Add a priority field to availability attributes to prioritize explicit attributes from declaration over attributes from '#pragma clang attribute'

[arphaman]

Summary

We have an issue when using #pragma clang attribute with availability attributes:

• The explicit attribute that's specified next to the declaration is not guaranteed to be preferred over the attribute specified in the pragma.

This patch fixes this by introducing a priority field to the availability attribute to control how they're merged. Attributes with higher priority are applied over attributes with lower priority for the same platform. The implicitly inferred attributes are given the lower priority. This ensures that:

1. explicit attributes are preferred over all other attributes.
2. implicitly inferred attributes that are inferred from an explicit attribute are discarded if there's an explicit attribute or an attribute specified using a #pragma for the same platform.
3. implicitly inferred attributes that are inferred from an attribute in the #pragma are not used if there's an explicit, explicit #pragma, or an implicit attribute inferred from an explicit attribute for the declaration.

This is the resulting ranking:

platform availability > platform availability from pragma > inferred availability > inferred availability from pragma

rdar://46390243

[arphaman]

Reverse the priority numbering direction as suggested by @dexonsmith .

Now the lower priority is the most important, and the less important priorities have a higher numerical value.

[aaron.ballman]

I think the documentation for the attribute should be updated to explain this new behavior, otherwise this will be a very inexplicable feature to users.

One question I have is: will this feature also be needed by other attributes? This seems like a somewhat general problem between explicit attributes, implicit attributes, and attributes added via the pragma. It might not be worth generalizing yet, but I'm curious to know whether we'll need this for things like visibility and type_visibility as well.

include/clang/Sema/Sema.h

2467

We should have a comment somewhere (perhaps here) that makes it clear that the priorities we calculate and store on the semantic attribute object are not expected to match values in this enumeration, but instead be treated as a plain integer value. This enumeration is just giving a name to some of the addends used to calculate that value.

[erik.pilkington]

This seems pretty reasonable to me. I agree that a more general mechanism to override #pca (/implicit) attributes would be pretty useful, but I guess there is no need to jump the gun on that.

include/clang/Sema/Sema.h

2471

Maybe add = 0 (and =1 and =2) explicitly to indicate that their relative values matter here.

lib/Sema/SemaDeclAttr.cpp

2479

Use AP_Explicit here instead of 0?

2483

... And the avoid adding it back on here?

[arphaman]

• Add documentation and expand a comment as suggested by Aaron.
• Address Erik's comments.

[arphaman]

In D56892#1363089, @aaron.ballman wrote:

I think the documentation for the attribute should be updated to explain this new behavior, otherwise this will be a very inexplicable feature to users.

Done

One question I have is: will this feature also be needed by other attributes? This seems like a somewhat general problem between explicit attributes, implicit attributes, and attributes added via the pragma. It might not be worth generalizing yet, but I'm curious to know whether we'll need this for things like visibility and type_visibility as well.

Possibly. We haven't come across a use case that requires that yet, but we would be definitely willing to work on a more generalized solution in the future if such a need arises.

[erik.pilkington]

LGTM, thanks!

[aaron.ballman]

LGTM with a suggested edit for the docs (which look great, btw!).

include/clang/Basic/AttrDocs.td

1252 ↗

(On Diff #182608)

tvOS whose -> tvOS, whose

1253 ↗

(On Diff #182608)

attribute the logic -> attribute, the logic

include/clang/Basic/Attr.td

def AsmLabel : InheritableAttr {
   let Documentation = [Undocumented];
 }
 
 def Availability : InheritableAttr {
   let Spellings = [Clang<"availability">];
   let Args = [IdentifierArgument<"platform">, VersionArgument<"introduced">,
               VersionArgument<"deprecated">, VersionArgument<"obsoleted">,
               BoolArgument<"unavailable">, StringArgument<"message">,
-              BoolArgument<"strict">, StringArgument<"replacement">];
+              BoolArgument<"strict">, StringArgument<"replacement">,
+              IntArgument<"priority">];
   let AdditionalMembers =
 [{static llvm::StringRef getPrettyPlatformName(llvm::StringRef Platform) {
     return llvm::StringSwitch<llvm::StringRef>(Platform)
              .Case("android", "Android")
              .Case("ios", "iOS")
              .Case("macos", "macOS")
              .Case("tvos", "tvOS")
              .Case("watchos", "watchOS")

include/clang/Sema/ParsedAttr.h

private:
   unsigned HasParsedType : 1;
 
