Fix @available(macCatalyst N, iOS N+M) bug

Previously, availability checking computed a declaration’s availability as the intersection of all @available attributes with active platforms. This meant that Swift would not allow you to use an API that was available earlier in a child platform than in its parent until it was also available in the parent platform. That was incorrect.

This PR corrects availability checking to find the most specific @available attribute with an introduced version and use that instead.

Fixes rdar://60892534.

Author: beccadax

lib/AST/Availability.cpp

@@ -134,7 +134,7 @@ void AvailabilityInference::applyInferredAvailableAttrs(
 
 Optional<AvailabilityContext>
 AvailabilityInference::annotatedAvailableRange(const Decl *D, ASTContext &Ctx) {
-  Optional<AvailabilityContext> AnnotatedRange;
+  const AvailableAttr *bestAvailAttr = nullptr;
 
   for (auto Attr : D->getAttrs()) {
     auto *AvailAttr = dyn_cast<AvailableAttr>(Attr);
@@ -145,21 +145,19 @@ AvailabilityInference::annotatedAvailableRange(const Decl *D, ASTContext &Ctx) {
       continue;
     }
 
-    AvailabilityContext AttrRange{
-        VersionRange::allGTE(AvailAttr->Introduced.getValue())};
-
-    // If we have multiple introduction versions, we will conservatively
-    // assume the worst case scenario. We may want to be more precise here
-    // in the future or emit a diagnostic.
-
-    if (AnnotatedRange.hasValue()) {
-      AnnotatedRange.getValue().intersectWith(AttrRange);
-    } else {
-      AnnotatedRange = AttrRange;
+    // Okay, we have a candidate, but is it better than one we already found?
+    if (!bestAvailAttr ||
+        inheritsAvailabilityFromPlatform(AvailAttr->Platform,
+                                         bestAvailAttr->Platform)) {
+      bestAvailAttr = AvailAttr;
     }
   }
 
-  return AnnotatedRange;
+  if (!bestAvailAttr)
+    return None;
+
+  return AvailabilityContext{
+    VersionRange::allGTE(bestAvailAttr->Introduced.getValue())};
 }
 
 AvailabilityContext AvailabilityInference::availableRange(const Decl *D,

test/attr/attr_availability_maccatalyst.swift

@@ -48,34 +48,51 @@ deprecatedOnIOSButNotMacCatalyst() // no-warning
 func introducedLaterOnMacCatalyst() {
 }
 
+@available(iOS 57.0, macCatalyst 56.0, *)
+func introducedLaterOnIOS() {
+}
+
 // expected-note@+1 *{{add @available attribute to enclosing global function}}
 func testPoundAvailable() {
 
   if #available(macCatalyst 55.0, *) {
     introducedLaterOnMacCatalyst() // expected-error {{'introducedLaterOnMacCatalyst()' is only available in Mac Catalyst 56.0 or newer}}
     // expected-note@-1 {{add 'if #available' version check}}
+    introducedLaterOnIOS() // expected-error {{'introducedLaterOnIOS()' is only available in Mac Catalyst 56.0 or newer}}
+    // expected-note@-1 {{add 'if #available' version check}}
   }
 
   // macCatalyst should win over iOS when present
 
   if #available(iOS 56.0, macCatalyst 55.0, *) {
     introducedLaterOnMacCatalyst() // expected-error {{'introducedLaterOnMacCatalyst()' is only available in Mac Catalyst 56.0 or newer}}
     // expected-note@-1 {{add 'if #available' version check}}
+    introducedLaterOnIOS() // expected-error {{'introducedLaterOnIOS()' is only available in Mac Catalyst 56.0 or newer}}
+    // expected-note@-1 {{add 'if #available' version check}}
   }
 
   if #available(iOS 55.0, macCatalyst 56.0, *) {
     introducedLaterOnMacCatalyst() // no-warning
+    introducedLaterOnIOS() // no-error
+  }
+
+  if #available(iOS 57.0, macCatalyst 56.0, *) {
+    introducedLaterOnMacCatalyst() // no-warning
+    introducedLaterOnIOS() // no-error
   }
 
   // iOS availability should be inherited when macCatalyst is not present
 
   if #available(iOS 55.0, *) {
     introducedLaterOnMacCatalyst() // expected-error {{'introducedLaterOnMacCatalyst()' is only available in Mac Catalyst 56.0 or newer}}
     // expected-note@-1 {{add 'if #available' version check}}
+    introducedLaterOnIOS() // expected-error {{'introducedLaterOnIOS()' is only available in Mac Catalyst 56.0 or newer}}
+    // expected-note@-1 {{add 'if #available' version check}}
   }
 
   if #available(iOS 56.0, *) {
     introducedLaterOnMacCatalyst() // no-warning
+    introducedLaterOnIOS() // no-error
   }
 
   // macOS availability doesn't count on macCatalyst for Swift.