[C++20][Modules][Serialization] Delay marking pending incomplete decl chains until the end of finishPendingActions. (#121245)

The call to `hasBody` inside `finishPendingActions` that bumps the `PendingIncompleteDeclChains`
size from `0` to `1`, and also sets the `LazyVal->LastGeneration` to `6` which matches
the `LazyVal->ExternalSource->getGeneration()` value of `6`. Later, the iterations over `redecls()`
(which calls `getNextRedeclaration`) is expected to trigger the reload, but it **does not** since
the generation numbers match.

The proposed solution is to perform the marking of incomplete decl chains at the end of `finishPendingActions`.
This way, **all** of the incomplete decls are marked incomplete as a post-condition of `finishPendingActions`.
It's also safe to delay this operation since any operation being done within `finishPendingActions` has
`NumCurrentElementsDeserializing == 1`, which means that any calls to `CompleteDeclChain` would simply
add to the `PendingIncompleteDeclChains` without doing anything anyway.

Author: mpark

clang/lib/Serialization/ASTReader.cpp

@@ -10186,12 +10186,12 @@ void ASTReader::visitTopLevelModuleMaps(
 }
 
 void ASTReader::finishPendingActions() {
-  while (
-      !PendingIdentifierInfos.empty() || !PendingDeducedFunctionTypes.empty() ||
-      !PendingDeducedVarTypes.empty() || !PendingIncompleteDeclChains.empty() ||
-      !PendingDeclChains.empty() || !PendingMacroIDs.empty() ||
-      !PendingDeclContextInfos.empty() || !PendingUpdateRecords.empty() ||
-      !PendingObjCExtensionIvarRedeclarations.empty()) {
+  while (!PendingIdentifierInfos.empty() ||
+         !PendingDeducedFunctionTypes.empty() ||
+         !PendingDeducedVarTypes.empty() || !PendingDeclChains.empty() ||
+         !PendingMacroIDs.empty() || !PendingDeclContextInfos.empty() ||
+         !PendingUpdateRecords.empty() ||
+         !PendingObjCExtensionIvarRedeclarations.empty()) {
     // If any identifiers with corresponding top-level declarations have
     // been loaded, load those declarations now.
     using TopLevelDeclsMap =
@@ -10239,13 +10239,6 @@ void ASTReader::finishPendingActions() {
     }
     PendingDeducedVarTypes.clear();
 
-    // For each decl chain that we wanted to complete while deserializing, mark
-    // it as "still needs to be completed".
-    for (unsigned I = 0; I != PendingIncompleteDeclChains.size(); ++I) {
-      markIncompleteDeclChain(PendingIncompleteDeclChains[I]);
-    }
-    PendingIncompleteDeclChains.clear();
-
     // Load pending declaration chains.
     for (unsigned I = 0; I != PendingDeclChains.size(); ++I)
       loadPendingDeclChain(PendingDeclChains[I].first,
@@ -10483,6 +10476,12 @@ void ASTReader::finishPendingActions() {
   for (auto *ND : PendingMergedDefinitionsToDeduplicate)
     getContext().deduplicateMergedDefinitonsFor(ND);
   PendingMergedDefinitionsToDeduplicate.clear();
+
+  // For each decl chain that we wanted to complete while deserializing, mark
+  // it as "still needs to be completed".
+  for (Decl *D : PendingIncompleteDeclChains)
+    markIncompleteDeclChain(D);
+  PendingIncompleteDeclChains.clear();
 }
 
 void ASTReader::diagnoseOdrViolations() {

clang/test/Modules/pr121245.cpp

@@ -0,0 +1,93 @@
+// If this test fails, it should be investigated under Debug builds.
+// Before the PR, this test was encountering an `llvm_unreachable()`.
+
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+// RUN: cd %t
+
+// RUN: %clang_cc1 -std=c++20 -emit-header-unit -xc++-user-header %t/hu-01.h \
+// RUN:  -fcxx-exceptions -o %t/hu-01.pcm
+
+// RUN: %clang_cc1 -std=c++20 -emit-header-unit -xc++-user-header %t/hu-02.h \
+// RUN:  -Wno-experimental-header-units -fcxx-exceptions \
+// RUN:  -fmodule-file=%t/hu-01.pcm -o %t/hu-02.pcm
+
+// RUN: %clang_cc1 -std=c++20 -emit-header-unit -xc++-user-header %t/hu-03.h \
+// RUN:  -Wno-experimental-header-units -fcxx-exceptions \
+// RUN:  -fmodule-file=%t/hu-01.pcm -o %t/hu-03.pcm
+
+// RUN: %clang_cc1 -std=c++20 -emit-header-unit -xc++-user-header %t/hu-04.h \
+// RUN:  -Wno-experimental-header-units -fcxx-exceptions \
+// RUN:  -fmodule-file=%t/hu-01.pcm -o %t/hu-04.pcm
+
+// RUN: %clang_cc1 -std=c++20 -emit-header-unit -xc++-user-header %t/hu-05.h \
+// RUN:  -Wno-experimental-header-units -fcxx-exceptions \
+// RUN:  -fmodule-file=%t/hu-03.pcm -fmodule-file=%t/hu-04.pcm \
+// RUN:  -fmodule-file=%t/hu-01.pcm -o %t/hu-05.pcm
+
+// RUN: %clang_cc1 -std=c++20 -emit-obj %t/main.cpp \
+// RUN:  -Wno-experimental-header-units -fcxx-exceptions \
+// RUN:  -fmodule-file=%t/hu-02.pcm -fmodule-file=%t/hu-05.pcm \
+// RUN:  -fmodule-file=%t/hu-04.pcm -fmodule-file=%t/hu-03.pcm \
+// RUN:  -fmodule-file=%t/hu-01.pcm
+
+//--- hu-01.h
+template <typename T>
+struct A {
+  A() {}
+  ~A() {}
+};
+
+template <typename T>
+struct EBO : T {
+  EBO() = default;
+};
+
+template <typename T>
+struct HT : EBO<A<T>> {};
+
+//--- hu-02.h
+import "hu-01.h";
+
+inline void f() {
+  HT<int>();
+}
+
+//--- hu-03.h
+import "hu-01.h";
+
+struct C {
+  C();
+
+  HT<long> _;
+};
+
+//--- hu-04.h
+import "hu-01.h";
+
+void g(HT<long> = {});
+
+//--- hu-05.h
+import "hu-03.h";
+import "hu-04.h";
+import "hu-01.h";
+
+struct B {
+  virtual ~B() = default;
+
+  virtual void f() {
+    HT<long>();
+  }
+};
+
+//--- main.cpp
+import "hu-02.h";
+import "hu-05.h";
+import "hu-03.h";
+
+int main() {
+  f();
+  C();
+  B();
+}