   /// True if the processing cache is valid.
   mutable unsigned HasProcessingCache : 1;
 
   /// A cached value.
   mutable unsigned ProcessingCache : 8;
 
+  /// True if the attribute is specified using '#pragma clang attribute'.
+  mutable unsigned IsPragmaClangAttribute : 1;
+
   /// The location of the 'unavailable' keyword in an
   /// availability attribute.
   SourceLocation UnavailableLoc;
 
   const Expr *MessageExpr;
 
   ArgsUnion *getArgsBuffer() { return getTrailingObjects<ArgsUnion>(); }
   ArgsUnion const *getArgsBuffer() const {
private:
   ParsedAttr(IdentifierInfo *attrName, SourceRange attrRange,
              IdentifierInfo *scopeName, SourceLocation scopeLoc,
              ArgsUnion *args, unsigned numArgs, Syntax syntaxUsed,
              SourceLocation ellipsisLoc)
       : AttrName(attrName), ScopeName(scopeName), AttrRange(attrRange),
         ScopeLoc(scopeLoc), EllipsisLoc(ellipsisLoc), NumArgs(numArgs),
         SyntaxUsed(syntaxUsed), Invalid(false), UsedAsTypeAttr(false),
         IsAvailability(false), IsTypeTagForDatatype(false), IsProperty(false),
-        HasParsedType(false), HasProcessingCache(false) {
+        HasParsedType(false), HasProcessingCache(false),
+        IsPragmaClangAttribute(false) {
     if (numArgs) memcpy(getArgsBuffer(), args, numArgs * sizeof(ArgsUnion));
     AttrKind = getKind(getName(), getScopeName(), syntaxUsed);
   }
 
   /// Constructor for availability attributes.
   ParsedAttr(IdentifierInfo *attrName, SourceRange attrRange,
              IdentifierInfo *scopeName, SourceLocation scopeLoc,
              IdentifierLoc *Parm, const AvailabilityChange &introduced,
              const AvailabilityChange &deprecated,
              const AvailabilityChange &obsoleted, SourceLocation unavailable,
              const Expr *messageExpr, Syntax syntaxUsed, SourceLocation strict,
              const Expr *replacementExpr)
       : AttrName(attrName), ScopeName(scopeName), AttrRange(attrRange),
         ScopeLoc(scopeLoc), NumArgs(1), SyntaxUsed(syntaxUsed), Invalid(false),
         UsedAsTypeAttr(false), IsAvailability(true),
         IsTypeTagForDatatype(false), IsProperty(false), HasParsedType(false),
-        HasProcessingCache(false), UnavailableLoc(unavailable),
-        MessageExpr(messageExpr) {
+        HasProcessingCache(false), IsPragmaClangAttribute(false),
+        UnavailableLoc(unavailable), MessageExpr(messageExpr) {
     ArgsUnion PVal(Parm);
     memcpy(getArgsBuffer(), &PVal, sizeof(ArgsUnion));
     new (getAvailabilityData()) detail::AvailabilityData(
         introduced, deprecated, obsoleted, strict, replacementExpr);
     AttrKind = getKind(getName(), getScopeName(), syntaxUsed);
   }
 
