CodeQL library for Java
codeql/java-all 0.4.4 (changelog, source)

Member predicate MemberRefExpr::getReceiverType

Gets the receiver type whose member this expression refers to. The result might not be the type which actually declares the member. For example, for the member reference ArrayList::toString, this predicate has the result java.util.ArrayList, the type explicitly referred to, while getReferencedCallable will have java.util.AbstractCollection.toString as result, which ArrayList inherits.

RefType getReceiverType()