Expression Type Resolver Extensions

If you want to override the inferred type of any expression, you can write an expression type resolver extension. This is a catch-all extension point — more general than dynamic return type extensions which only work for specific method/function calls.

The implementation is all about applying the core concepts so check out that guide first and then continue here.

For overriding return types of specific method or function calls, dynamic return type extensions are a better fit. Use expression type resolver extensions when you need to override types for arbitrary expressions like property fetches, array accesses, or other constructs that don’t have a dedicated extension point.

This is the interface your extension needs to implement:

namespace PHPStan\Type;

use PhpParser\Node\Expr;
use PHPStan\Analyser\Scope;

interface ExpressionTypeResolverExtension
{

	public function getType(Expr $expr, Scope $scope): ?Type;

}

The getType() method receives any expression node and the current scope. Return a Type to override the inferred type, or return null when your extension doesn’t care about the given expression.

Here’s an example extension that narrows the type of array dimension fetches on a config object — $config['database'] always returns a string:

namespace App\PHPStan;

use PhpParser\Node\Expr;
use PhpParser\Node\Expr\ArrayDimFetch;
use PhpParser\Node\Scalar\String_;
use PHPStan\Analyser\Scope;
use PHPStan\Type\Constant\ConstantStringType;
use PHPStan\Type\ExpressionTypeResolverExtension;
use PHPStan\Type\ObjectType;
use PHPStan\Type\StringType;
use PHPStan\Type\Type;

class ConfigArrayAccessExtension implements ExpressionTypeResolverExtension
{

	public function getType(Expr $expr, Scope $scope): ?Type
	{
		if (!$expr instanceof ArrayDimFetch) {
			return null;
		}

		if ($expr->dim === null) {
			return null;
		}

		$varType = $scope->getType($expr->var);
		if (!(new ObjectType(\App\Config::class))->isSuperTypeOf($varType)->yes()) {
			return null;
		}

		$dimType = $scope->getType($expr->dim);
		if ($dimType instanceof ConstantStringType && $dimType->getValue() === 'database') {
			return new StringType();
		}

		return null;
	}

}

With this extension, PHPStan narrows the type of $config['database']:

function foo(Config $config): void
{
	// Without extension: mixed
	// With extension: string
	$db = $config['database'];
}

The implementation needs to be registered in your configuration file:

services:
	-
		class: App\PHPStan\ConfigArrayAccessExtension
		tags:
			- phpstan.broker.expressionTypeResolverExtension

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