diff --git a/clang/include/clang/Basic/LangOptions.h b/clang/include/clang/Basic/LangOptions.h
index fb23ff6647eaf..6dc5ca20cbb99 100644
--- a/clang/include/clang/Basic/LangOptions.h
+++ b/clang/include/clang/Basic/LangOptions.h
@@ -64,7 +64,13 @@ enum class PointerAuthenticationMode : unsigned {
   SignAndAuth
 };
 
-enum class FeatureAvailKind { None, Available, Unavailable, Dynamic };
+enum class FeatureAvailKind {
+  None,
+  Available,
+  Unavailable,
+  Dynamic,
+  AlwaysAvailable,
+};
 
 /// Bitfields of LangOptions, split out from LangOptions in order to ensure that
 /// this large collection of bitfields is a trivial class type.
diff --git a/clang/lib/AST/ASTContext.cpp b/clang/lib/AST/ASTContext.cpp
index 3a24b234263ab..5a9b118bf86bc 100644
--- a/clang/lib/AST/ASTContext.cpp
+++ b/clang/lib/AST/ASTContext.cpp
@@ -948,6 +948,9 @@ ASTContext::getFeatureAvailInfo(Decl *D) const {
   case 2:
     Kind = FeatureAvailKind::Dynamic;
     break;
+  case 3:
+    Kind = FeatureAvailKind::AlwaysAvailable;
+    break;
   default:
     llvm_unreachable("invalid feature kind");
   }
@@ -990,10 +993,20 @@ bool ASTContext::hasUnavailableFeature(const Decl *D) const {
   for (auto *AA : D->specific_attrs<DomainAvailabilityAttr>()) {
     auto FeatureName = AA->getDomain();
     auto FeatureInfo = getFeatureAvailInfo(FeatureName);
-    if (FeatureInfo.Kind == (AA->getUnavailable()
-                                 ? FeatureAvailKind::Available
-                                 : FeatureAvailKind::Unavailable))
-      return true;
+    switch (FeatureInfo.Kind) {
+    case FeatureAvailKind::Available:
+    case FeatureAvailKind::AlwaysAvailable:
+      if (AA->getUnavailable())
+        return true;
+      break;
+    case FeatureAvailKind::Unavailable:
+      if (!AA->getUnavailable())
+        return true;
+      break;
+    case FeatureAvailKind::Dynamic:
+    case FeatureAvailKind::None:
+      break;
+    }
   }
 
   return false;
diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index d89b9ef6548b9..bb62373f3f86f 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -11995,6 +11995,7 @@ class IntExprEvaluator
 
     switch (FeatureInfo.Kind) {
     case FeatureAvailKind::Available:
+    case FeatureAvailKind::AlwaysAvailable:
       ResultInt = 1;
       break;
     case FeatureAvailKind::Unavailable:
diff --git a/clang/lib/Headers/availability_domain.h b/clang/lib/Headers/availability_domain.h
index ede75841e2b97..2806f58f97821 100644
--- a/clang/lib/Headers/availability_domain.h
+++ b/clang/lib/Headers/availability_domain.h
@@ -15,6 +15,7 @@
 #define __AVAILABILITY_DOMAIN_ENABLED 0
 #define __AVAILABILITY_DOMAIN_DISABLED 1
 #define __AVAILABILITY_DOMAIN_DYNAMIC 2
+#define __AVAILABILITY_DOMAIN_ALWAYS_ENABLED 3
 
 /// Describes the fields of a Clang availability domain. This struct is an
 /// implementation detail of the compiler and is subject to change so don't
@@ -43,11 +44,8 @@ struct __AvailabilityDomain {
       availability_domain(domain))) = {__AVAILABILITY_DOMAIN_DISABLED, 0}
 
 #define CLANG_ALWAYS_ENABLED_AVAILABILITY_DOMAIN(domain)                       \
-  static struct __AvailabilityDomain domain __attribute__((                    \
-      availability_domain(domain))) = {__AVAILABILITY_DOMAIN_ENABLED, 0}
-
-#define CLANG_ALWAYS_DISABLED_AVAILABILITY_DOMAIN(domain)                      \
-  static struct __AvailabilityDomain domain __attribute__((                    \
-      availability_domain(domain))) = {__AVAILABILITY_DOMAIN_DISABLED, 0}
+  static struct __AvailabilityDomain domain                                    \
+      __attribute__((availability_domain(domain))) = {                         \
+          __AVAILABILITY_DOMAIN_ALWAYS_ENABLED, 0}
 
 #endif /* __AVAILABILITY_DOMAIN_H */
diff --git a/clang/test/Sema/feature-availability.c b/clang/test/Sema/feature-availability.c
index 7ce82e8b0cd5e..c493f02a98eed 100644
--- a/clang/test/Sema/feature-availability.c
+++ b/clang/test/Sema/feature-availability.c
@@ -13,6 +13,7 @@ CLANG_ENABLED_AVAILABILITY_DOMAIN(feature1);
 CLANG_DISABLED_AVAILABILITY_DOMAIN(feature2);
 CLANG_ENABLED_AVAILABILITY_DOMAIN(feature3);
 CLANG_DYNAMIC_AVAILABILITY_DOMAIN(feature4, pred1);
+CLANG_ALWAYS_ENABLED_AVAILABILITY_DOMAIN(feature5);
 #endif
 
 __attribute__((availability(domain:feature1, AVAIL))) void func12(void);
@@ -31,6 +32,8 @@ __attribute__((availability(domain:feature4, AVAIL))) void func10(void);
 __attribute__((availability(domain:feature4, UNAVAIL))) void func11(void);
 __attribute__((availability(domain:feature4, AVAIL))) int g4;
 __attribute__((availability(domain:feature4, UNAVAIL))) int g5;
+__attribute__((availability(domain:feature5, AVAIL))) void func21(void);
+__attribute__((availability(domain:feature5, UNAVAIL))) void func22(void);
 #endif
 
 void test_unreachable_code(void) {
@@ -237,3 +240,19 @@ void test7(void) {
     e = EB; // expected-error {{use of 'EB' requires feature 'feature1' to be available}}
   }
 }
+
+#ifdef USE_DOMAIN
+void test8(void) {
+  // FIXME: Use of 'func21()' should not be diagnosed because feature5 is always available.
+  func21(); // expected-error {{use of 'func21' requires feature 'feature5' to be available}}
+  func22(); // expected-error {{use of 'func22' requires feature 'feature5' to be unavailable}}
+
+  if (__builtin_available(domain:feature5)) {
+    func21();
+    func22(); // expected-error {{use of 'func22' requires feature 'feature5' to be unavailable}}
+  } else {
+    func21(); // expected-error {{use of 'func21' requires feature 'feature5' to be available}}
+    func22();
+  }
+}
+#endif