diff --git a/lib/AST/Decl.cpp b/lib/AST/Decl.cpp
index b065d6d82ade0..1cb4ab7b3a341 100644
--- a/lib/AST/Decl.cpp
+++ b/lib/AST/Decl.cpp
@@ -1126,6 +1126,12 @@ bool Decl::isAlwaysWeakImported() const {
   if (getAttrs().hasAttribute<WeakLinkedAttr>())
     return true;
 
+  // FIXME: Weak linking on Windows is not yet supported
+  // https://github.com/apple/swift/issues/53303
+  if (getSemanticUnavailableAttr() &&
+      !getASTContext().LangOpts.Target.isOSWindows())
+    return true;
+
   if (auto *accessor = dyn_cast<AccessorDecl>(this))
     return accessor->getStorage()->isAlwaysWeakImported();
 
diff --git a/test/IRGen/Inputs/weak_import_availability_helper.swift b/test/IRGen/Inputs/weak_import_availability_helper.swift
index 3a74e9651b8a3..a5d1f1db0e126 100644
--- a/test/IRGen/Inputs/weak_import_availability_helper.swift
+++ b/test/IRGen/Inputs/weak_import_availability_helper.swift
@@ -1,17 +1,35 @@
 @available(macOS 10.50, *)
 public func conditionallyAvailableFunction() {}
 
+@available(macOS, unavailable)
+public func unavailableFunction() {}
+
 @available(macOS 10.50, *)
 public var conditionallyAvailableGlobal: Int {
   get {return 0}
   set {}
 }
 
+@available(macOS, unavailable)
+public var unavailableGlobal: Int {
+  get {return 0}
+  set {}
+}
+
 @available(macOS 10.50, *)
 public struct ConditionallyAvailableStruct {
   public func conditionallyAvailableMethod() {}
 }
 
+extension ConditionallyAvailableStruct {
+  public struct NestedStruct {}
+}
+
+@available(macOS, unavailable)
+public struct UnvailableStruct {
+  public func unavailableMethod() {}
+}
+
 public protocol AlwaysAvailableProtocol {}
 
 public struct AlwaysAvailableStruct {}
@@ -19,9 +37,15 @@ public struct AlwaysAvailableStruct {}
 @available(macOS 10.50, *)
 extension AlwaysAvailableStruct : AlwaysAvailableProtocol {}
 
+@available(macOS, unavailable)
+public protocol UnavailableProtocol {}
+
+@available(macOS, unavailable)
+extension AlwaysAvailableStruct : UnavailableProtocol {}
+
 public enum AlwaysAvailableEnum {
   case alwaysAvailableCase
 
   @available(macOS 10.50, *)
   case conditionallyAvailableCase
-}
\ No newline at end of file
+}
diff --git a/test/IRGen/float16_macos.swift b/test/IRGen/float16_macos.swift
index 7293254fe495f..e9e518a09013b 100644
--- a/test/IRGen/float16_macos.swift
+++ b/test/IRGen/float16_macos.swift
@@ -1,5 +1,5 @@
-// RUN: %target-swift-frontend -emit-ir %s -target x86_64-apple-macos10.15 | %FileCheck %s --check-prefix=CHECK10
-// RUN: %target-swift-frontend -emit-ir %s -target x86_64-apple-macos11 | %FileCheck %s --check-prefix=CHECK11
+// RUN: %target-swift-frontend -emit-ir %s -target x86_64-apple-macos10.15 | %FileCheck %s
+// RUN: %target-swift-frontend -emit-ir %s -target x86_64-apple-macos11 | %FileCheck %s
 
 // REQUIRES: OS=macosx
 // REQUIRES: CPU=x86_64
@@ -11,5 +11,4 @@ public struct Float16Wrapper {
   var x: Float16
 }
 
