Support missing Sendable conformances everywhere in the requirement machine

test/Concurrency/sendable_conformance_checking.swift

@@ -168,8 +168,26 @@ extension MultiConformance: @unchecked Sendable {} // expected-error {{redundant
 
 // rdar://91174106 - allow missing Sendable conformances when extending a
 // type generically.
-// FIXME: Should warn because of missing Sendable, but currently is silent.
-// We don't want a hard error except in Swift 6.
+// FIXME: Should warn because of missing Sendable, but currently is silent
+// because we aren't checking conformance availability here yet.
 struct X<T: Sendable> { }
 enum Y {}
 extension X where T == Y {}
+
+protocol P2 {
+  associatedtype A: Sendable
+}
+
+enum Y2: P2, P3 {
+  typealias A = Y
+}
+
+struct X2<T: P2> { }
+extension X2 where T == Y2 { }
+
+protocol P3 {
+  associatedtype A
+}
+
+struct X3<T: P3> where T.A: Sendable { }
+extension X3 where T == Y2 { }