[Clang][Modules] Add added implicit members to record upon deserialization

Previously, we were only called RD->addedMember(MD) to notify RD that MD
(an implicit member) was added, but MD is never actually added as
a child of RD. (Interestingly, MD still thinks RD is its parent.)

We end up with a broken AST that Clang itself seems to be robust to, but
causes issues for users of libclang like Swift's ClangImporter.

This patch tries to maintain a well-formed AST by replacing calls to
RD->addedMember(MD) with RD->addHiddenDecl(MD) for member function decls
(addHiddenDecl() internally calls addedMember() to keep member metadata
up to date). Most of the code is there to check that RD doesn't already
contain the same decl as MD, which may be declared in and deserialized
from another module. We fall back to the original behavior of calling
addedMember() for non-function members: it's unclear what scenario leads
to that (if any), nor how we should best handle that.

Note that we call addHiddenDecl() instead of addDecl() because the
latter can sometimes lead to an assertion failure.

rdar://161931408

Author: j-hui

clang/lib/Serialization/ASTReader.cpp

@@ -10671,6 +10671,70 @@ void ASTReader::finishPendingActions() {
 
   // Inform any classes that had members added that they now have more members.
   for (auto [RD, MD] : PendingAddedClassMembers) {
+    // The module we just imported added MD to RD so we should do the same, but
+    // only RD doesn't already contain the same member. This can happen when two
+    // modules independent add and serialize the same implicit member.
+
+    // Fast path: we can check for the presence of implicit special members
+    // (default/copy/move constructors + copy/move assignments + destructors)
+    // with RD's needsImplicit*() methods (which are constant time).
+    if (const auto *Ctor = dyn_cast<CXXConstructorDecl>(MD)) {
+      if (!Ctor->isInheritingConstructor()) {
+        if (Ctor->isDefaultConstructor()) {
+          if (RD->needsImplicitDefaultConstructor())
+            RD->addHiddenDecl(MD);
+          continue;
+        }
+        if (Ctor->isCopyConstructor()) {
+          if (RD->needsImplicitCopyConstructor())
+            RD->addHiddenDecl(MD);
+          continue;
+        }
+        if (Ctor->isMoveConstructor()) {
+          if (RD->needsImplicitMoveConstructor())
+            RD->addHiddenDecl(MD);
+          continue;
+        }
+      }
+    } else if (const auto *Mthd = dyn_cast<CXXMethodDecl>(MD)) {
+      if (isa<CXXDestructorDecl>(Mthd)) {
+        if (RD->needsImplicitDestructor())
+          RD->addHiddenDecl(MD);
+        continue;
+      }
+      if (Mthd->isCopyAssignmentOperator()) {
+        if (RD->needsImplicitCopyAssignment())
+          RD->addHiddenDecl(MD);
+        continue;
+      }
+      if (Mthd->isMoveAssignmentOperator()) {
+        if (RD->needsImplicitMoveAssignment())
+          RD->addHiddenDecl(MD);
+        continue;
+      }
+    }
+
+    if (auto *FD = dyn_cast<FunctionDecl>(MD)) {
+      // Slow path: this is some other implicit added member (e.g., inherited
+      // constructor). We need iterate through all of RD's members and compare
+      // their ODR hash against MD's.
+      auto needToAddMD = true;
+      for (auto *RDD : RD->noload_decls()) {
+        if (auto *RDFD = dyn_cast<FunctionDecl>(RDD)) {
+          if (FD->getODRHash() == RDFD->getODRHash()) {
+            needToAddMD = false;
+            break;
+          }
+        }
+      }
+      if (needToAddMD)
+        RD->addHiddenDecl(MD);
+      continue;
+    }
+
+    // This branch is only reachable if MD is not a function decl (which means
+    // it doesn't have an ODR hash to compare). Call addedMember() to preserve
+    // historical behavior.
     RD->addedMember(MD);
   }
   PendingAddedClassMembers.clear();