-// CHECK10-LABEL: @"$ss7Float16VMn" = extern_weak global %swift.type_descriptor
-// CHECK11-LABEL: @"$ss7Float16VMn" = external global %swift.type_descriptor
+// CHECK-LABEL: @"$ss7Float16VMn" = extern_weak global %swift.type_descriptor
diff --git a/test/IRGen/weak_import_availability.swift b/test/IRGen/weak_import_availability.swift
index 2dc7f3f8f0ec2..abf0f0d01aa8d 100644
--- a/test/IRGen/weak_import_availability.swift
+++ b/test/IRGen/weak_import_availability.swift
@@ -1,12 +1,12 @@
 // RUN: %empty-directory(%t)
-// RUN: %target-swift-frontend -emit-module -emit-module-path %t/weak_import_availability_helper.swiftmodule -parse-as-library %S/Inputs/weak_import_availability_helper.swift -enable-library-evolution
+// RUN: %target-swift-frontend -target %target-cpu-apple-macosx10.50 -emit-module -emit-module-path %t/weak_import_availability_helper.swiftmodule -parse-as-library %S/Inputs/weak_import_availability_helper.swift -enable-library-evolution
 //
-// RUN: %target-swift-frontend -primary-file %s -I %t -emit-ir | %FileCheck %s --check-prefix=CHECK-OLD
-// RUN: %target-swift-frontend -primary-file %s -I %t -emit-ir -target %target-cpu-apple-macosx10.50 | %FileCheck %s --check-prefix=CHECK-NEW
-// RUN: %target-swift-frontend -primary-file %s -I %t -emit-ir -target %target-cpu-apple-macosx10.60 | %FileCheck %s --check-prefix=CHECK-NEW
+// RUN: %target-swift-frontend -primary-file %s -I %t -emit-ir | %FileCheck %s --check-prefixes=CHECK,CHECK-OLD
+// RUN: %target-swift-frontend -primary-file %s -I %t -emit-ir -target %target-cpu-apple-macosx10.50 | %FileCheck %s --check-prefixes=CHECK,CHECK-NEW
+// RUN: %target-swift-frontend -primary-file %s -I %t -emit-ir -target %target-cpu-apple-macosx10.60 | %FileCheck %s --check-prefixes=CHECK,CHECK-NEW
 
-// RUN: %target-swift-frontend -primary-file %s -I %t -emit-ir -target %target-cpu-apple-macosx10.50 -weak-link-at-target | %FileCheck %s --check-prefix=CHECK-OLD
-// RUN: %target-swift-frontend -primary-file %s -I %t -emit-ir -target %target-cpu-apple-macosx10.60 -weak-link-at-target | %FileCheck %s --check-prefix=CHECK-NEW
+// RUN: %target-swift-frontend -primary-file %s -I %t -emit-ir -target %target-cpu-apple-macosx10.50 -weak-link-at-target | %FileCheck %s --check-prefixes=CHECK,CHECK-OLD
+// RUN: %target-swift-frontend -primary-file %s -I %t -emit-ir -target %target-cpu-apple-macosx10.60 -weak-link-at-target | %FileCheck %s --check-prefixes=CHECK,CHECK-NEW
 
 // REQUIRES: OS=macosx
 
@@ -35,6 +35,16 @@ public func useConditionallyAvailableConformance() {
 // CHECK-OLD-LABEL: @"$s31weak_import_availability_helper21AlwaysAvailableStructVAA0eF8ProtocolAAWP" = extern_weak global i8*
 // CHECK-NEW-LABEL: @"$s31weak_import_availability_helper21AlwaysAvailableStructVAA0eF8ProtocolAAWP" = external global i8*
 
