Warn about async-imported selector conflicts

The change in #59479 inadvertently fixed another bug in selector conflict checking: Swift did not notice when an ObjC header declared an async-imported method, but a Swift extension to the same class added another method with the same selector. Unfortunately, fixing this bug was source-breaking, and it also caused Swift to diagnose spurious conflicts between the various names that a single ObjC method might be imported with. Soften the error to a warning in Swift 5 mode and suppress the spurious diagnostics.

Fixes rdar://95887113.

Author: beccadax

lib/Sema/TypeCheckDeclObjC.cpp

@@ -2257,6 +2257,12 @@ namespace {
   /// Produce a deterministic ordering of the given declarations.
   struct OrderDeclarations {
     bool operator()(ValueDecl *lhs, ValueDecl *rhs) const {
+      // If one declaration is imported from ObjC and the other is native Swift,
+      // put the imported Clang one first.
+      if (lhs->hasClangNode() != rhs->hasClangNode()) {
+        return lhs->hasClangNode();
+      }
+
       // If the declarations come from different modules, order based on the
       // module.
       ModuleDecl *lhsModule = lhs->getDeclContext()->getParentModule();
@@ -2506,6 +2512,10 @@ bool swift::diagnoseObjCMethodConflicts(SourceFile &sf) {
       RULE(!isa<ExtensionDecl>(a->getDeclContext()),
            !isa<ExtensionDecl>(b->getDeclContext()));
 
+      // Is one of these imported from Objective-C?
+      RULE(a->hasClangNode(),
+           b->hasClangNode());
+
       // Are these from different source files? If so, fall back to the order in
       // which the declarations were type checked.
       // FIXME: This is gross and nondeterministic.
@@ -2534,16 +2544,28 @@ bool swift::diagnoseObjCMethodConflicts(SourceFile &sf) {
         if (auto accessor = dyn_cast<AccessorDecl>(originalMethod))
           originalDecl = accessor->getStorage();
 
+      // Conflicts between two imported ObjC methods are caused by the same
+      // clang decl having several Swift names.
+      if (originalDecl->hasClangNode() && conflictingDecl->hasClangNode())
+        continue;
+
+      bool breakingInSwift5 = false;
+      // Imported ObjC async methods weren't fully checked for selector
+      // conflicts until 5.7.
+      if (originalMethod->hasClangNode() && originalMethod->hasAsync())
+        breakingInSwift5 = true;
+      // Protocols weren't checked for selector conflicts in 5.0.
+      if (!isa<ClassDecl>(conflict.typeDecl))
+        breakingInSwift5 = true;
+
       bool redeclSame = (diagInfo == origDiagInfo);
       auto diag = Ctx.Diags.diagnose(conflictingDecl,
                                      redeclSame ? diag::objc_redecl_same
                                                 : diag::objc_redecl,
                                      diagInfo.first, diagInfo.second,
                                      origDiagInfo.first, origDiagInfo.second,
                                      conflict.selector);
-
-      // Protocols weren't checked for selector conflicts in 5.0.
-      diag.warnUntilSwiftVersionIf(!isa<ClassDecl>(conflict.typeDecl), 6);
+      diag.warnUntilSwiftVersionIf(breakingInSwift5, 6);
 
       auto objcAttr = getObjCAttrIfFromAccessNote(conflictingDecl);
       swift::softenIfAccessNote(conflictingDecl, objcAttr, diag);

test/Concurrency/Inputs/Delegate.h

@@ -21,4 +21,10 @@
 -(void)myAsyncMethod:(void (^ _Nullable)(NSError * _Nullable, NSString * _Nullable))completionHandler;
 @end
 
+@interface Delegate (SwiftImpls)
+
+- (void)makeRequestFromSwift:(Request * _Nonnull)request completionHandler:(void (^ _Nullable)(void))handler;
+
+@end
+
 #endif

test/Concurrency/objc_async_overload.swift

@@ -53,3 +53,10 @@ extension Delegate {
     }
   }
 }
+
+// rdar://95887113 - Implementing an ObjC category method in Swift is not strictly valid, but should be tolerated
+
+extension Delegate {
+  @objc public func makeRequest(fromSwift: Request, completionHandler: (() -> Void)?) {}
+  // expected-warning@-1 {{method 'makeRequest(fromSwift:completionHandler:)' with Objective-C selector 'makeRequestFromSwift:completionHandler:' conflicts with method 'makeRequest(fromSwift:)' with the same Objective-C selector; this is an error in Swift 6}}
+}