[CSBindings] New binding formed by join operation should refer to constraint that triggered join

Using existing binding with updated type would result in incorrect
behavior in incremental mode since when "originating" (new) constraint
gets retracted it would leave a stale binding behind.

Author: xedin

lib/Sema/CSBindings.cpp

@@ -597,7 +597,10 @@ bool PotentialBindings::addPotentialBinding(PotentialBinding binding,
             Type::join(existingBinding->BindingType, binding.BindingType);
 
         if (join && isAcceptableJoin(*join)) {
-          joined.push_back(existingBinding->withType(*join));
+          // Result of the join has to use new binding because it refers
+          // to the constraint that triggered the join that replaced the
+          // existing binding.
+          joined.push_back(binding.withType(*join));
           // Remove existing binding from the set.
           // It has to be re-introduced later, since its type has been changed.
           existingBinding = Bindings.erase(existingBinding);

test/Sema/type_join.swift

@@ -120,3 +120,14 @@ func rdar37241221(_ a: C?, _ b: D?) {
   let inferred = [a!, b]
   expectEqualType(type(of: array_c_opt).self, type(of: inferred).self)
 }
+
+extension FixedWidthInteger {
+  public static func test_nonstale_join_result<Other: BinaryInteger>(_ lhs: inout Self, _ rhs: Other) {
+    let shift = rhs < -Self.bitWidth ? -Self.bitWidth
+               : rhs > Self.bitWidth ? Self.bitWidth
+               : Int(rhs) // `shift` is `Int`
+
+    func accepts_int(_: Int) {}
+    accepts_int(shift) // Ok
+  }
+}