[C++20] [Modules] Don't import initializer/pending implicit instantiations from other named module

clang/lib/Serialization/ASTReader.cpp

@@ -4087,10 +4087,14 @@ llvm::Error ASTReader::ReadASTBlock(ModuleFile &F,
             std::errc::illegal_byte_sequence,
             "Invalid PENDING_IMPLICIT_INSTANTIATIONS block");
 
-      for (unsigned I = 0, N = Record.size(); I != N; /* in loop */) {
-        PendingInstantiations.push_back(
-            {ReadDeclID(F, Record, I),
-             ReadSourceLocation(F, Record, I).getRawEncoding()});
+      // For standard C++20 module, we will only reads the instantiations
+      // if it is the main file.
+      if (!F.StandardCXXModule || F.Kind == MK_MainFile) {
+        for (unsigned I = 0, N = Record.size(); I != N; /* in loop */) {
+          PendingInstantiations.push_back(
+              {ReadDeclID(F, Record, I),
+               ReadSourceLocation(F, Record, I).getRawEncoding()});
+        }
       }
       break;
 
@@ -6438,10 +6442,13 @@ llvm::Error ASTReader::ReadSubmoduleBlock(ModuleFile &F,
     case SUBMODULE_INITIALIZERS: {
       if (!ContextObj)
         break;
-      SmallVector<GlobalDeclID, 16> Inits;
-      for (unsigned I = 0; I < Record.size(); /*in loop*/)
-        Inits.push_back(ReadDeclID(F, Record, I));
-      ContextObj->addLazyModuleInitializers(CurrentModule, Inits);
+      // Standard C++ module has its own way to initialize variables.
+      if (!F.StandardCXXModule || F.Kind == MK_MainFile) {
+        SmallVector<GlobalDeclID, 16> Inits;
+        for (unsigned I = 0; I < Record.size(); /*in loop*/)
+          Inits.push_back(ReadDeclID(F, Record, I));
+        ContextObj->addLazyModuleInitializers(CurrentModule, Inits);
+      }
       break;
     }
 

clang/lib/Serialization/ASTWriter.cpp

@@ -3247,7 +3247,7 @@ void ASTWriter::WriteSubmodules(Module *WritingModule, ASTContext *Context) {
 
     // Emit the reachable initializers.
     // The initializer may only be unreachable in reduced BMI.
-    if (Context) {
+    if (Context && !GeneratingReducedBMI) {
       RecordData Inits;
       for (Decl *D : Context->getModuleInitializers(Mod))
         if (wasDeclEmitted(D))
@@ -5827,17 +5827,19 @@ void ASTWriter::WriteSpecialDeclRecords(Sema &SemaRef) {
     Stream.EmitRecord(UNUSED_LOCAL_TYPEDEF_NAME_CANDIDATES,
                       UnusedLocalTypedefNameCandidates);
 
-  // Write the record containing pending implicit instantiations.
-  RecordData PendingInstantiations;
-  for (const auto &I : SemaRef.PendingInstantiations) {
-    if (!wasDeclEmitted(I.first))
-      continue;
+  if (!GeneratingReducedBMI) {
+    // Write the record containing pending implicit instantiations.
+    RecordData PendingInstantiations;
+    for (const auto &I : SemaRef.PendingInstantiations) {
+      if (!wasDeclEmitted(I.first))
+        continue;
 
-    AddDeclRef(I.first, PendingInstantiations);
-    AddSourceLocation(I.second, PendingInstantiations);
+      AddDeclRef(I.first, PendingInstantiations);
+      AddSourceLocation(I.second, PendingInstantiations);
+    }
+    if (!PendingInstantiations.empty())
+      Stream.EmitRecord(PENDING_IMPLICIT_INSTANTIATIONS, PendingInstantiations);
   }
-  if (!PendingInstantiations.empty())
-    Stream.EmitRecord(PENDING_IMPLICIT_INSTANTIATIONS, PendingInstantiations);
 
   // Write the record containing declaration references of Sema.
   RecordData SemaDeclRefs;

clang/test/Modules/pr166068.cppm

@@ -0,0 +1,38 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 %t/flyweight.cppm -emit-reduced-module-interface -o %t/flyweight.pcm
+// RUN: %clang_cc1 -std=c++20 %t/account.cppm -emit-reduced-module-interface -o %t/account.pcm -fprebuilt-module-path=%t
+// RUN: %clang_cc1 -std=c++20 %t/core.cppm -emit-reduced-module-interface -o %t/core.pcm -fprebuilt-module-path=%t
+// RUN: %clang_cc1 -std=c++20 %t/core.cppm -fprebuilt-module-path=%t -emit-llvm -disable-llvm-passes -o - | FileCheck %t/core.cppm
+
+//--- flyweight.cppm
+module;
+template <typename> struct flyweight_core {
+  static bool init() { (void)__builtin_operator_new(2); return true; }
+  static bool static_initializer;
+};
+template <typename T> bool flyweight_core<T>::static_initializer = init();
+export module flyweight;
+export template <class> void flyweight() {
+  (void)flyweight_core<int>::static_initializer;
+}
+
+//--- account.cppm
+export module account;
+import flyweight;
+export void account() {
+  (void)::flyweight<int>;
+}
+
+//--- core.cppm
+export module core;
+import account;
+
+extern "C" void core() {}
+
+// Fine enough to check it won't crash.
+// CHECK-NOT: init
+// CHECK-NOT: static_initializer
+// CHECK: define {{.*}}@core(