[C++20] [Modules] Merge lambdas from Sema to imported one

clang/lib/Serialization/ASTReader.cpp

@@ -8962,17 +8962,6 @@ void ASTReader::ReadLateParsedTemplates(
   LateParsedTemplates.clear();
 }
 
-void ASTReader::AssignedLambdaNumbering(const CXXRecordDecl *Lambda) {
-  if (Lambda->getLambdaContextDecl()) {
-    // Keep track of this lambda so it can be merged with another lambda that
-    // is loaded later.
-    LambdaDeclarationsForMerging.insert(
-        {{Lambda->getLambdaContextDecl()->getCanonicalDecl(),
-          Lambda->getLambdaIndexInContext()},
-         const_cast<CXXRecordDecl *>(Lambda)});
-  }
-}
-
 void ASTReader::LoadSelector(Selector Sel) {
   // It would be complicated to avoid reading the methods anyway. So don't.
   ReadMethodPool(Sel);

clang/lib/Serialization/ASTReaderDecl.cpp

@@ -4506,6 +4506,34 @@ void ASTReader::loadObjCCategories(GlobalDeclID ID, ObjCInterfaceDecl *D,
   ModuleMgr.visit(Visitor);
 }
 
+void ASTReader::AssignedLambdaNumbering(const CXXRecordDecl *Lambda) {
+  if (Lambda->getLambdaContextDecl()) {
+    auto ContextAndIndexPair = std::make_pair<const Decl *, unsigned>(
+        Lambda->getLambdaContextDecl()->getCanonicalDecl(),
+        Lambda->getLambdaIndexInContext());
+
+    auto Iter = LambdaDeclarationsForMerging.find(ContextAndIndexPair);
+    // If we import first and then include second, try to merge the second
+    // one with the first one.
+    if (Iter != LambdaDeclarationsForMerging.end() &&
+        Iter->second->isFromASTFile()) {
+      NamedDecl *Slot = Iter->second;
+      ASTDeclMerger Merger(*this);
+      // The lambda shouldn't be a key declaration as the lambda wouldn't
+      // be a canonical decl for its owning module.
+      Merger.mergeRedeclarableImpl(const_cast<CXXRecordDecl *>(Lambda),
+                                   cast<TagDecl>(Slot),
+                                   /*KeyDeclID*/ GlobalDeclID());
+      return;
+    }
+
+    // Keep track of this lambda so it can be merged with another lambda that
+    // is loaded later.
+    LambdaDeclarationsForMerging.insert(
+        {ContextAndIndexPair, const_cast<CXXRecordDecl *>(Lambda)});
+  }
+}
+
 template<typename DeclT, typename Fn>
 static void forAllLaterRedecls(DeclT *D, Fn F) {
   F(D);

clang/test/Modules/pr102721.cppm

@@ -0,0 +1,33 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 %t/a.cppm -emit-module-interface -o %t/a.pcm
+// RUN: %clang_cc1 -std=c++20 %t/b.cppm -emit-module-interface -o %t/b.pcm \
+// RUN:   -fprebuilt-module-path=%t
+// RUN: %clang_cc1 -std=c++20 %t/test.cc -fsyntax-only -verify \
+// RUN:   -fprebuilt-module-path=%t
+
+//--- foo.h
+inline auto x = []{};
+
+//--- a.cppm
+module;
+#include "foo.h"
+export module a;
+export using ::x;
+
+//--- b.cppm
+module;
+import a;
+#include "foo.h"
+export module b;
+export using ::x;
+
+//--- test.cc
+// expected-no-diagnostics
+import a;
+import b;
+void test() {
+  x();
+}