   /// Constructor for objc_bridge_related attributes.
   ParsedAttr(IdentifierInfo *attrName, SourceRange attrRange,
              IdentifierInfo *scopeName, SourceLocation scopeLoc,
              IdentifierLoc *Parm1, IdentifierLoc *Parm2, IdentifierLoc *Parm3,
              Syntax syntaxUsed)
       : AttrName(attrName), ScopeName(scopeName), AttrRange(attrRange),
         ScopeLoc(scopeLoc), NumArgs(3), SyntaxUsed(syntaxUsed), Invalid(false),
         UsedAsTypeAttr(false), IsAvailability(false),
         IsTypeTagForDatatype(false), IsProperty(false), HasParsedType(false),
-        HasProcessingCache(false) {
+        HasProcessingCache(false), IsPragmaClangAttribute(false) {
     ArgsUnion *Args = getArgsBuffer();
     Args[0] = Parm1;
     Args[1] = Parm2;
     Args[2] = Parm3;
     AttrKind = getKind(getName(), getScopeName(), syntaxUsed);
   }
 
   /// Constructor for type_tag_for_datatype attribute.
   ParsedAttr(IdentifierInfo *attrName, SourceRange attrRange,
              IdentifierInfo *scopeName, SourceLocation scopeLoc,
              IdentifierLoc *ArgKind, ParsedType matchingCType,
              bool layoutCompatible, bool mustBeNull, Syntax syntaxUsed)
       : AttrName(attrName), ScopeName(scopeName), AttrRange(attrRange),
         ScopeLoc(scopeLoc), NumArgs(1), SyntaxUsed(syntaxUsed), Invalid(false),
         UsedAsTypeAttr(false), IsAvailability(false),
         IsTypeTagForDatatype(true), IsProperty(false), HasParsedType(false),
-        HasProcessingCache(false) {
+        HasProcessingCache(false), IsPragmaClangAttribute(false) {
     ArgsUnion PVal(ArgKind);
     memcpy(getArgsBuffer(), &PVal, sizeof(ArgsUnion));
     detail::TypeTagForDatatypeData &ExtraData = getTypeTagForDatatypeDataSlot();
     new (&ExtraData.MatchingCType) ParsedType(matchingCType);
     ExtraData.LayoutCompatible = layoutCompatible;
     ExtraData.MustBeNull = mustBeNull;
     AttrKind = getKind(getName(), getScopeName(), syntaxUsed);
   }
 
   /// Constructor for attributes with a single type argument.
   ParsedAttr(IdentifierInfo *attrName, SourceRange attrRange,
              IdentifierInfo *scopeName, SourceLocation scopeLoc,
              ParsedType typeArg, Syntax syntaxUsed)
       : AttrName(attrName), ScopeName(scopeName), AttrRange(attrRange),
         ScopeLoc(scopeLoc), NumArgs(0), SyntaxUsed(syntaxUsed), Invalid(false),
         UsedAsTypeAttr(false), IsAvailability(false),
         IsTypeTagForDatatype(false), IsProperty(false), HasParsedType(true),
-        HasProcessingCache(false) {
+        HasProcessingCache(false), IsPragmaClangAttribute(false) {
     new (&getTypeBuffer()) ParsedType(typeArg);
     AttrKind = getKind(getName(), getScopeName(), syntaxUsed);
   }
 
   /// Constructor for microsoft __declspec(property) attribute.
   ParsedAttr(IdentifierInfo *attrName, SourceRange attrRange,
              IdentifierInfo *scopeName, SourceLocation scopeLoc,
              IdentifierInfo *getterId, IdentifierInfo *setterId,
              Syntax syntaxUsed)
       : AttrName(attrName), ScopeName(scopeName), AttrRange(attrRange),
         ScopeLoc(scopeLoc), NumArgs(0), SyntaxUsed(syntaxUsed), Invalid(false),
         UsedAsTypeAttr(false), IsAvailability(false),
         IsTypeTagForDatatype(false), IsProperty(true), HasParsedType(false),
-        HasProcessingCache(false) {
+        HasProcessingCache(false), IsPragmaClangAttribute(false) {
     new (&getPropertyDataBuffer()) detail::PropertyData(getterId, setterId);
     AttrKind = getKind(getName(), getScopeName(), syntaxUsed);
   }
 
