Look to enclosing declarations for @available attributes on @_nonSendable types

`@_nonSendable` types get a synthesized, unavailable extension that
declares the Sendable conformance. This extension also needs to have
appropriate platform availability for the type that is being marked
non-Sendable, so the platform-specific attributes are copied from the
nominal type declaration. However, for nested types, we might need to
copy those attributes from an enclosing declarations. Do so when
appropriate.

Fixes rdar://90330588.

Author: DougGregor

lib/Sema/TypeCheckConcurrency.cpp

@@ -4341,23 +4341,37 @@ static void addUnavailableAttrs(ExtensionDecl *ext, NominalTypeDecl *nominal) {
   ASTContext &ctx = nominal->getASTContext();
   llvm::VersionTuple noVersion;
 
-  // Add platform-version-specific @available attributes.
-  for (auto available : nominal->getAttrs().getAttributes<AvailableAttr>()) {
-    if (available->Platform == PlatformKind::none)
-      continue;
+  // Add platform-version-specific @available attributes. Search from nominal
+  // type declaration through its enclosing declarations to find the first one
+  // with platform-specific attributes.
+  for (Decl *enclosing = nominal;
+       enclosing;
+       enclosing = enclosing->getDeclContext()
+           ? enclosing->getDeclContext()->getAsDecl()
+           : nullptr) {
+    bool anyPlatformSpecificAttrs = false;
+    for (auto available: enclosing->getAttrs().getAttributes<AvailableAttr>()) {
+      if (available->Platform == PlatformKind::none)
+        continue;
 
-    auto attr = new (ctx) AvailableAttr(
-        SourceLoc(), SourceRange(),
-        available->Platform,
-        available->Message,
-        "", nullptr,
-        available->Introduced.getValueOr(noVersion), SourceRange(),
-        available->Deprecated.getValueOr(noVersion), SourceRange(),
-        available->Obsoleted.getValueOr(noVersion), SourceRange(),
-        PlatformAgnosticAvailabilityKind::Unavailable,
-        /*implicit=*/true,
-        available->IsSPI);
-    ext->getAttrs().add(attr);
+      auto attr = new (ctx) AvailableAttr(
+          SourceLoc(), SourceRange(),
+          available->Platform,
+          available->Message,
+          "", nullptr,
+          available->Introduced.getValueOr(noVersion), SourceRange(),
+          available->Deprecated.getValueOr(noVersion), SourceRange(),
+          available->Obsoleted.getValueOr(noVersion), SourceRange(),
+          PlatformAgnosticAvailabilityKind::Unavailable,
+          /*implicit=*/true,
+          available->IsSPI);
+      ext->getAttrs().add(attr);
+      anyPlatformSpecificAttrs = true;
+    }
+
+    // If we found any platform-specific availability attributes, we're done.
+    if (anyPlatformSpecificAttrs)
+      break;
   }
 
   // Add the blanket "unavailable".

test/ModuleInterface/sendable_availability.swift

@@ -12,6 +12,16 @@ public struct X { }
 @_nonSendable
 public struct Y { }
 
+@available(macOS 11.0, *)
+extension X {
+  @available(macOS 12.0, *)
+  @_nonSendable
+  public struct A { }
+
+  @_nonSendable
+  public struct B { }
+}
+
 // RUN: %FileCheck %s <%t/Library.swiftinterface
 // CHECK: @available(macOS 11.0, *)
 // CHECK-NEXT: public struct X
@@ -23,6 +33,14 @@ public struct Y { }
 // CHECK: @available(*, unavailable)
 // CHECK-NEXT: extension Library.Y{{( )?}}: @unchecked Swift.Sendable {
 
+// CHECK: @available(macOS, unavailable, introduced: 12.0)
+// CHECK-NEXT: @available(*, unavailable)
+// CHECK-NEXT: extension Library.X.A{{( )?}}: @unchecked Swift.Sendable {
+
+// CHECK: @available(macOS, unavailable, introduced: 11.0)
+// CHECK-NEXT: @available(*, unavailable)
+// CHECK-NEXT: extension Library.X.B{{( )?}}: @unchecked Swift.Sendable {
+
 // RUN: %target-swift-emit-module-interface(%t/Library.swiftinterface) %s -target %target-cpu-apple-macosx12.0 -DLIBRARY -module-name Library -module-interface-preserve-types-as-written
 // RUN: %target-swift-typecheck-module-from-interface(%t/Library.swiftinterface) -module-name Library
 // RUN: %FileCheck %s <%t/Library.swiftinterface