[SR-4611] Runtime crash with nested type used as superclass generic argument

[swift-ci]

Previous ID	SR-4611
Radar	None
Original Reporter	mlwoollard (JIRA User)
Type	Bug
Status	Resolved
Resolution	Duplicate

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, RunTimeCrash, Runtime
Assignee	None
Priority	Medium

md5: cae105004f0afe8b0cad98e9ecd852b9

duplicates:

• SR-5650 Remove parent field from type metadata

relates to:

• SR-3779 Program with struct nested in generic struct crashes because of “cyclic metadata dependency”
• SR-5432 Using nested type as a superclass generic parameter leads to a crash

Issue Description:

The following code compiles without warning but when run crashes instantiating Subclass instance, prior to entering Subclass.init. Would appear to be due to having nested struct A defined within Subclass and this being used as generic parameter. Refactoring struct A so its not nested and no crash. If this is not valid to use nested type as generic parameter like this should not the compiler give an error? If it is valid code the runtime crash should be fixed.

$ swiftc --version
Apple Swift version 3.1 (swiftlang-802.0.51 clang-802.0.41)

class Base<T> {}

class Subclass: Base<Subclass.A> {
    
    struct A {}
    
    override init() {
        super.init()
    }    
}

let _ = Subclass()

[belkadan]

Yep, the nested type's runtime metadata has "parent" pointer that refers to the outer type, but the outer type has to record its superclass using the nested type.