AST: Introduce Decl::isSemanticallyUnavailable().

Adopt it in a few places as a replacement for `Decl::getSemanticUnavailableAttr()`.

Author: tshortli

include/swift/AST/Decl.h

@@ -1455,9 +1455,15 @@ class alignas(1 << DeclAlignInBits) Decl : public ASTAllocated<Decl>, public Swi
   std::optional<std::pair<const AvailableAttr *, const Decl *>>
   getSemanticUnavailableAttr(bool ignoreAppExtensions = false) const;
 
+  /// Returns true if the decl is effectively always unavailable in the current
+  /// compilation context. This query differs from \c isUnavailable() because it
+  /// takes the availability of parent declarations into account.
+  bool isSemanticallyUnavailable() const;
+
   /// Returns true if code associated with this declaration should be considerd
   /// unreachable at runtime because the declaration is unavailable in all
-  /// execution contexts in which the code may run.
+  /// execution contexts in which the code may run. This result takes the
+  /// availability of parent declarations into account.
   bool isUnreachableAtRuntime() const;
 
   /// Returns true if this declaration should be considered available during

lib/AST/Availability.cpp

@@ -763,6 +763,13 @@ SemanticDeclAvailabilityRequest::evaluate(Evaluator &evaluator,
   return SemanticDeclAvailability::PotentiallyAvailable;
 }
 
+bool Decl::isSemanticallyUnavailable() const {
+  auto availability = evaluateOrDefault(
+      getASTContext().evaluator, SemanticDeclAvailabilityRequest{this},
+      SemanticDeclAvailability::PotentiallyAvailable);
+  return availability != SemanticDeclAvailability::PotentiallyAvailable;
+}
+
 bool Decl::isUnreachableAtRuntime() const {
   auto availability = evaluateOrDefault(
       getASTContext().evaluator, SemanticDeclAvailabilityRequest{this},

lib/SIL/IR/SILProfiler.cpp

@@ -117,7 +117,7 @@ static bool shouldProfile(SILDeclRef Constant) {
 
   if (auto *D = DC->getInnermostDeclarationDeclContext()) {
     // Do not profile AST nodes in unavailable contexts.
-    if (D->getSemanticUnavailableAttr()) {
+    if (D->isSemanticallyUnavailable()) {
       LLVM_DEBUG(llvm::dbgs() << "Skipping ASTNode: unavailable context\n");
       return false;
     }

lib/Sema/TypeCheckAttr.cpp

@@ -5007,7 +5007,7 @@ TypeChecker::diagnosticIfDeclCannotBeUnavailable(const Decl *D) {
   auto parentIsUnavailable = [](const Decl *D) -> bool {
     if (auto *parent =
             AvailabilityInference::parentDeclForInferredAvailability(D)) {
-      return parent->getSemanticUnavailableAttr() != std::nullopt;
+      return parent->isSemanticallyUnavailable();
     }
     return false;
   };

lib/Sema/TypeCheckDeclOverride.cpp

@@ -1924,7 +1924,7 @@ checkOverrideUnavailability(ValueDecl *override, ValueDecl *base) {
   if (auto *overrideParent = override->getDeclContext()->getAsDecl()) {
     // If the parent of the override is unavailable, then the unavailability of
     // the override decl is irrelevant.
-    if (overrideParent->getSemanticUnavailableAttr())
+    if (overrideParent->isSemanticallyUnavailable())
       return {OverrideUnavailabilityStatus::Ignored, nullptr};
   }
 

lib/Sema/TypeCheckProtocol.cpp

@@ -1881,7 +1881,7 @@ RequirementCheck WitnessChecker::checkWitness(ValueDecl *requirement,
       }
 
       if (auto adoptingNominal = DC->getSelfNominalTypeDecl()) {
-        if (adoptingNominal->getSemanticUnavailableAttr())
+        if (adoptingNominal->isSemanticallyUnavailable())
           return true;
       }