Implement ArrayAccess->offsetExists narrowing

conf/config.neon

@@ -1097,6 +1097,16 @@ services:
 		tags:
 			- phpstan.broker.dynamicFunctionReturnTypeExtension
 
+	-
+		class: PHPStan\Type\Php\ArrayAccessOffsetExistsMethodTypeSpecifyingExtension
+		tags:
+			- phpstan.typeSpecifier.methodTypeSpecifyingExtension
+
+	-
+		class: PHPStan\Type\Php\ArrayAccessOffsetGetMethodReturnTypeExtension
+		tags:
+			- phpstan.broker.dynamicMethodReturnTypeExtension
+
 	-
 		class: PHPStan\Type\Php\ArrayIntersectKeyFunctionReturnTypeExtension
 		tags:

src/Type/ObjectType.php

@@ -1149,14 +1149,14 @@ public function hasOffsetValueType(Type $offsetType): TrinaryLogic
 
 	public function getOffsetValueType(Type $offsetType): Type
 	{
-		if (!$this->isExtraOffsetAccessibleClass()->no()) {
-			return new MixedType();
-		}
-
 		if ($this->isInstanceOf(ArrayAccess::class)->yes()) {
 			return RecursionGuard::run($this, fn (): Type => $this->getMethod('offsetGet', new OutOfClassScope())->getOnlyVariant()->getReturnType());
 		}
 
+		if (!$this->isExtraOffsetAccessibleClass()->no()) {
+			return new MixedType();
+		}
+
 		return new ErrorType();
 	}
 

src/Type/Php/ArrayAccessOffsetExistsMethodTypeSpecifyingExtension.php

@@ -0,0 +1,86 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\Type\Php;
+
+use ArrayAccess;
+use PhpParser\Node\Expr\MethodCall;
+use PHPStan\Analyser\Scope;
+use PHPStan\Analyser\SpecifiedTypes;
+use PHPStan\Analyser\TypeSpecifier;
+use PHPStan\Analyser\TypeSpecifierAwareExtension;
+use PHPStan\Analyser\TypeSpecifierContext;
+use PHPStan\Reflection\MethodReflection;
+use PHPStan\Type\Accessory\HasOffsetValueType;
+use PHPStan\Type\Constant\ConstantIntegerType;
+use PHPStan\Type\Constant\ConstantStringType;
+use PHPStan\Type\Generic\GenericObjectType;
+use PHPStan\Type\MethodTypeSpecifyingExtension;
+use function count;
+
+final class ArrayAccessOffsetExistsMethodTypeSpecifyingExtension implements MethodTypeSpecifyingExtension, TypeSpecifierAwareExtension
+{
+
+	private TypeSpecifier $typeSpecifier;
+
+	public function setTypeSpecifier(TypeSpecifier $typeSpecifier): void
+	{
+		$this->typeSpecifier = $typeSpecifier;
+	}
+
+	public function getClass(): string
+	{
+		return ArrayAccess::class;
+	}
+
+	public function isMethodSupported(
+		MethodReflection $methodReflection,
+		MethodCall $node,
+		TypeSpecifierContext $context,
+	): bool
+	{
+		return $methodReflection->getName() === 'offsetExists' && $context->true();
+	}
+
+	public function specifyTypes(MethodReflection $methodReflection, MethodCall $node, Scope $scope, TypeSpecifierContext $context): SpecifiedTypes
+	{
+		if (count($node->getArgs()) < 1) {
+			return new SpecifiedTypes();
+		}
+		$key = $node->getArgs()[0]->value;
+		$keyType = $scope->getType($key);
+
+		if (
+			!$keyType instanceof ConstantStringType
+			&& !$keyType instanceof ConstantIntegerType
+		) {
+			return new SpecifiedTypes();
+		}
+
+		foreach ($scope->getType($node->var)->getObjectClassReflections() as $classReflection) {
+			$implementsTags = $classReflection->getImplementsTags();
+
+			if (
+				!isset($implementsTags[ArrayAccess::class])
+				|| !$implementsTags[ArrayAccess::class]->getType() instanceof GenericObjectType
+			) {
+				continue;
+			}
+
+			$implementsType = $implementsTags[ArrayAccess::class]->getType();
+			$arrayAccessGenericTypes = $implementsType->getTypes();
+			if (!isset($arrayAccessGenericTypes[1])) {
+				continue;
+			}
+
+			return $this->typeSpecifier->create(
+				$node->var,
+				new HasOffsetValueType($keyType, $arrayAccessGenericTypes[1]),
+				$context,
+				$scope,
+			);
+		}
+
+		return new SpecifiedTypes();
+	}
+
+}

