[SR-11503] Regression on self-referential generic where clauses in Swift 5.1

[swift-ci]

Previous ID	SR-11503
Radar	rdar://problem/55625744
Original Reporter	andrewchang-bird (JIRA User)
Type	Bug

Environment

• macOS 10.14.6 (18G95)

• Apple Swift version 5.1 (swiftlang-1100.0.270.13 clang-1100.0.33.7)

• Xcode 11 (11A420a)

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, 5.1Regression, CompilerCrash, Generics, Serialization
Assignee	None
Priority	Medium

md5: a10484874437e4d02dc076e577990dae

Issue Description:

In Swift 5.0.1 this declaration compiled successfully:

protocol Type {
  associatedtype Element
}
class GenericType<T: Type> where T.Element == GenericType {}
// also fails: class GenericType<T> where T: Type, T.Element == GenericType {}
// also fails: class GenericType<T> where T: Type, T.Element: GenericType {}

But in Swift 5.1, it now results in an error:

error: Illegal instruction: 4

[theblixguy]

This doesn't crash for me on master and on Xcode 11 GM (I don't have GM2 installed yet, only GM1 aka build 11A419c).

[belkadan]

It crashes in merge-modules, which makes sense given some of the changes in Serialization this release.

[belkadan]

@swift-ci create

[harlanhaskins]

This is overflowing the stack trying to resolve these decls