   /// Type tag information is stored immediately following the arguments, if
   /// any, at the end of the object.  They are mutually exclusive with
   /// availability slots.
   detail::TypeTagForDatatypeData &getTypeTagForDatatypeDataSlot() {
public:
   void setProcessingCache(unsigned value) const {
     ProcessingCache = value;
     HasProcessingCache = true;
   }
 
   bool isUsedAsTypeAttr() const { return UsedAsTypeAttr; }
   void setUsedAsTypeAttr() { UsedAsTypeAttr = true; }
 
+  /// True if the attribute is specified using '#pragma clang attribute'.
+  bool isPragmaClangAttribute() const { return IsPragmaClangAttribute; }
+
+  void setIsPragmaClangAttribute() { IsPragmaClangAttribute = true; }
+
   bool isPackExpansion() const { return EllipsisLoc.isValid(); }
   SourceLocation getEllipsisLoc() const { return EllipsisLoc; }
 
   Kind getKind() const { return Kind(AttrKind); }
   static Kind getKind(const IdentifierInfo *Name, const IdentifierInfo *Scope,
                       Syntax SyntaxUsed);
 
   /// getNumArgs - Return the number of actual arguments to this attribute.

include/clang/Sema/Sema.h

enum AvailabilityMergeKind {
     /// Merge availability attributes for an override, which requires
     /// an exact match or a weakening of constraints.
     AMK_Override,
     /// Merge availability attributes for an implementation of
     /// a protocol requirement.
     AMK_ProtocolImplementation,
   };
 
+  /// Describes the kind of priority given to an availability attribute.
+  ///
+  /// The sum of priorities deteremines the final priority of the attribute.
+  /// The final priority determines how the attribute will be merged.
+  /// An attribute with a higher priority will always remove lower priority
+  /// attributes for the specified platform when it is being applied. An
+  /// attribute with a lower priority will not be applied if the declaration
+  /// already has an availability attribute with a higher priority for the
+  /// specified platform.
+  enum AvailabilityPriority : int {
+    /// The availability attribute was specified explicitly next to the
+    /// declaration.
+    AP_Explicit = 0,
+
+    /// The availability attribute was applied using '#pragma clang attribute'.
+    AP_PragmaClangAttribute = -1,
+
+    /// The availability attribute for a specific platform was inferred from
+    /// an availability attribute for another platform.
+    AP_InferredFromOtherPlatform = -2
+  };
+
   /// Attribute merging methods. Return true if a new attribute was added.
-  AvailabilityAttr *mergeAvailabilityAttr(NamedDecl *D, SourceRange Range,
-                                          IdentifierInfo *Platform,
-                                          bool Implicit,
-                                          VersionTuple Introduced,
-                                          VersionTuple Deprecated,
-                                          VersionTuple Obsoleted,
-                                          bool IsUnavailable,
-                                          StringRef Message,
-                                          bool IsStrict, StringRef Replacement,
-                                          AvailabilityMergeKind AMK,
-                                          unsigned AttrSpellingListIndex);
+  AvailabilityAttr *mergeAvailabilityAttr(
+      NamedDecl *D, SourceRange Range, IdentifierInfo *Platform, bool Implicit,
+      VersionTuple Introduced, VersionTuple Deprecated, VersionTuple Obsoleted,
+      bool IsUnavailable, StringRef Message, bool IsStrict,
+      StringRef Replacement, AvailabilityMergeKind AMK, int Priority,
   TypeVisibilityAttr *mergeTypeVisibilityAttr(Decl *D, SourceRange Range,
                                        TypeVisibilityAttr::VisibilityType Vis,
                                               unsigned AttrSpellingListIndex);
   VisibilityAttr *mergeVisibilityAttr(Decl *D, SourceRange Range,
                                       VisibilityAttr::VisibilityType Vis,
                                       unsigned AttrSpellingListIndex);
   UuidAttr *mergeUuidAttr(Decl *D, SourceRange Range,
                           unsigned AttrSpellingListIndex, StringRef Uuid);

lib/Sema/SemaAttr.cpp

attrMatcherRuleListToString(ArrayRef<attr::SubjectMatchRule> Rules) {
   return OS.str();
 }
 