src/Type/Php/ArrayAccessOffsetGetMethodReturnTypeExtension.php

@@ -0,0 +1,44 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\Type\Php;
+
+use ArrayAccess;
+use PhpParser\Node\Expr\MethodCall;
+use PHPStan\Analyser\Scope;
+use PHPStan\Reflection\MethodReflection;
+use PHPStan\Type\DynamicMethodReturnTypeExtension;
+use PHPStan\Type\Type;
+use function count;
+
+final class ArrayAccessOffsetGetMethodReturnTypeExtension implements DynamicMethodReturnTypeExtension
+{
+
+	public function getClass(): string
+	{
+		return ArrayAccess::class;
+	}
+
+	public function isMethodSupported(
+		MethodReflection $methodReflection,
+	): bool
+	{
+		return $methodReflection->getName() === 'offsetGet';
+	}
+
+	public function getTypeFromMethodCall(MethodReflection $methodReflection, MethodCall $methodCall, Scope $scope): ?Type
+	{
+		if (count($methodCall->getArgs()) < 1) {
+			return null;
+		}
+		$key = $methodCall->getArgs()[0]->value;
+		$keyType = $scope->getType($key);
+		$objectType = $scope->getType($methodCall->var);
+
+		if (!$objectType->hasOffsetValueType($keyType)->yes()) {
+			return null;
+		}
+
+		return $objectType->getOffsetValueType($keyType);
+	}
+
+}

tests/PHPStan/Analyser/nsrt/bug-3323.php

@@ -0,0 +1,54 @@
+<?php
+
+namespace Bug3323;
+
+use function PHPStan\Testing\assertType;
+use ArrayAccess;
+
+/**
+ * @implements ArrayAccess<string, self>
+ */
+class FormView implements \ArrayAccess
+{
+	public array $vars = [];
+
+	public function offsetExists($offset) {
+		return array_key_exists($offset, $this->vars);
+	}
+	public function offsetGet($offset) {
+		return $this->vars[$offset] ?? null;
+	}
+	public function offsetSet($offset, $value) {
+		$this->vars[$offset] = $value;
+	}
+	public function offsetUnset($offset) {
+		unset($this->vars[$offset]);
+	}
+}
+
+function doFoo() {
+	$formView = new FormView();
+	assertType('Bug3323\FormView', $formView);
+	if ($formView->offsetExists('_token')) {
+		assertType("Bug3323\FormView&hasOffsetValue('_token', Bug3323\FormView)", $formView);
+
+		$a = $formView->offsetGet('_token');
+		assertType("Bug3323\FormView", $a);
+
+		$a = $formView->offsetGet(123);
+		assertType("Bug3323\FormView|null", $a);
+	} else {
+		assertType('Bug3323\FormView', $formView);
+
+		$a = $formView->offsetGet('_token');
+		assertType("Bug3323\FormView|null", $a); // could be "null" only
+	}
+	assertType('Bug3323\FormView', $formView);
+
+	$a = $formView->offsetGet('_token');
+	assertType("Bug3323\FormView|null", $a);
+
+	$a = $formView->offsetGet(123);
+	assertType("Bug3323\FormView|null", $a);
+}
+