AST: Introduce UnmetAvailabilityRequirement::isConditionallySatisfiable().

Author: tshortli

include/swift/AST/Availability.h

@@ -401,6 +401,10 @@ class UnmetAvailabilityRequirement {
   /// `std::nullopt` otherwise.
   std::optional<AvailabilityRange>
   getRequiredNewerAvailabilityRange(ASTContext &ctx) const;
+
+  /// Returns true if this unmet requirement can be satisfied by introducing an
+  /// `if #available(...)` condition in source.
+  bool isConditionallySatisfiable() const;
 };
 
 class AvailabilityInference {

lib/AST/Availability.cpp

@@ -77,6 +77,17 @@ UnmetAvailabilityRequirement::getRequiredNewerAvailabilityRange(
   }
 }
 
+bool UnmetAvailabilityRequirement::isConditionallySatisfiable() const {
+  switch (kind) {
+  case Kind::AlwaysUnavailable:
+  case Kind::RequiresVersion:
+  case Kind::Obsoleted:
+    return false;
+  case Kind::IntroducedInNewerVersion:
+    return true;
+  }
+}
+
 namespace {
 
 /// The inferred availability required to access a group of declarations

lib/Sema/TypeCheckAvailability.cpp

@@ -4140,12 +4140,8 @@ bool swift::diagnoseDeclAvailability(const ValueDecl *D, SourceRange R,
 
   auto unmetRequirement =
       getUnmetDeclAvailabilityRequirement(D, DC, Where.getAvailability());
-  auto requiredRange =
-      unmetRequirement
-          ? unmetRequirement->getRequiredNewerAvailabilityRange(ctx)
-          : std::nullopt;
 
-  if (unmetRequirement && !requiredRange) {
+  if (unmetRequirement && !unmetRequirement->isConditionallySatisfiable()) {
     // FIXME: diagnoseExplicitUnavailability should take an unmet requirement
     if (diagnoseExplicitUnavailability(D, R, Where, call, Flags))
       return true;
@@ -4169,6 +4165,11 @@ bool swift::diagnoseDeclAvailability(const ValueDecl *D, SourceRange R,
         && isa<ProtocolDecl>(D))
     return false;
 
+  if (!unmetRequirement)
+    return false;
+
+  auto requiredRange = unmetRequirement->getRequiredNewerAvailabilityRange(ctx);
+
   // Diagnose (and possibly signal) for potential unavailability
   if (!requiredRange)
     return false;

lib/Sema/TypeCheckPattern.cpp

@@ -39,12 +39,11 @@ using namespace swift;
 static EnumElementDecl *
 extractEnumElement(DeclContext *DC, SourceLoc UseLoc,
                    const VarDecl *constant) {
-  auto &ctx = DC->getASTContext();
   auto availabilityContext = TypeChecker::availabilityAtLocation(UseLoc, DC);
   if (auto requirement = getUnmetDeclAvailabilityRequirement(
           constant, DC, availabilityContext)) {
     // Only diagnose explicit unavailability.
-    if (!requirement->getRequiredNewerAvailabilityRange(ctx))
+    if (!requirement->isConditionallySatisfiable())
       diagnoseDeclAvailability(constant, UseLoc, nullptr,
                                ExportContext::forFunctionBody(DC, UseLoc));
   }