 } // end anonymous namespace
 
 void Sema::ActOnPragmaAttributeAttribute(
     ParsedAttr &Attribute, SourceLocation PragmaLoc,
     attr::ParsedSubjectMatchRuleSet Rules) {
+  Attribute.setIsPragmaClangAttribute();
   SmallVector<attr::SubjectMatchRule, 4> SubjectMatchRules;
   // Gather the subject match rules that are supported by the attribute.
   SmallVector<std::pair<attr::SubjectMatchRule, bool>, 4>
       StrictSubjectMatchRuleSet;
   Attribute.getMatchRules(LangOpts, StrictSubjectMatchRuleSet);
 
   // Figure out which subject matching rules are valid.
   if (StrictSubjectMatchRuleSet.empty()) {

 
 void Sema::AddPragmaAttributes(Scope *S, Decl *D) {
   if (PragmaAttributeStack.empty())
     return;
   for (auto &Group : PragmaAttributeStack) {
     for (auto &Entry : Group.Entries) {
       ParsedAttr *Attribute = Entry.Attribute;
       assert(Attribute && "Expected an attribute");
+      assert(Attribute->isPragmaClangAttribute() &&
+             "expected #pragma clang attribute");
 
       // Ensure that the attribute can be applied to the given declaration.
       bool Applies = false;
       for (const auto &Rule : Entry.MatchRules) {
         if (Attribute->appliesToDecl(D, Rule)) {
           Applies = true;
           break;
         }

lib/Sema/SemaDecl.cpp

static bool mergeDeclAttribute(Sema &S, NamedDecl *D,
   // yet or if that attribute allows duplicates.
   // If you're adding a new attribute that requires logic different from
   // "use explicit attribute on decl if present, else use attribute from
   // previous decl", for example if the attribute needs to be consistent
   // between redeclarations, you need to call a custom merge function here.
   InheritableAttr *NewAttr = nullptr;
   unsigned AttrSpellingListIndex = Attr->getSpellingListIndex();
   if (const auto *AA = dyn_cast<AvailabilityAttr>(Attr))
-    NewAttr = S.mergeAvailabilityAttr(D, AA->getRange(), AA->getPlatform(),
-                                      AA->isImplicit(), AA->getIntroduced(),
-                                      AA->getDeprecated(),
-                                      AA->getObsoleted(), AA->getUnavailable(),
-                                      AA->getMessage(), AA->getStrict(),
-                                      AA->getReplacement(), AMK,
-                                      AttrSpellingListIndex);
+    NewAttr = S.mergeAvailabilityAttr(
+        D, AA->getRange(), AA->getPlatform(), AA->isImplicit(),
+        AA->getIntroduced(), AA->getDeprecated(), AA->getObsoleted(),
+        AA->getUnavailable(), AA->getMessage(), AA->getStrict(),
+        AA->getReplacement(), AMK, AA->getPriority(), AttrSpellingListIndex);
   else if (const auto *VA = dyn_cast<VisibilityAttr>(Attr))
     NewAttr = S.mergeVisibilityAttr(D, VA->getRange(), VA->getVisibility(),
                                     AttrSpellingListIndex);
   else if (const auto *VA = dyn_cast<TypeVisibilityAttr>(Attr))
     NewAttr = S.mergeTypeVisibilityAttr(D, VA->getRange(), VA->getVisibility(),
                                         AttrSpellingListIndex);
   else if (const auto *ImportA = dyn_cast<DLLImportAttr>(Attr))
     NewAttr = S.mergeDLLImportAttr(D, ImportA->getRange(),

