Sema: Fix availability of inherited designated initializers

Inherited designated initializers got the same availability
as the corresponding initialier in the superclass.

However if the superclass was more available than the subclass,
we would generate a diagnostic that a member cannot be more
available than its containing type.

This diagnostic had an unknown source location, since the
location was for a synthesized declaration, causing confusion.

Fixes <https://bugs.swift.org/browse/SR-7881> aka
<rdar://problem/40853731>.

lib/Sema/CodeSynthesis.cpp

@@ -2030,9 +2030,12 @@ static void configureDesignatedInitAttributes(TypeChecker &tc,
     ctor->getAttrs().add(clonedAttr);
   }
 
-  // Make sure the constructor is only as available as its superclass's
-  // constructor.
-  AvailabilityInference::applyInferredAvailableAttrs(ctor, superclassCtor, ctx);
+  // If the superclass has its own availability, make sure the synthesized
+  // constructor is only as available as its superclass's constructor.
+  if (superclassCtor->getAttrs().hasAttribute<AvailableAttr>()) {
+    AvailabilityInference::applyInferredAvailableAttrs(
+        ctor, {classDecl, superclassCtor}, ctx);
+  }
 
   if (superclassCtor->isObjC()) {
     // Inherit the @objc name from the superclass initializer, if it

test/Sema/availability_versions.swift

@@ -985,6 +985,19 @@ func useUnavailableExtension() {
       // expected-note@-1 {{add 'if #available' version check}}
 }
 
+// Availability of synthesized designated initializers.
+
+@available(OSX, introduced: 10.51)
+class WidelyAvailableBase {
+  init() {}
+
+  @available(OSX, introduced: 10.52)
+  init(thing: ()) {}
+}
+
+@available(OSX, introduced: 10.53)
+class EsotericSmallBatchHipsterThing : WidelyAvailableBase {}
+
 // Useless #available(...) checks
 
 func functionWithDefaultAvailabilityAndUselessCheck(_ p: Bool) {