Sema: Diagnose availability of availability domains in if #available queries.

When emitting statement diagnostics for `if #available` queries, diagnose the
availability of the decls representing the referenced availability domains.
Among other things, this checks that the domains are sufficiently visible to be
used in the containing function body context.

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;

test/ClangImporter/availability_custom_domains_access.swift

@@ -0,0 +1,30 @@
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -verify \
+// RUN:   -import-objc-header %S/Inputs/availability_domains_bridging_header.h \
+// RUN:   -I %S/../Inputs/custom-modules/availability-domains \
+// RUN:   -enable-experimental-feature CustomAvailability \
+// RUN:   %s
+
+// REQUIRES: swift_feature_CustomAvailability
+
+private import Rivers // also re-exported by Oceans
+internal import Oceans
+// expected-note@-1 {{availability domain 'Arctic' imported as 'internal' from 'Oceans' here}}
+// expected-note@-2 {{availability domain 'Colorado' imported as 'internal' from 'Oceans' here}}
+// expected-note@-3 {{availability domain 'Grand' imported as 'internal' from 'Oceans' here}}
+public import Seas
+
+@inlinable public func inlinableFunc() {
+  if #available(Colorado) { } // expected-error {{availability domain 'Colorado' is internal and cannot be referenced from an '@inlinable' function}}
+  if #available(Grand) { } // expected-error {{availability domain 'Grand' is internal and cannot be referenced from an '@inlinable' function}}
+  if #available(Arctic) { } // expected-error {{availability domain 'Arctic' is internal and cannot be referenced from an '@inlinable' function}}
+  if #available(Baltic) { }
+  if #available(BayBridge) { }
+}
+
+public func nonInlinablePublicFunc() {
+  if #available(Colorado) { }
+  if #available(Grand) { } // expected-warning {{availability domain 'Grand' is deprecated: Use Colorado instead}}
+  if #available(Arctic) { }
+  if #available(Baltic) { }
+  if #available(BayBridge) { }
+}

test/Inputs/custom-modules/availability-domains/Rivers.h

@@ -3,6 +3,10 @@
 static struct __AvailabilityDomain colorado_domain __attribute__((
     availability_domain(Colorado))) = {__AVAILABILITY_DOMAIN_DISABLED, 0};
 
+__attribute__((deprecated("Use Colorado instead")))
+static struct __AvailabilityDomain grand_domain __attribute__((
+    availability_domain(Grand))) = {__AVAILABILITY_DOMAIN_DISABLED, 0};
+
 #define AVAIL 0
 #define UNAVAIL 1