lib/Sema/SemaDeclAttr.cpp

if (X == Y)
     return true;
 
   if (BeforeIsOkay && X < Y)
     return true;
 
   return false;
 }
 
-AvailabilityAttr *Sema::mergeAvailabilityAttr(NamedDecl *D, SourceRange Range,
-                                              IdentifierInfo *Platform,
-                                              bool Implicit,
-                                              VersionTuple Introduced,
-                                              VersionTuple Deprecated,
-                                              VersionTuple Obsoleted,
-                                              bool IsUnavailable,
-                                              StringRef Message,
-                                              bool IsStrict,
-                                              StringRef Replacement,
-                                              AvailabilityMergeKind AMK,
-                                              unsigned AttrSpellingListIndex) {
+AvailabilityAttr *Sema::mergeAvailabilityAttr(
+    NamedDecl *D, SourceRange Range, IdentifierInfo *Platform, bool Implicit,
+    VersionTuple Introduced, VersionTuple Deprecated, VersionTuple Obsoleted,
+    bool IsUnavailable, StringRef Message, bool IsStrict, StringRef Replacement,
+    AvailabilityMergeKind AMK, int Priority, unsigned AttrSpellingListIndex) {
   VersionTuple MergedIntroduced = Introduced;
   VersionTuple MergedDeprecated = Deprecated;
   VersionTuple MergedObsoleted = Obsoleted;
   bool FoundAny = false;
   bool OverrideOrImpl = false;
   switch (AMK) {
   case AMK_None:
   case AMK_Redeclaration:
for (unsigned i = 0, e = Attrs.size(); i != e;) {
 
       IdentifierInfo *OldPlatform = OldAA->getPlatform();
       if (OldPlatform != Platform) {
         ++i;
         continue;
       }
 
       // If there is an existing availability attribute for this platform that
-      // is explicit and the new one is implicit use the explicit one and
-      // discard the new implicit attribute.
-      if (!OldAA->isImplicit() && Implicit) {
+      // has a higher priority use the existing one and discard the new
+      // attribute.
+      if (OldAA->getPriority() > Priority)
         return nullptr;
-      }
 
-      // If there is an existing attribute for this platform that is implicit
-      // and the new attribute is explicit then erase the old one and
-      // continue processing the attributes.
-      if (!Implicit && OldAA->isImplicit()) {
+      // If there is an existing attribute for this platform that has a lower
+      // priority than the new attribute then erase the old one and continue
+      // processing the attributes.
+      if (OldAA->getPriority() < Priority) {
         Attrs.erase(Attrs.begin() + i);
         --e;
         continue;
       }
 
       FoundAny = true;
       VersionTuple OldIntroduced = OldAA->getIntroduced();
       VersionTuple OldDeprecated = OldAA->getDeprecated();
if (FoundAny &&
       MergedObsoleted == Obsoleted)
     return nullptr;
 
   // Only create a new attribute if !OverrideOrImpl, but we want to do
   // the checking.
   if (!checkAvailabilityAttr(*this, Range, Platform, MergedIntroduced,
                              MergedDeprecated, MergedObsoleted) &&
       !OverrideOrImpl) {
-    auto *Avail =  ::new (Context) AvailabilityAttr(Range, Context, Platform,
-                                            Introduced, Deprecated,
-                                            Obsoleted, IsUnavailable, Message,
-                                            IsStrict, Replacement,
-                                            AttrSpellingListIndex);
+    auto *Avail = ::new (Context)
+        AvailabilityAttr(Range, Context, Platform, Introduced, Deprecated,
+                         Obsoleted, IsUnavailable, Message, IsStrict,
+                         Replacement, Priority, AttrSpellingListIndex);
     Avail->setImplicit(Implicit);
     return Avail;
   }
   return nullptr;
 }
 
 static void handleAvailabilityAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
   if (!checkAttributeNumArgs(S, AL, 1))
if (II->isStr("swift")) {
     if (Introduced.isValid() || Obsoleted.isValid() ||
         (!IsUnavailable && !Deprecated.isValid())) {
       S.Diag(AL.getLoc(),
              diag::warn_availability_swift_unavailable_deprecated_only);
       return;
     }
   }
 
