Sema: Require experimental feature to use custom availability domains.

Author: tshortli

include/swift/AST/DiagnosticsSema.def

@@ -6731,6 +6731,10 @@ WARNING(attr_availability_cannot_be_used_for_domain, none,
 WARNING(attr_availability_expected_version_spec, none,
        "expected 'introduced', 'deprecated', or 'obsoleted' in '%0' attribute "
        "for platform '%1'", (StringRef, StringRef))
+ERROR(attr_availability_requires_custom_availability, none,
+      "specifying '%0' in '%1' attribute requires "
+      "-enable-experimental-feature CustomAvailability",
+      (StringRef, const DeclAttribute))
 
 ERROR(availability_decl_unavailable, none,
       "%0 is unavailable%select{ in %2|}1%select{|: %3}3",

lib/Sema/TypeCheckAttr.cpp

@@ -8386,6 +8386,15 @@ SemanticAvailableAttrRequest::evaluate(swift::Evaluator &evaluator,
       return std::nullopt;
     }
 
+    if (domain->isCustom() &&
+        !ctx.LangOpts.hasFeature(Feature::CustomAvailability) &&
+        !declContext->isInSwiftinterface()) {
+      diags.diagnose(domainLoc,
+                     diag::attr_availability_requires_custom_availability,
+                     *domainName, attr);
+      return std::nullopt;
+    }
+
     mutableAttr->setCachedDomain(*domain);
   }
 

test/attr/attr_availability_custom_domains_experimental_feature_required.swift

@@ -0,0 +1,5 @@
+// RUN: %target-typecheck-verify-swift \
+// RUN:  -define-enabled-availability-domain SomeDomain
+
+@available(SomeDomain, unavailable) // expected-error {{specifying 'SomeDomain' in '@available' attribute requires -enable-experimental-feature CustomAvailability}}
+func availableInSomeDomain() { }