[SR-14633] Generic structs sometimes cause a crash when using existential types (only in Xcode Playground)

[swift-ci]

Previous ID	SR-14633
Radar	rdar://problem/78224316
Original Reporter	fermion17 (JIRA User)
Type	Bug

Environment

iMac (Retina 5K, 27-inch, 2019), macOS 11.3, Xcode 12.5

MacBook Pro (Retina, 15-inch, Mid 2015), macOS 10.15.7, Xcode 12.3

Additional Detail from JIRA

Votes	0
Component/s	Xcode Playground Support
Labels	Bug
Assignee	None
Priority	Medium

md5: e78de01bb76e1378e78ad689d5720824

Issue Description:

Generic structs S<T> cause a crash when:

• it contains a field with type T?

• it is specialized by an existential type, e.g. S<P> where P is a protocol.

For example, the following code in Xcode Playground causes a crash.

protocol P {}
struct S<T> {
    let field: T? = nil
}
let x = S<P>() // error: Execution was interrupted, reason: EXC_BAD_ACCESS (code=EXC_I386_GPFLT).

This issue only occurs in Xcode Playground.

[typesanitizer]

@swift-ci create