Sema: Check access, availability, and exportability of availability domains

include/swift/AST/DiagnosticsSema.def

@@ -6989,6 +6989,17 @@ WARNING(attr_availability_expected_version_spec, none,
 ERROR(attr_availability_requires_custom_availability, none,
       "%0 requires '-enable-experimental-feature CustomAvailability'",
       (AvailabilityDomain))
+ERROR(attr_availability_domain_access, none,
+      "availability domain '%0' is "
+      "%select{private|fileprivate|internal|package|%error|%error}1 "
+      "and cannot be used in '%2' on "
+      "%select{private|fileprivate|internal|package|public|%error}3 %kind4",
+      (AvailabilityDomain, AccessLevel, DeclAttribute, AccessLevel,
+       const Decl *))
+ERROR(attr_availability_domain_not_usable_from_inline, none,
+      "availability domain '%0' used in '%1' on %kind2 must be "
+      "'@usableFromInline' or public",
+      (AvailabilityDomain, DeclAttribute, const Decl *))
 
 ERROR(availability_decl_unavailable, none,
       "%0 is unavailable%select{ in %2|}1%select{|: %3}3",

lib/AST/AvailabilityDomain.cpp

@@ -41,9 +41,7 @@ getCustomDomainKind(clang::FeatureAvailKind featureAvailKind) {
 static const CustomAvailabilityDomain *
 customDomainForClangDecl(ValueDecl *decl) {
   auto *clangDecl = decl->getClangDecl();
-  ASSERT(clangDecl);
-
-  auto *varDecl = dyn_cast<clang::VarDecl>(clangDecl);
+  auto *varDecl = dyn_cast_or_null<clang::VarDecl>(clangDecl);
   if (!varDecl)
     return nullptr;
 

lib/AST/DiagnosticEngine.cpp

@@ -16,6 +16,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "swift/AST/DiagnosticEngine.h"
+#include "swift/AST/AvailabilityDomain.h"
 #include "swift/AST/ASTContext.h"
 #include "swift/AST/ASTPrinter.h"
 #include "swift/AST/Decl.h"
@@ -877,6 +878,20 @@ static void formatDiagnosticArgument(StringRef Modifier,
       assert(Modifier.empty() && "Improper modifier for ValueDecl argument");
     }
 
+    // Handle declarations representing an AvailabilityDomain specially.
+    if (auto VD = dyn_cast<ValueDecl>(D)) {
+      if (auto domain = AvailabilityDomain::forCustom(const_cast<ValueDecl *>(VD))) {
+        Out << "availability domain";
+
+        if (includeName) {
+          Out << " " << FormatOpts.OpeningQuotationMark;
+          Out << domain->getNameForDiagnostics();
+          Out << FormatOpts.ClosingQuotationMark;
+        }
+        break;
+      }
+    }
+
     if (includeName) {
       if (auto accessor = dyn_cast<AccessorDecl>(D)) {
         // If it's an accessor, describe that and then switch to discussing its

lib/Sema/MiscDiagnostics.cpp

@@ -5257,6 +5257,12 @@ static bool diagnoseAvailabilityCondition(PoundAvailableInfo *info,
       return true;
     }
 
+    // Check the availability of the domain decl.
+    if (auto *domainDecl = spec.getDomain().getDecl()) {
+      auto where = ExportContext::forFunctionBody(DC, loc);
+      diagnoseDeclAvailability(domainDecl, loc, nullptr, where);
+    }
+
     hasValidSpecs = true;
     if (!domain.isPlatform())
       allValidSpecsArePlatform = false;