Static compilation requires casting inside instanceof check

There are many times I find myself having to cast variables directly inside an instanceof Class check.

In addition, I have found a specific problem with a generic <T extends SomeClass> where a method in the class that returns T, Groovy believes a collection is being returned. This is the line in question: https://github.com/grails/gorm-graphql/blob/ea6887bef0ced75d9cc0d01dc7b94d4fc0520266/core/src/main/groovy/org/grails/gorm/graphql/fetcher/impl/EntityDataFetcher.groovy#L45

The goal of this issue is to have the following codebase compile without changes.

git clone -b broken_compilation https://github.com/grails/gorm-graphql

Attempt to compile with ./gradlew clean classes