Fix return type of arrow function

Author: ondrejmirtes

src/Analyser/MutatingScope.php

@@ -1308,7 +1308,7 @@ private function resolveType(Expr $node): Type
 			}
 
 			if ($node instanceof Expr\ArrowFunction) {
-				$returnType = $this->getType($node->expr);
+				$returnType = $this->enterArrowFunctionWithoutReflection($node)->getType($node->expr);
 				if ($node->returnType !== null) {
 					$returnType = TypehintHelper::decideType($this->getFunctionType($node->returnType, false, false), $returnType);
 				}
@@ -2789,6 +2789,35 @@ public function enterAnonymousFunction(
 	}
 
 	public function enterArrowFunction(Expr\ArrowFunction $arrowFunction): self
+	{
+		$anonymousFunctionReflection = $this->getType($arrowFunction);
+		if (!$anonymousFunctionReflection instanceof ClosureType) {
+			throw new \PHPStan\ShouldNotHappenException();
+		}
+
+		$scope = $this->enterArrowFunctionWithoutReflection($arrowFunction);
+
+		return $this->scopeFactory->create(
+			$scope->context,
+			$scope->isDeclareStrictTypes(),
+			$scope->constantTypes,
+			$scope->getFunction(),
+			$scope->getNamespace(),
+			$scope->variableTypes,
+			$scope->moreSpecificTypes,
+			$scope->conditionalExpressions,
+			$scope->inClosureBindScopeClass,
+			$anonymousFunctionReflection,
+			true,
+			[],
+			[],
+			[],
+			$scope->afterExtractCall,
+			$scope->parentScope
+		);
+	}
+
+	private function enterArrowFunctionWithoutReflection(Expr\ArrowFunction $arrowFunction): self
 	{
 		$variableTypes = $this->variableTypes;
 		$mixed = new MixedType();
@@ -2813,11 +2842,6 @@ public function enterArrowFunction(Expr\ArrowFunction $arrowFunction): self
 			unset($variableTypes['this']);
 		}
 
-		$anonymousFunctionReflection = $this->getType($arrowFunction);
-		if (!$anonymousFunctionReflection instanceof ClosureType) {
-			throw new \PHPStan\ShouldNotHappenException();
-		}
-
 		$conditionalExpressions = [];
 		foreach ($this->conditionalExpressions as $conditionalExprString => $holders) {
 			$newHolders = [];
@@ -2883,7 +2907,7 @@ public function enterArrowFunction(Expr\ArrowFunction $arrowFunction): self
 			$this->moreSpecificTypes,
 			$conditionalExpressions,
 			$this->inClosureBindScopeClass,
-			$anonymousFunctionReflection,
+			null,
 			true,
 			[],
 			[],

tests/PHPStan/Analyser/NodeScopeResolverTest.php

@@ -10594,6 +10594,15 @@ public function dataBug4188(): array
 		return $this->gatherAssertTypes(__DIR__ . '/data/bug-4188.php');
 	}
 
+	public function dataBug4339(): array
+	{
+		if (PHP_VERSION_ID < 70400) {
+			return [];
+		}
+
+		return $this->gatherAssertTypes(__DIR__ . '/data/bug-4339.php');
+	}
+
 	/**
 	 * @param string $file
 	 * @return array<string, mixed[]>
@@ -10797,6 +10806,7 @@ private function gatherAssertTypes(string $file): array
 	 * @dataProvider dataBug2980
 	 * @dataProvider dataBug3986
 	 * @dataProvider dataBug4188
+	 * @dataProvider dataBug4339
 	 * @param string $assertType
 	 * @param string $file
 	 * @param mixed ...$args

tests/PHPStan/Analyser/data/bug-4339.php

@@ -0,0 +1,15 @@
+<?php // lint >= 7.4
+
+namespace Bug4339;
+
+use function PHPStan\Analyser\assertType;
+
+function (?string $v) {
+	assertType('string', $v ?? '-');
+	fn (?string $value): string => assertType('string', $value ?? '-');
+	fn (?string $value): void => assertType('string|null', $value);
+
+	$f = fn (?string $value): string => $value ?? '-';
+
+	assertType('string', $f($v));
+};

tests/PHPStan/Rules/Functions/data/arrow-functions-return-type.php

@@ -27,6 +27,7 @@ public function doFoo(): void
 	public function doBar(): void
 	{
 		fn () => $this->doFoo();
+		fn (?string $value): string => $value ?? '-';
 	}
 
 }