[C++20] [Modules] Avoid use-but-not-defined error

See the attached test for example.

1 files changed, 12 insertions, 0 deletions

diff --git a/clang/lib/Serialization/ASTWriterDecl.cpp b/clang/lib/Serialization/ASTWriterDecl.cpp
index a181000..88dd212 100644
--- a/clang/lib/Serialization/ASTWriterDecl.cpp
+++ b/clang/lib/Serialization/ASTWriterDecl.cpp
@@ -330,6 +330,12 @@ namespace clang {
 }
 
 bool clang::CanElideDeclDef(const Decl *D) {
+  bool isExternalWithNoLinkageType = false;
+  if (auto *VD = dyn_cast<ValueDecl>(D))
+    if (VD->hasExternalFormalLinkage() &&
+        !isExternalFormalLinkage(VD->getType()->getLinkage()))
+      isExternalWithNoLinkageType = true;
+
   if (auto *FD = dyn_cast<FunctionDecl>(D)) {
     if (FD->isInlined() || FD->isConstexpr())
       return false;
@@ -339,6 +345,9 @@ bool clang::CanElideDeclDef(const Decl *D) {
 
     if (FD->getTemplateSpecializationKind() == TSK_ImplicitInstantiation)
       return false;
+
+    if (isExternalWithNoLinkageType && !FD->isExternC())
+      return false;
   }
 
   if (auto *VD = dyn_cast<VarDecl>(D)) {
@@ -352,6 +361,9 @@ bool clang::CanElideDeclDef(const Decl *D) {
 
     if (VD->getTemplateSpecializationKind() == TSK_ImplicitInstantiation)
       return false;
+
+    if (isExternalWithNoLinkageType && !VD->isExternC())
+      return false;
   }
 
   return true;