+@available(macOS, unavailable)
+func useUnavailableConformance<T : UnavailableProtocol>(_: T.Type) {}
+
+@available(macOS, unavailable)
+public func useUnavailableConformance() {
+  useUnavailableConformance(AlwaysAvailableStruct.self)
+}
+
+// CHECK-LABEL: @"$s31weak_import_availability_helper21AlwaysAvailableStructVAA19UnavailableProtocolAAWP" = extern_weak global i8*, align 8
+
 @available(macOS 10.50, *)
 public func callConditionallyAvailableFunction() {
   conditionallyAvailableFunction()
@@ -43,6 +53,13 @@ public func callConditionallyAvailableFunction() {
 // CHECK-OLD-LABEL: declare extern_weak swiftcc void @"$s31weak_import_availability_helper30conditionallyAvailableFunctionyyF"()
 // CHECK-NEW-LABEL: declare swiftcc void @"$s31weak_import_availability_helper30conditionallyAvailableFunctionyyF"()
 
+@available(macOS, unavailable)
+public func callUnavailableFunction() {
+  unavailableFunction()
+}
+
+// CHECK-LABEL: declare extern_weak swiftcc void @"$s31weak_import_availability_helper19unavailableFunctionyyF"()
+
 @available(macOS 10.50, *)
 public func useConditionallyAvailableGlobal() {
   _ = conditionallyAvailableGlobal
@@ -56,6 +73,20 @@ public func useConditionallyAvailableGlobal() {
 // CHECK-OLD-LABEL: declare extern_weak swiftcc void @"$s31weak_import_availability_helper28conditionallyAvailableGlobalSivs"(i64)
 // CHECK-NEW-LABEL: declare swiftcc void @"$s31weak_import_availability_helper28conditionallyAvailableGlobalSivs"(i64)
 
+// CHECK-OLD-LABEL: declare extern_weak swiftcc { i8*, %TSi* } @"$s31weak_import_availability_helper28conditionallyAvailableGlobalSivM"(i8* noalias dereferenceable(32))
+// CHECK-NEW-LABEL: declare swiftcc { i8*, %TSi* } @"$s31weak_import_availability_helper28conditionallyAvailableGlobalSivM"(i8* noalias dereferenceable(32))
+
+@available(macOS, unavailable)
+public func useUnavailableGlobal() {
+  _ = unavailableGlobal
+  unavailableGlobal = 0
+  unavailableGlobal += 1
+}
+
+// CHECK-LABEL: declare extern_weak swiftcc i64 @"$s31weak_import_availability_helper17unavailableGlobalSivg"()
+// CHECK-LABEL: declare extern_weak swiftcc void @"$s31weak_import_availability_helper17unavailableGlobalSivs"(i64)
+// CHECK-LABEL: declare extern_weak swiftcc { i8*, %TSi* } @"$s31weak_import_availability_helper17unavailableGlobalSivM"(i8* noalias dereferenceable(32))
+
 func blackHole<T>(_: T) {}
 
 @available(macOS 10.50, *)
@@ -63,6 +94,17 @@ public func useConditionallyAvailableStruct() {
   blackHole(ConditionallyAvailableStruct.self)
 }
 
+// CHECK-OLD-LABEL: declare extern_weak swiftcc %swift.metadata_response @"$s31weak_import_availability_helper28ConditionallyAvailableStructVMa"(i64)
+// CHECK-NEW-LABEL: declare swiftcc %swift.metadata_response @"$s31weak_import_availability_helper28ConditionallyAvailableStructVMa"(i64)
+
+@available(macOS 10.50, *)
+public func useNestedConditionallyAvailableStruct() {
+  blackHole(ConditionallyAvailableStruct.NestedStruct.self)
+}
+
+// CHECK-OLD-LABEL: declare extern_weak swiftcc %swift.metadata_response @"$s31weak_import_availability_helper28ConditionallyAvailableStructV06NestedG0VMa"(i64)
+// CHECK-NEW-LABEL: declare swiftcc %swift.metadata_response @"$s31weak_import_availability_helper28ConditionallyAvailableStructV06NestedG0VMa"(i64)
+
 @available(macOS 10.50, *)
 public func useConditionallyAvailableMethod(s: ConditionallyAvailableStruct) {
   s.conditionallyAvailableMethod()
@@ -70,3 +112,17 @@ public func useConditionallyAvailableMethod(s: ConditionallyAvailableStruct) {
 
