diff --git a/lib/Sema/TypeCheckAvailability.cpp b/lib/Sema/TypeCheckAvailability.cpp
index b8784d9e62978..f99a9415f551c 100644
--- a/lib/Sema/TypeCheckAvailability.cpp
+++ b/lib/Sema/TypeCheckAvailability.cpp
@@ -1566,7 +1566,10 @@ static bool isInsideCompatibleUnavailableDeclaration(
   auto IsUnavailable = [platform](const Decl *D) {
     auto EnclosingUnavailable =
         D->getAttrs().getUnavailable(D->getASTContext());
-    return EnclosingUnavailable && EnclosingUnavailable->Platform == platform;
+    return EnclosingUnavailable &&
+        (EnclosingUnavailable->Platform == platform ||
+         inheritsAvailabilityFromPlatform(platform,
+           EnclosingUnavailable->Platform));
   };
 
   return someEnclosingDeclMatches(ReferenceRange, ReferenceDC, IsUnavailable);
diff --git a/stdlib/public/Darwin/Foundation/URL.swift b/stdlib/public/Darwin/Foundation/URL.swift
index cef7f3a4d4f42..76b264398e055 100644
--- a/stdlib/public/Darwin/Foundation/URL.swift
+++ b/stdlib/public/Darwin/Foundation/URL.swift
@@ -827,7 +827,8 @@ public struct URL : ReferenceConvertible, Equatable {
         } else {
             // Now we need to do something more expensive
             if var c = URLComponents(url: self, resolvingAgainstBaseURL: true) {
-                c.path = (c.path as NSString).appendingPathComponent(pathComponent)
+                let path = (c.path as NSString).appendingPathComponent(pathComponent)
+                c.path = isDirectory ? path + "/" : path
                 
                 if let result = c.url {
                     return result
diff --git a/test/attr/attr_availability_maccatalyst.swift b/test/attr/attr_availability_maccatalyst.swift
index 07fd5229471d1..f69cc8aea1b3d 100644
--- a/test/attr/attr_availability_maccatalyst.swift
+++ b/test/attr/attr_availability_maccatalyst.swift
@@ -140,3 +140,27 @@ protocol P: Builtin.AnyObject {
 }
 
 extension X: P {}
+
+// Test platform inheritance for iOS unavailability.
+// rdar://68597591
+
+@available(iOS, unavailable)
+public struct UnavailableOniOS { } // expected-note 2 {{'UnavailableOniOS' has been explicitly marked unavailable here}}
+
+@available(iOS, unavailable)
+func unavailableOniOS(_ p: UnavailableOniOS) { } // ok
+
+func functionUsingAnUnavailableType(_ p: UnavailableOniOS) { } // expected-error {{'UnavailableOniOS' is unavailable in iOS}}
+
+public extension UnavailableOniOS { } // expected-error {{'UnavailableOniOS' is unavailable in iOS}}
+
+@available(iOS, unavailable)
+public extension UnavailableOniOS { // ok
+  func someMethod(_ p: UnavailableOniOS) { }
+}
+
+@available(iOS, unavailable)
+@available(macCatalyst, introduced: 13.0)
+public struct AvailableOnMacCatalyst { }
+
+public extension AvailableOnMacCatalyst { } // ok