diff --git a/lib/Sema/TypeCheckDecl.cpp b/lib/Sema/TypeCheckDecl.cpp
index e1f93c5c5f928..446eed89db868 100644
--- a/lib/Sema/TypeCheckDecl.cpp
+++ b/lib/Sema/TypeCheckDecl.cpp
@@ -1688,6 +1688,14 @@ bool TypeChecker::isAvailabilitySafeForConformance(
   assert(dc->getSelfNominalTypeDecl() &&
          "Must have a nominal or extension context");
 
+  AvailabilityContext contextForConformingDecl =
+      availabilityForDeclSignature(dc->getAsDecl());
+
+  // If the conformance is unavailable then it's irrelevant whether the witness
+  // is potentially unavailable.
+  if (contextForConformingDecl.isUnavailable())
+    return true;
+
   // Make sure that any access of the witness through the protocol
   // can only occur when the witness is available. That is, make sure that
   // on every version where the conforming declaration is available, if the
@@ -1703,7 +1711,7 @@ bool TypeChecker::isAvailabilitySafeForConformance(
   requirementInfo = AvailabilityInference::availableRange(requirement);
 
   AvailabilityRange infoForConformingDecl =
-      overApproximateAvailabilityAtLocation(dc->getAsDecl()->getLoc(), dc);
+      contextForConformingDecl.getPlatformRange();
 
   // Relax the requirements for @_spi witnesses by treating the requirement as
   // if it were introduced at the deployment target. This is not strictly sound
diff --git a/test/Sema/availability_scopes.swift b/test/Sema/availability_scopes.swift
index 69d1d672fb754..aba23d053038c 100644
--- a/test/Sema/availability_scopes.swift
+++ b/test/Sema/availability_scopes.swift
@@ -373,6 +373,12 @@ extension SomeEnum {
   func neverAvailable() {}
 }
 
+// CHECK-NEXT: {{^}}  (decl version=50 unavailable=macOS decl=unavailableOnMacOSAndIntroduced()
+
+@available(macOS, unavailable, introduced: 52)
+func unavailableOnMacOSAndIntroduced() {
+}
+
 // CHECK-NEXT: {{^}}  (decl version=50 unavailable=* decl=NeverAvailable
 // CHECK-NEXT: {{^}}    (decl version=50 unavailable=* decl=unavailableOnMacOS()
 
diff --git a/test/Sema/conformance_availability.swift b/test/Sema/conformance_availability.swift
index c73adabcddce3..ead1d41f89d8e 100644
--- a/test/Sema/conformance_availability.swift
+++ b/test/Sema/conformance_availability.swift
@@ -466,6 +466,14 @@ extension Bike : Vehicle {
   func move() {}
 }
 
+@available(macOS, introduced: 100)
+struct Wagon {}
+
+@available(macOS, introduced: 100)
+extension Wagon : Vehicle {
+  func move() {}
+}
+
 class Car {}
 class ClownCar : Car {}
 
@@ -478,6 +486,63 @@ extension Car {
 extension ClownCar : Vehicle {}
 // expected-error@-1 {{protocol 'Vehicle' requires 'move()' to be available in macOS 100 and newer}}
 
+@available(macOS, unavailable)
+struct Truck : Vehicle {
+  func move() {}
+}
+
+struct Scooter {}
+
+@available(macOS, unavailable)
+extension Scooter : Vehicle {
+  func move() {}
+}
+
+struct Motorcycle {}
+
+@available(macOS, unavailable)
+extension Motorcycle : Vehicle {
+  @available(macOS, introduced: 100)
+  func move() {}
+}
+
+@available(macOS, unavailable)
+struct AircraftCarrier {
+  struct Jet : Vehicle {
+    @available(macOS, introduced: 100)
+    func move() {}
+  }
+}
+
+struct Unicycle {
+  @available(macOS, introduced: 100)
+  func move() {}
+}
+
+@available(macOS, unavailable)
+extension Unicycle : Vehicle {}
+
+@available(macOS, unavailable, introduced: 100)
+struct Train : Vehicle {
+  func move() {}
+}
+
+struct Blimp {}
+
+@available(macOS, unavailable, introduced: 100)
+extension Blimp : Vehicle {
+  func move() {}
+}
+
+@available(macOS, unavailable, introduced: 100)
+struct Spaceship {
+}
+
+@available(macOS, unavailable, introduced: 100)
+extension Spaceship : Vehicle {
+  func move() {}
+}
+
 // rdar://problem/75430966 - Allow using unavailable conformances from unavailable contexts.
 @available(*, unavailable)
 public enum UnavailableEnum {