 // CHECK-OLD-LABEL: declare extern_weak swiftcc void @"$s31weak_import_availability_helper28ConditionallyAvailableStructV013conditionallyF6MethodyyF"(%swift.opaque* noalias nocapture swiftself)
 // CHECK-NEW-LABEL: declare swiftcc void @"$s31weak_import_availability_helper28ConditionallyAvailableStructV013conditionallyF6MethodyyF"(%swift.opaque* noalias nocapture swiftself)
+
+@available(macOS, unavailable)
+public func useUnavailableStruct() {
+  blackHole(UnvailableStruct.self)
+}
+
+// CHECK-LABEL: declare extern_weak swiftcc %swift.metadata_response @"$s31weak_import_availability_helper16UnvailableStructVMa"(i64)
+
+@available(macOS, unavailable)
+public func useUnavailableMethod(s: UnvailableStruct) {
+  s.unavailableMethod()
+}
+
+// CHECK-LABEL: declare extern_weak swiftcc void @"$s31weak_import_availability_helper16UnvailableStructV17unavailableMethodyyF"(%swift.opaque* noalias nocapture swiftself)
diff --git a/test/SILGen/objc_init_unavailable.swift b/test/SILGen/objc_init_unavailable.swift
index 7d3f30accff67..04f791ebfd7df 100644
--- a/test/SILGen/objc_init_unavailable.swift
+++ b/test/SILGen/objc_init_unavailable.swift
@@ -1,12 +1,13 @@
 // RUN: %target-swift-emit-silgen(mock-sdk: %clang-importer-sdk) -enable-objc-interop -import-objc-header %S/Inputs/objc_init_unavailable.h %s | %FileCheck %s
 // REQUIRES: objc_interop
+// REQUIRES: OS=macosx
 
 @available(macOS, unavailable)
 public func callUnavailableInit(name: String) -> ClassWithUnavailableInit {
   return ClassWithUnavailableInit(bundleID: name)
 }
 
-// CHECK-LABEL: sil [ossa] @$s21objc_init_unavailable19callUnavailableInit4nameSo09ClassWitheF0CSS_tF : $@convention(thin) (@guaranteed String) -> @owned ClassWithUnavailableInit {
+// CHECK-LABEL: sil [weak_imported] [ossa] @$s21objc_init_unavailable19callUnavailableInit4nameSo09ClassWitheF0CSS_tF : $@convention(thin) (@guaranteed String) -> @owned ClassWithUnavailableInit {
 // CHECK: function_ref @$sSo24ClassWithUnavailableInitC8bundleIDABSgSSSg_tcfC : $@convention(method) (@owned Optional<String>, @thick ClassWithUnavailableInit.Type) -> @owned Optional<ClassWithUnavailableInit>
 // CHECK: return
 
diff --git a/validation-test/execution/unavailable_decls.swift b/validation-test/execution/unavailable_decls.swift
new file mode 100644
index 0000000000000..244045d14c72b
--- /dev/null
+++ b/validation-test/execution/unavailable_decls.swift
@@ -0,0 +1,30 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+// RUN: %target-build-swift-dylib(%t/%target-library-name(Library)) -emit-module -emit-module-path %t/Library.swiftmodule -module-name Library -parse-as-library %t/Library.swift -swift-version 5 -enable-library-evolution
+// RUN: %target-build-swift-dylib(%t/%target-library-name(ClientLibrary)) %t/ClientA.swift %t/ClientB.swift -I%t -L%t -swift-version 5 -lLibrary
+
+//--- Library.swift
+
+@available(*, unavailable)
+public struct UnavailableStruct {}
+
+//--- ClientA.swift
+
+import Library
+
+@inline(never)
+func blackHole<T>(_ t: T) {}
+
+@available(*, unavailable)
+public func foo() {
+  blackHole(UnavailableStruct.self)
+}
+
+//--- ClientB.swift
+
+import Library
+
+@available(*, unavailable)
+public func bar() {
+  blackHole(UnavailableStruct.self)
+}