrowFunctions(): array
 	{
 		return [
 			[
-				'Closure(string): int',
+				'Closure(string): 1',
 				'$x',
 			],
 			[
-				'int',
+				'1',
 				'$x()',
 			],
+			[
+				'array(\'a\' => 1, \'b\' => 2)',
+				'$y()',
+			],
 		];
 	}
 

tests/PHPStan/Analyser/data/arrow-functions.php

@@ -8,6 +8,7 @@ class Foo
 	public function doFoo()
 	{
 		$x = fn(string $str): int => 1;
+		$y = fn(): array => ['a' => 1, 'b' => 2];
 		die;
 	}