Sema: Relax @backDeployed availability conflict diagnostics.

Only diagnose `@backDeployed` as conflicting with unavailability if the
attribute that is making the declaration unavailable is unconditional or it is
for the same base platform. For example, it should be allowed to back deploy a
function on macOS while making that function unavailable for application
extensions on macOS.

Resolves rdar://107291474

Author: tshortli

lib/Sema/TypeCheckAttr.cpp

@@ -4320,15 +4320,19 @@ void AttributeChecker::checkBackDeployedAttrs(
     // Unavailable decls cannot be back deployed.
     if (auto unavailableAttrPair = VD->getSemanticUnavailableAttr()) {
       auto unavailableAttr = unavailableAttrPair.value().first;
-      DeclName name;
-      unsigned accessorKind;
-      std::tie(accessorKind, name) = getAccessorKindAndNameForDiagnostics(VD);
-      diagnose(AtLoc, diag::attr_has_no_effect_on_unavailable_decl, Attr,
-               accessorKind, name, prettyPlatformString(Platform));
-      diagnose(unavailableAttr->AtLoc, diag::availability_marked_unavailable,
-               accessorKind, name)
-          .highlight(unavailableAttr->getRange());
-      continue;
+
+      if (unavailableAttr->Platform == PlatformKind::none ||
+          unavailableAttr->Platform == Attr->Platform) {
+        DeclName name;
+        unsigned accessorKind;
+        std::tie(accessorKind, name) = getAccessorKindAndNameForDiagnostics(VD);
+        diagnose(AtLoc, diag::attr_has_no_effect_on_unavailable_decl, Attr,
+                 accessorKind, name, prettyPlatformString(Platform));
+        diagnose(unavailableAttr->AtLoc, diag::availability_marked_unavailable,
+                 accessorKind, name)
+            .highlight(unavailableAttr->getRange());
+        continue;
+      }
     }
 
     // Verify that the decl is available before the back deployment boundary.

test/attr/File

@@ -0,0 +1,9 @@
+// RUN: %target-typecheck-verify-swift -parse-as-library
+// RUN: %target-typecheck-verify-swift -parse-as-library -application-extension
+
+@available(macOS 11, *)
+@available(macOSApplicationExtension, unavailable)
+public struct UnavailableMacOSExtensionsStruct {
+  @backDeployed(before: macOS 12)
+  public func memberFunc() {}
+}