Add a couple of regression tests

test/Generics/sr15792_2.swift

@@ -0,0 +1,37 @@
+// RUN: %target-swift-frontend -typecheck %s
+
+// Reduced from Realm open source project: https://github.com/realm/realm-swift.git
+
+internal protocol RealmCollectionImpl: RealmCollection where Index == Int, SubSequence == Slice<Self> {}
+
+public protocol RealmCollection: RealmCollectionBase, Equatable {}
+
+public protocol RealmCollectionBase: RandomAccessCollection, LazyCollectionProtocol, CustomStringConvertible, ThreadConfined where Element: RealmCollectionValue {
+  typealias ElementType = Element
+}
+
+public protocol RealmCollectionValue: Hashable, _HasPersistedType where PersistedType: RealmCollectionValue {}
+
+public protocol _HasPersistedType: _ObjcBridgeable {
+  associatedtype PersistedType: _ObjcBridgeable
+}
+
+public protocol _ObjcBridgeable {}
+
+public protocol ThreadConfined {}
+
+public protocol PersistableEnum: _PersistableInsideOptional, RawRepresentable, CaseIterable, RealmEnum, _RealmCollectionValueInsideOptional, MinMaxType, Comparable where RawValue: Comparable {}
+
+public protocol _PersistableInsideOptional: _Persistable where PersistedType: _PersistableInsideOptional {} 
+
+public protocol _Persistable: _RealmSchemaDiscoverable, _HasPersistedType where PersistedType: _Persistable, PersistedType.PersistedType.PersistedType == PersistedType.PersistedType {}
+
+public protocol RealmEnum: RealmOptionalType, _RealmSchemaDiscoverable {}
+
+public protocol _RealmCollectionValueInsideOptional: RealmCollectionValue where PersistedType: _RealmCollectionValueInsideOptional {}
+
+public protocol MinMaxType {}
+
+public protocol _RealmSchemaDiscoverable {}
+
+public protocol RealmOptionalType: _ObjcBridgeable {}

validation-test/compiler_crashers_2_fixed/sr14668.swift

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+// RUN: %target-swift-frontend -emit-ir %s
+
+class A { }
+class B<T> : A { }
+class C {
+  func bar(x : (A?) -> Void) { }
+}
+class D<T> : C {
+  override func bar(x : (B<T>?) -> Void) { }
+}