diff --git a/lib/Sema/TypeCheckAttr.cpp b/lib/Sema/TypeCheckAttr.cpp
index ec81f0b5327ae..93df148d0df2f 100644
--- a/lib/Sema/TypeCheckAttr.cpp
+++ b/lib/Sema/TypeCheckAttr.cpp
@@ -1964,7 +1964,9 @@ void AttributeChecker::visitAvailableAttr(AvailableAttr *attr) {
       if (!AttrRange.isContainedIn(*EnclosingAnnotatedRange)) {
         // Members of extensions of nominal types with available ranges were
         // not diagnosed previously, so only emit a warning in that case.
-        auto limit = (enclosingDecl != parent && isa<ExtensionDecl>(parent))
+        bool inExtension = isa<ExtensionDecl>(
+            D->getDeclContext()->getTopmostDeclarationDeclContext());
+        auto limit = (enclosingDecl != parent && inExtension)
                          ? DiagnosticBehavior::Warning
                          : DiagnosticBehavior::Unspecified;
         diagnose(D->isImplicit() ? enclosingDecl->getLoc()
diff --git a/test/attr/attr_availability_osx.swift b/test/attr/attr_availability_osx.swift
index c80e90a626f9d..9e0da7419b0d2 100644
--- a/test/attr/attr_availability_osx.swift
+++ b/test/attr/attr_availability_osx.swift
@@ -81,7 +81,7 @@ func doSomethingDeprecatedOniOS() { }
 doSomethingDeprecatedOniOS() // okay
 
 @available(macOS 10.10, *)
-struct TestStruct {} // expected-note {{enclosing scope requires availability of macOS 10.10 or newer}}
+struct TestStruct {} // expected-note 2 {{enclosing scope requires availability of macOS 10.10 or newer}}
 
 @available(macOS 10.10, *)
 extension TestStruct { // expected-note {{enclosing scope requires availability of macOS 10.10 or newer}}
@@ -107,6 +107,11 @@ extension TestStruct { // expected-note {{enclosing scope requires availability
 extension TestStruct {
   @available(macOS 10.9, *) // expected-warning {{instance method cannot be more available than enclosing scope}}
   func doFifthThing() {}
+
+  struct NestedStruct {
+    @available(macOS 10.9, *) // expected-warning {{instance method cannot be more available than enclosing scope}}
+    func doSixthThing() {}
+  }
 }
 
 @available(macOS 10.11, *)