ABI-checker: diagnose missing available attributes for members of extensions instead of the extensions themselves

test/api-digester/Inputs/cake_current/cake.swift

@@ -241,3 +241,7 @@ open class AddingNewDesignatedInit {
     print(foo)
   }
 }
+
+public extension Float {
+  func floatHigher() {}
+}

test/api-digester/Outputs/Cake-abi.txt

@@ -67,6 +67,7 @@ cake: Func C1.foo1() is now not static
 cake: Func C5.dy_foo() is now with dynamic
 cake: Func FinalFuncContainer.NewFinalFunc() is now with final
 cake: Func FinalFuncContainer.NoLongerFinalFunc() is now without final
+cake: Func Float.floatHigher() is a new API without @available attribute
 cake: Func HasMutatingMethodClone.foo() has self access kind changing from Mutating to NonMutating
 cake: Func RequiementChanges.addedFunc() is a new API without @available attribute
 cake: Func S1.foo1() has self access kind changing from NonMutating to Mutating

tools/swift-api-digester/swift-api-digester.cpp

@@ -1112,6 +1112,28 @@ class PrunePass : public MatchedNodeListener, public SDKTreeDiffPass {
     UpdateMap(Ctx.getNodeUpdateMap()),
     ProtocolReqWhitelist(std::move(prWhitelist)) {}
 
+  void diagnoseMissingAvailable(SDKNodeDecl *D) {
+    // For extensions of external types, we diagnose individual member's missing
+    // available attribute instead of the extension itself.
+    // The reason is we may merge several extensions into a single one; some
+    // attributes are missing.
+    if (auto *DT = dyn_cast<SDKNodeDeclType>(D)) {
+      if (DT->isExtension()) {
+        for(auto MD: DT->getChildren()) {
+          diagnoseMissingAvailable(cast<SDKNodeDecl>(MD));
+        }
+        return;
+      }
+    }
+    // Diagnose the missing of @available attributes.
+    // Decls with @_alwaysEmitIntoClient aren't required to have an
+    // @available attribute.
+    if (!Ctx.getOpts().SkipOSCheck &&
+        !D->getIntroducingVersion().hasOSAvailability() &&
+        !D->hasDeclAttribute(DeclAttrKind::DAK_AlwaysEmitIntoClient)) {
+      D->emitDiag(D->getLoc(), diag::new_decl_without_intro);
+    }
+  }
   void foundMatch(NodePtr Left, NodePtr Right, NodeMatchReason Reason) override {
     if (options::DebugMapping)
       debugMatch(Left, Right, Reason, llvm::errs());
@@ -1125,14 +1147,7 @@ class PrunePass : public MatchedNodeListener, public SDKTreeDiffPass {
           if (D->hasFixedBinaryOrder()) {
             D->emitDiag(D->getLoc(), diag::decl_added);
           }
-          // Diagnose the missing of @available attributes.
-          // Decls with @_alwaysEmitIntoClient aren't required to have an
-          // @available attribute.
-          if (!Ctx.getOpts().SkipOSCheck &&
-              !D->getIntroducingVersion().hasOSAvailability() &&
-              !D->hasDeclAttribute(DeclAttrKind::DAK_AlwaysEmitIntoClient)) {
-            D->emitDiag(D->getLoc(), diag::new_decl_without_intro);
-          }
+          diagnoseMissingAvailable(D);
         }
       }
       // Complain about added protocol requirements