[Sema] Accept unavailable constructors override in unavailable types

Preserve the old behavior of accepted unavailable override of
constructors as this change would be source breaking.

Author: xymus

lib/Sema/TypeCheckDeclOverride.cpp

@@ -1687,7 +1687,8 @@ static bool diagnoseOverrideForAvailability(ValueDecl *override,
   if (isRedundantAccessorOverrideAvailabilityDiagnostic(override, base))
     return false;
 
-  auto &diags = override->getASTContext().Diags;
+  auto &ctx = override->getASTContext();
+  auto &diags = ctx.Diags;
   if (auto *accessor = dyn_cast<AccessorDecl>(override)) {
     diags.diagnose(override, diag::override_accessor_less_available,
                    accessor->getDescriptiveKind(),
@@ -1696,6 +1697,15 @@ static bool diagnoseOverrideForAvailability(ValueDecl *override,
     return true;
   }
 
+  // Don't report constructors that are marked unavailable as being less
+  // available than their introduction. This was previously allowed and
+  // can be used to forbid the direct use of a constructor in a subclass.
+  // Note that even when marked unavailable the constructor could be called
+  // by other inherited constructors.
+  if (isa<ConstructorDecl>(override) &&
+      override->getAttrs().isUnavailable(ctx))
+    return false;
+
   diags.diagnose(override, diag::override_less_available,
                  override->getBaseName());
   diags.diagnose(base, diag::overridden_here);

test/Sema/availability_versions.swift

@@ -788,6 +788,9 @@ class UnavailableClassExtendingUnavailableClass : ClassAvailableOn10_51 {
 // Method availability is contravariant
 
 class SuperWithAlwaysAvailableMembers {
+
+  required init() {} // expected-note {{overridden declaration is here}}
+
   func shouldAlwaysBeAvailableMethod() { // expected-note 2 {{overridden declaration is here}}
   }
 
@@ -808,6 +811,10 @@ class SuperWithAlwaysAvailableMembers {
 }
 
 class SubWithLimitedMemberAvailability : SuperWithAlwaysAvailableMembers {
+
+  @available(OSX, introduced: 10.51)
+  required init() {} // expected-error {{overriding 'init' must be as available as declaration it overrides}}
+
   @available(OSX, introduced: 10.51)
   override func shouldAlwaysBeAvailableMethod() { // expected-error {{overriding 'shouldAlwaysBeAvailableMethod' must be as available as declaration it overrides}}
   }
@@ -833,6 +840,10 @@ class SubWithLimitedMemberAvailability : SuperWithAlwaysAvailableMembers {
 }
 
 class SubWithUnavailableMembers : SuperWithAlwaysAvailableMembers {
+
+  @available(OSX, unavailable)
+  required init() {}
+
   @available(OSX, unavailable)
   override func shouldAlwaysBeAvailableMethod() { // expected-error {{overriding 'shouldAlwaysBeAvailableMethod' must be as available as declaration it overrides}}
   }