CustomAvailability: avoid exposing the synthesized function of dynamic availablity checking as an API entry

When exposing the function body of the synthesized function that checks whether a dynmic domain is available, the
downstream client may hit a deserialization issue.

Author: nkcsgexi

lib/ClangImporter/SwiftDeclSynthesizer.cpp

@@ -3183,9 +3183,9 @@ FuncDecl *SwiftDeclSynthesizer::makeAvailabilityDomainPredicate(
       BuiltinIntegerType::get(1, ctx), ImporterImpl.ImportedHeaderUnit);
   funcDecl->setBodySynthesizer(synthesizeAvailabilityDomainPredicateBody,
                                (void *)var);
-  funcDecl->setAccess(AccessLevel::Public);
+  funcDecl->setAccess(AccessLevel::Internal);
   funcDecl->addAttribute(
-      new (ctx) ExportAttr(ExportKind::Implementation, /*IsImplicit=*/true));
+      new (ctx) ExportAttr(ExportKind::Interface, /*IsImplicit=*/true));
 
   ImporterImpl.availabilityDomainPredicates[var] = funcDecl;
 

test/Availability/Inputs/AvailabilityDomains.h

@@ -0,0 +1,28 @@
+#include <availability_domain.h>
+
+int dynamic_domain_pred();
+
+CLANG_ENABLED_AVAILABILITY_DOMAIN(EnabledDomain);
+CLANG_DISABLED_AVAILABILITY_DOMAIN(DisabledDomain);
+CLANG_DYNAMIC_AVAILABILITY_DOMAIN(DynamicDomain, dynamic_domain_pred);
+
+#define AVAIL 0
+#define UNAVAIL 1
+
+__attribute__((availability(domain : EnabledDomain, AVAIL))) void
+available_in_enabled_domain(void);
+
+__attribute__((availability(domain : EnabledDomain, UNAVAIL))) void
+unavailable_in_enabled_domain(void);
+
+__attribute__((availability(domain : DisabledDomain, AVAIL))) void
+available_in_disabled_domain(void);
+
+__attribute__((availability(domain : DisabledDomain, UNAVAIL))) void
+unavailable_in_disabled_domain(void);
+
+__attribute__((availability(domain : DynamicDomain, AVAIL))) void
+available_in_dynamic_domain(void);
+
+__attribute__((availability(domain : DynamicDomain, UNAVAIL))) void
+unavailable_in_dynamic_domain(void);

test/Availability/availability_custom_domains_clang_serialization.swift

@@ -0,0 +1,42 @@
+// RUN: %empty-directory(%t/mod)
+// RUN: %target-swift-frontend -emit-module %s -o %t/mod/Foo.swiftmodule -parse-as-library -enable-library-evolution -module-name Foo -import-bridging-header %S/Inputs/AvailabilityDomains.h -enable-experimental-feature CustomAvailability -D FOO
+// RUN: %target-swift-frontend -emit-module %s -o %t/mod/Foo.swiftmodule -parse-as-library -enable-library-evolution -module-name Bar -import-bridging-header %S/Inputs/AvailabilityDomains.h -enable-experimental-feature CustomAvailability -I %t/mod -D BAR
+
+#if FOO
+
+@available(DynamicDomain)
+public struct X {
+    public init() { }
+}
+
+public struct Z {
+    public init() {
+        if #available(DynamicDomain) {
+            print("#available")
+            print(X())
+        } else {
+            print("#unavailable")
+        }
+    }
+}
+
+#endif
+
+#if BAR
+
+import Foo
+
+public struct Y {
+    init() {
+        if #available(DynamicDomain) {
+            print("#available")
+            print(X())
+            print(Z())
+        } else {
+            print("#unavailable")
+            print(Z())
+        }
+    }
+
+}
+#endif