AST: Fix a regression in constraining an AvailabilityContext for a decl.

When building up AvailabilityContexts, we assume that all of the enclosing
decls have already been accounted for in the AvailabilityContext that we are
constraining. Therefore, it doesn't make sense to merge availability
constraints from the enclosing extension of the target decl.

Author: tshortli

include/swift/AST/AvailabilityConstraint.h

@@ -22,6 +22,7 @@
 #include "swift/AST/AvailabilityRange.h"
 #include "swift/AST/PlatformKind.h"
 #include "swift/Basic/LLVM.h"
+#include "swift/Basic/OptionSet.h"
 
 namespace swift {
 
@@ -165,12 +166,22 @@ class DeclAvailabilityConstraints {
   const_iterator end() const { return constraints.end(); }
 };
 
+enum class AvailabilityConstraintFlag : uint8_t {
+  /// By default, the availability constraints for the members of extensions
+  /// include the constraints for `@available` attributes that were written on
+  /// the enclosing extension, since these members can be referred to without
+  /// referencing the extension. When this flag is specified, though, only the
+  /// attributes directly attached to the declaration are considered.
+  SkipEnclosingExtension = 1 << 0,
+};
+using AvailabilityConstraintFlags = OptionSet<AvailabilityConstraintFlag>;
+
 /// Returns the set of availability constraints that restrict use of \p decl
 /// when it is referenced from the given context. In other words, it is the
 /// collection of of `@available` attributes with unsatisfied conditions.
-DeclAvailabilityConstraints
-getAvailabilityConstraintsForDecl(const Decl *decl,
-                                  const AvailabilityContext &context);
+DeclAvailabilityConstraints getAvailabilityConstraintsForDecl(
+    const Decl *decl, const AvailabilityContext &context,
+    AvailabilityConstraintFlags flags = std::nullopt);
 } // end namespace swift
 
 #endif

lib/AST/AvailabilityConstraint.cpp

@@ -228,7 +228,8 @@ static void getAvailabilityConstraintsForDecl(
 
 DeclAvailabilityConstraints
 swift::getAvailabilityConstraintsForDecl(const Decl *decl,
-                                         const AvailabilityContext &context) {
+                                         const AvailabilityContext &context,
+                                         AvailabilityConstraintFlags flags) {
   llvm::SmallVector<AvailabilityConstraint, 4> constraints;
 
   // Generic parameters are always available.
@@ -239,6 +240,9 @@ swift::getAvailabilityConstraintsForDecl(const Decl *decl,
 
   getAvailabilityConstraintsForDecl(constraints, decl, context);
 
+  if (flags.contains(AvailabilityConstraintFlag::SkipEnclosingExtension))
+    return constraints;
+
   // If decl is an extension member, query the attributes of the extension, too.
   //
   // Skip decls imported from Clang, though, as they could be associated to the

lib/AST/AvailabilityContext.cpp

@@ -202,7 +202,10 @@ void AvailabilityContext::constrainWithDeclAndPlatformRange(
   bool isConstrained = false;
 
   Info info{storage->info};
-  auto constraints = swift::getAvailabilityConstraintsForDecl(decl, *this);
+  AvailabilityConstraintFlags flags =
+      AvailabilityConstraintFlag::SkipEnclosingExtension;
+  auto constraints =
+      swift::getAvailabilityConstraintsForDecl(decl, *this, flags);
   isConstrained |= info.constrainWith(constraints, decl->getASTContext());
   isConstrained |= CONSTRAIN_BOOL(info.IsDeprecated, decl->isDeprecated());
   isConstrained |= constrainRange(info.Range, platformRange);

test/Sema/availability_scopes.swift

@@ -358,7 +358,7 @@ extension SomeEnum {
 // CHECK-NEXT: {{^}}  (decl_implicit version=50 decl=extension.SomeEnum
 // CHECK-NEXT: {{^}}    (decl version=50 unavailable=macOS decl=extension.SomeEnum
 // CHECK-NEXT: {{^}}      (decl_implicit version=50 unavailable=macOS decl=availableMacOS_52
-// CHECK-NEXT: {{^}}        (decl version=50 unavailable=macOS decl=availableMacOS_52
+// CHECK-NEXT: {{^}}        (decl version=52 unavailable=macOS decl=availableMacOS_52
 // CHECK-NEXT: {{^}}      (decl version=50 unavailable=* decl=neverAvailable()
 
 @available(macOS, unavailable)