[ReturnTypeExtension] add test case for narrow typed json_decode#993

ondrejmirtes merged 21 commits into

phpstan:1.6.xphpstan/phpstan-src:1.6.xfrom

TomasVotruba:tv-json-decode-narrow-typeTomasVotruba/phpstan-src:tv-json-decode-narrow-typeCopy head branch name to clipboard