-fmodules-codegen should not emit extern templates

If a header contains 'extern template', then the template should be provided
somewhere by an explicit instantiation, so it is not necessary to generate
a copy. Worse, this can lead to an unresolved symbol, because the codegen's
object file will not actually contain functions from such a template
because of the GVA_AvailableExternally, but the object file for the explicit
instantiation will not contain them either because it will be blocked
by the information provided by the module.

Differential Revision: https://reviews.llvm.org/D69779

1 files changed, 3 insertions, 2 deletions

diff --git a/clang/lib/Serialization/ASTWriterDecl.cpp b/clang/lib/Serialization/ASTWriterDecl.cpp
index a553936570b..74a49fa813a 100644
--- a/clang/lib/Serialization/ASTWriterDecl.cpp
+++ b/clang/lib/Serialization/ASTWriterDecl.cpp
@@ -2437,11 +2437,12 @@ void ASTRecordWriter::AddFunctionDefinition(const FunctionDecl *FD) {
     }
     if (Writer->Context->getLangOpts().ModulesCodegen) {
       // Under -fmodules-codegen, codegen is performed for all non-internal,
-      // non-always_inline functions.
+      // non-always_inline functions, unless they are available elsewhere.
       if (!FD->hasAttr<AlwaysInlineAttr>()) {
         if (!Linkage)
           Linkage = Writer->Context->GetGVALinkageForFunction(FD);
-        ModulesCodegen = *Linkage != GVA_Internal;
+        ModulesCodegen =
+            *Linkage != GVA_Internal && *Linkage != GVA_AvailableExternally;
       }
     }
   }