Sema: Generalize more-available-than-enclosing-decl diagnostics.

NFC.

lib/Sema/TypeCheckAttr.cpp

@@ -2200,13 +2200,19 @@ static Decl *getEnclosingDeclForDecl(Decl *D) {
 }
 
 static std::optional<std::pair<SemanticAvailableAttr, const Decl *>>
-getSemanticAvailableRangeDeclAndAttr(const Decl *decl) {
-  if (auto attr = decl->getAvailableAttrForPlatformIntroduction(
-          /*checkExtension=*/false))
-    return std::make_pair(*attr, decl);
+getSemanticAvailableRangeDeclAndAttr(const Decl *decl,
+                                     AvailabilityDomain domain) {
+  auto &ctx = decl->getASTContext();
+  AvailabilityConstraintFlags flags =
+      AvailabilityConstraintFlag::SkipEnclosingExtension;
+  if (auto constraint = swift::getAvailabilityConstraintForDeclInDomain(
+          decl, AvailabilityContext::forAlwaysAvailable(ctx), domain, flags)) {
+    if (constraint->isPotentiallyAvailable())
+      return std::make_pair(constraint->getAttr(), decl);
+  }
 
   if (auto *parent = decl->parentDeclForAvailability())
-    return getSemanticAvailableRangeDeclAndAttr(parent);
+    return getSemanticAvailableRangeDeclAndAttr(parent, domain);
 
   return std::nullopt;
 }
@@ -2300,7 +2306,7 @@ void AttributeChecker::visitAvailableAttr(AvailableAttr *parsedAttr) {
 
   if (auto *parent = getEnclosingDeclForDecl(D)) {
     if (auto enclosingAvailable =
-            getSemanticAvailableRangeDeclAndAttr(parent)) {
+            getSemanticAvailableRangeDeclAndAttr(parent, attr->getDomain())) {
       SemanticAvailableAttr enclosingAttr = enclosingAvailable->first;
       const Decl *enclosingDecl = enclosingAvailable->second;
       enclosingIntroducedRange = enclosingAttr.getIntroducedRange(Ctx);
@@ -5245,8 +5251,8 @@ void AttributeChecker::checkBackDeployedAttrs(
     // If it's not, the attribute doesn't make sense since the back deployment
     // fallback could never be executed at runtime.
     if (auto availableRangeAttrPair =
-            getSemanticAvailableRangeDeclAndAttr(VD)) {
-      auto beforeDomain = Attr->getAvailabilityDomain();
+            getSemanticAvailableRangeDeclAndAttr(VD, Domain)) {
+      auto beforeDomain = Domain;
       auto beforeVersion = Attr->getVersion();
       auto availableAttr = availableRangeAttrPair.value().first;
       auto introVersion = availableAttr.getIntroduced().value();

test/Availability/availability_custom_domains.swift

@@ -231,18 +231,29 @@ func testFixIts() {
   // expected-note@-1 {{add 'if #available' version check}}{{3-29=if #available(EnabledDomain) {\n      availableInEnabledDomain()\n  \} else {\n      // Fallback\n  \}}}
 }
 
+struct Container { }
+
+@available(EnabledDomain)
+extension Container {
+  @available(EnabledDomain)
+  func redundantlyAvailableInEnabledDomain() { }
+
+  @available(EnabledDomain, unavailable)
+  func unavailableInEnabledDomain() { }
+}
+
 protocol P { }
 
 @available(EnabledDomain)
-struct AvailableInEnabledDomain: P { }
+struct AvailableConformsToP: P { }
 
 @available(EnabledDomain, unavailable)
-struct UnavailableInEnabledDomain: P { }
+struct UnavailableConformsToP: P { }
 
 func testOpaqueReturnType() -> some P {
   if #available(EnabledDomain) { // expected-error {{opaque return type cannot depend on EnabledDomain availability}}
-    return AvailableInEnabledDomain()
+    return AvailableConformsToP()
   } else {
-    return UnavailableInEnabledDomain()
+    return UnavailableConformsToP()
   }
 }