[CSBindings] Mark type variables that represent type parameter packs as "involving type variables"

Such type variables are always dependent on pack expansion type variables
which should be bound first.

Resolves: rdar://109381194

(cherry picked from commit 5b5b3dd)
(cherry picked from commit ce826c2)
(cherry picked from commit 1728cca)

Author: xedin

lib/Sema/CSBindings.cpp

@@ -150,7 +150,8 @@ bool BindingSet::isDelayed() const {
 bool BindingSet::involvesTypeVariables() const {
   // This type variable always depends on a pack expansion variable
   // which should be inferred first if possible.
-  if (TypeVar->getImpl().canBindToPack())
+  if (TypeVar->getImpl().getGenericParameter() &&
+      TypeVar->getImpl().canBindToPack())
     return true;
 
   // This is effectively O(1) right now since bindings are re-computed

test/Constraints/casts.swift

@@ -718,3 +718,26 @@ func isHashable_is(_ error: Error) -> Bool {
 func isHashable_composition(_ error: Error & AnyObject) -> Bool {
   error is AnyHashable // OK
 }
+
+// rdar://109381194 - incorrect ambiguity while comparing collections
+do {
+  class A : Hashable, Equatable {
+    func hash(into hasher: inout Hasher) {
+    }
+
+    static func == (lhs: A, rhs: A) -> Bool {
+      false
+    }
+  }
+
+  class B : A {}
+
+  func test(a: [B], b: [String: B]) {
+    assert(Set(a) == Set(b.values.map { $0 as A })) // ok
+  }
+
+  func test(a: [A], b: [B]) {
+    assert(Set(a) == Set(b.map { $0 as A })) // Ok
+    assert(Set(b.map { $0 as A }) == Set(a)) // Ok
+  }
+}