AST: Introduce AvailabilityContext::forDeploymentTarget()

Author: slavapestov

include/swift/AST/Availability.h

@@ -240,6 +240,10 @@ class AvailabilityContext {
   /// Creates a context that requires certain versions of the target OS.
   explicit AvailabilityContext(VersionRange OSVersion) : OSVersion(OSVersion) {}
 
+  /// Creates a context that imposes the constraints of the ASTContext's
+  /// deployment target.
+  static AvailabilityContext forDeploymentTarget(ASTContext &Ctx);
+
   /// Creates a context that imposes no constraints.
   ///
   /// \see isAlwaysAvailable

lib/AST/Availability.cpp

@@ -14,6 +14,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "swift/AST/ASTContext.h"
 #include "swift/AST/Attr.h"
 #include "swift/AST/Decl.h"
 #include "swift/AST/Types.h"
@@ -24,6 +25,11 @@
 
 using namespace swift;
 
+AvailabilityContext AvailabilityContext::forDeploymentTarget(ASTContext &Ctx) {
+  return AvailabilityContext(
+      VersionRange::allGTE(Ctx.LangOpts.getMinPlatformVersion()));
+}
+
 namespace {
 
 /// The inferred availability required to access a group of declarations

lib/Sema/TypeCheckAvailability.cpp

@@ -592,8 +592,7 @@ void TypeChecker::buildTypeRefinementContextHierarchy(SourceFile &SF,
     // The root type refinement context reflects the fact that all parts of
     // the source file are guaranteed to be executing on at least the minimum
     // platform version.
-    AvailabilityContext MinPlatformReq{
-        VersionRange::allGTE(AC.LangOpts.getMinPlatformVersion())};
+    auto MinPlatformReq = AvailabilityContext::forDeploymentTarget(AC);
     RootTRC = TypeRefinementContext::createRoot(&SF, MinPlatformReq);
     SF.setTypeRefinementContext(RootTRC);
   }
@@ -638,8 +637,8 @@ TypeChecker::overApproximateAvailabilityAtLocation(SourceLoc loc,
   // this will be a real problem.
 
   // We can assume we are running on at least the minimum deployment target.
-  AvailabilityContext OverApproximateContext{
-    VersionRange::allGTE(getLangOpts().getMinPlatformVersion())};
+  auto OverApproximateContext =
+    AvailabilityContext::forDeploymentTarget(Context);
 
   while (DC && loc.isInvalid()) {
     const Decl *D = DC->getInnermostDeclarationDeclContext();