AST: Weak-link declarations in extensions of weak-linked types

This is a workaround to fix weak linking of frameworks that define
types with availability, but then define extensions of those types
without availability.

This can come up if the framework itself is built with a newer
deployment target than the client that uses the framework. Since the
type checker only enforces that an extension has availability if
the extension is less available than the deployment target, we were
failing to weak link the members of the extension in this case.

This is not a perfect fix; ideally such frameworks should be built
with -require-explicit-availability, and all reported warnings
fixed by adding explicit availability.

However, it allows clients to weak link when using existing
swiftinterface files that have already shipped in the mean time,
and it should not cause any problems once the frameworks are properly
annotated in the future.

Fixes <rdar://problem/58490723>.

Author: slavapestov

lib/AST/Decl.cpp

@@ -785,6 +785,10 @@ AvailabilityContext Decl::getAvailabilityForLinkage() const {
   if (auto *accessor = dyn_cast<AccessorDecl>(this))
     return accessor->getStorage()->getAvailabilityForLinkage();
 
+  if (auto *ext = dyn_cast<ExtensionDecl>(this))
+    if (auto *nominal = ext->getExtendedNominal())
+      return nominal->getAvailabilityForLinkage();
+
   auto *dc = getDeclContext();
   if (auto *ext = dyn_cast<ExtensionDecl>(dc))
     return ext->getAvailabilityForLinkage();
@@ -806,6 +810,10 @@ bool Decl::isAlwaysWeakImported() const {
   if (auto *accessor = dyn_cast<AccessorDecl>(this))
     return accessor->getStorage()->isAlwaysWeakImported();
 
+  if (auto *ext = dyn_cast<ExtensionDecl>(this))
+    if (auto *nominal = ext->getExtendedNominal())
+      return nominal->isAlwaysWeakImported();
+
   auto *dc = getDeclContext();
   if (auto *ext = dyn_cast<ExtensionDecl>(dc))
     return ext->isAlwaysWeakImported();

test/IRGen/Inputs/weak_import_extension_helper.swift

@@ -0,0 +1,8 @@
+@available(macOS 10.60, *)
+public struct Foo {
+  public init() {}
+}
+
+extension Foo {
+  public func extensionMethod() {}
+}

test/IRGen/weak_import_extension.swift

@@ -0,0 +1,22 @@
+// RUN: %empty-directory(%t)
+//
+// RUN: %target-swift-frontend -enable-library-evolution -emit-module -target x86_64-apple-macosx10.60 -emit-module-path %t/weak_import_extension_helper.swiftmodule -parse-as-library %S/Inputs/weak_import_extension_helper.swift
+// RUN: %target-swift-frontend -primary-file %s -I %t -emit-ir -target x86_64-apple-macosx10.50 | %FileCheck %s --check-prefix=CHECK-OLD
+// RUN: %target-swift-frontend -primary-file %s -I %t -emit-ir -target x86_64-apple-macosx10.60 | %FileCheck %s --check-prefix=CHECK-NEW
+//
+// REQUIRES: OS=macosx
+
+import weak_import_extension_helper
+
+@available(macOS 10.60, *)
+public func callsExtensionMethod() {
+  Foo().extensionMethod()
+}
+
+// CHECK-OLD: declare extern_weak swiftcc %swift.metadata_response @"$s28weak_import_extension_helper3FooVMa"
+// CHECK-OLD: declare extern_weak swiftcc void @"$s28weak_import_extension_helper3FooVACycfC"
+// CHECK-OLD: declare extern_weak swiftcc void @"$s28weak_import_extension_helper3FooV0C6MethodyyF"
+
+// CHECK-NEW: declare swiftcc %swift.metadata_response @"$s28weak_import_extension_helper3FooVMa"
+// CHECK-NEW: declare swiftcc void @"$s28weak_import_extension_helper3FooVACycfC"
+// CHECK-NEW: declare swiftcc void @"$s28weak_import_extension_helper3FooV0C6MethodyyF"