-  AvailabilityAttr *NewAttr = S.mergeAvailabilityAttr(ND, AL.getRange(), II,
-                                                      false/*Implicit*/,
-                                                      Introduced.Version,
-                                                      Deprecated.Version,
-                                                      Obsoleted.Version,
-                                                      IsUnavailable, Str,
-                                                      IsStrict, Replacement,
-                                                      Sema::AMK_None,
-                                                      Index);
+  int PriorityModifier =
+      AL.isPragmaClangAttribute() ? Sema::AP_PragmaClangAttribute : 0;
+  AvailabilityAttr *NewAttr = S.mergeAvailabilityAttr(
+      ND, AL.getRange(), II, false /*Implicit*/, Introduced.Version,
+      Deprecated.Version, Obsoleted.Version, IsUnavailable, Str, IsStrict,
+      Replacement, Sema::AMK_None, PriorityModifier + Sema::AP_Explicit, Index);
   if (NewAttr)
     D->addAttr(NewAttr);
 
   // Transcribe "ios" to "watchos" (and add a new attribute) if the versioning
   // matches before the start of the watchOS platform.
   if (S.Context.getTargetInfo().getTriple().isWatchOS()) {
     IdentifierInfo *NewII = nullptr;
     if (II->getName() == "ios")
if (NewII) {
 
           return VersionTuple(2, 0);
         };
 
         auto NewIntroduced = adjustWatchOSVersion(Introduced.Version);
         auto NewDeprecated = adjustWatchOSVersion(Deprecated.Version);
         auto NewObsoleted = adjustWatchOSVersion(Obsoleted.Version);
 
-        AvailabilityAttr *NewAttr = S.mergeAvailabilityAttr(ND,
-                                                            AL.getRange(),
-                                                            NewII,
-                                                            true/*Implicit*/,
-                                                            NewIntroduced,
-                                                            NewDeprecated,
-                                                            NewObsoleted,
-                                                            IsUnavailable, Str,
-                                                            IsStrict,
-                                                            Replacement,
-                                                            Sema::AMK_None,
-                                                            Index);
+        AvailabilityAttr *NewAttr = S.mergeAvailabilityAttr(
+            ND, AL.getRange(), NewII, true /*Implicit*/, NewIntroduced,
+            NewDeprecated, NewObsoleted, IsUnavailable, Str, IsStrict,
+            Replacement, Sema::AMK_None,
+            PriorityModifier + Sema::AP_InferredFromOtherPlatform, Index);
         if (NewAttr)
           D->addAttr(NewAttr);
       }
   } else if (S.Context.getTargetInfo().getTriple().isTvOS()) {
     // Transcribe "ios" to "tvos" (and add a new attribute) if the versioning
     // matches before the start of the tvOS platform.
     IdentifierInfo *NewII = nullptr;
     if (II->getName() == "ios")
       NewII = &S.Context.Idents.get("tvos");
     else if (II->getName() == "ios_app_extension")
       NewII = &S.Context.Idents.get("tvos_app_extension");
 
     if (NewII) {
-        AvailabilityAttr *NewAttr = S.mergeAvailabilityAttr(ND,
-                                                            AL.getRange(),
-                                                            NewII,
-                                                            true/*Implicit*/,
-                                                            Introduced.Version,
-                                                            Deprecated.Version,
-                                                            Obsoleted.Version,
-                                                            IsUnavailable, Str,
-                                                            IsStrict,
-                                                            Replacement,
-                                                            Sema::AMK_None,
-                                                            Index);
-        if (NewAttr)
-          D->addAttr(NewAttr);
+      AvailabilityAttr *NewAttr = S.mergeAvailabilityAttr(
+          ND, AL.getRange(), NewII, true /*Implicit*/, Introduced.Version,
+          Deprecated.Version, Obsoleted.Version, IsUnavailable, Str, IsStrict,
+          Replacement, Sema::AMK_None,
+          PriorityModifier + Sema::AP_InferredFromOtherPlatform, Index);
       }
   }
 }
 
 static void handleExternalSourceSymbolAttr(Sema &S, Decl *D,
                                            const ParsedAttr &AL) {
   if (!checkAttributeAtLeastNumArgs(S, AL, 1))
     return;

test/SemaObjC/attr-availability-priority.m

+// RUN: %clang_cc1 -triple arm64-apple-tvos12.0 -fsyntax-only -verify %s
+
+void explicit() __attribute__((availability(tvos, introduced=11.0, deprecated=12.0))); // expected-note {{marked deprecated here}}
+void inferred() __attribute__((availability(ios, introduced=11.0, deprecated=12.0))); // expected-note {{marked deprecated here}}
+void explicitOverInferred()
+__attribute__((availability(ios, introduced=11.0, deprecated=12.0)))
+__attribute__((availability(tvos, introduced=11.0)));
+void explicitOverInferred2()
+__attribute__((availability(tvos, introduced=11.0)))
+__attribute__((availability(ios, introduced=11.0, deprecated=12.0)));
+
+void simpleUsage() {
+  explicit(); // expected-warning{{'explicit' is deprecated: first deprecated in tvOS 12.0}}
+  inferred(); // expected-warning{{'inferred' is deprecated: first deprecated in tvOS 12.0}}
+  // ok, not deprecated for tvOS.
+  explicitOverInferred();
+  explicitOverInferred2();
+}
+
+#pragma clang attribute push (__attribute__((availability(tvos, introduced=11.0, deprecated=12.0))), apply_to=function)
+
+void explicitFromPragma(); // expected-note {{marked deprecated here}}
+void explicitWinsOverExplicitFromPragma() __attribute__((availability(tvos, introduced=11.0)));
+void implicitLosesOverExplicitFromPragma() __attribute__((availability(ios, introduced=11.0))); // expected-note {{marked deprecated here}}
+
+#pragma clang attribute pop
+
+#pragma clang attribute push (__attribute__((availability(ios, introduced=11.0, deprecated=12.0))), apply_to=function)
+
+void implicitFromPragma(); // expected-note {{marked deprecated here}}
+void explicitWinsOverImplicitFromPragma() __attribute__((availability(tvos, introduced=11.0)));
+void implicitWinsOverImplicitFromPragma() __attribute__((availability(ios, introduced=11.0)));
+
+#pragma clang attribute pop
+
+#pragma clang attribute push (__attribute__((availability(tvos, introduced=11.0, deprecated=12.0))), apply_to=function)
+#pragma clang attribute push (__attribute__((availability(ios, introduced=11.0, deprecated=11.3))), apply_to=function)
+
+void pragmaExplicitWinsOverPragmaImplicit(); // expected-note {{marked deprecated here}}
+
+#pragma clang attribute pop
+#pragma clang attribute pop
+
+void pragmaUsage() {
+  explicitFromPragma(); // expected-warning {{'explicitFromPragma' is deprecated: first deprecated in tvOS 12.0}}
+  explicitWinsOverExplicitFromPragma(); // ok
+  implicitLosesOverExplicitFromPragma(); // expected-warning {{'implicitLosesOverExplicitFromPragma' is deprecated: first deprecated in tvOS 12.0}}
+    
+  implicitFromPragma(); // expected-warning {{'implicitFromPragma' is deprecated: first deprecated in tvOS 12.0}}
+  explicitWinsOverImplicitFromPragma(); // ok
+  implicitWinsOverImplicitFromPragma(); // ok
+  pragmaExplicitWinsOverPragmaImplicit(); // expected-warning {{'pragmaExplicitWinsOverPragmaImplicit' is deprecated: first deprecated in tvOS 12.0}}
+}