-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

3 files changed, 22 insertions, 0 deletions

diff --git a/clang/test/Modules/codegen-extern-template.cpp b/clang/test/Modules/codegen-extern-template.cpp
new file mode 100644
index 00000000000..3f0bd15f26c
--- /dev/null
+++ b/clang/test/Modules/codegen-extern-template.cpp
@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 -triple=x86_64-linux-gnu -fmodules -fmodules-codegen -emit-module -fmodule-name=foo %S/codegen-extern-template.modulemap -x c++ -o %t.pcm
+// RUN: %clang_cc1 -triple x86_64-linux-gnu -fmodules -fmodule-file=%t.pcm %s -emit-llvm -o - | FileCheck %s
+// expected-no-diagnostics
+
+#include "codegen-extern-template.h"
+
+template int foo<int>();
+
+// CHECK: define weak_odr i32 @_Z3fooIiET_v
diff --git a/clang/test/Modules/codegen-extern-template.h b/clang/test/Modules/codegen-extern-template.h
new file mode 100644
index 00000000000..56ee0369ad8
--- /dev/null
+++ b/clang/test/Modules/codegen-extern-template.h
@@ -0,0 +1,12 @@
+// header for codegen-extern-template.cpp
+#ifndef CODEGEN_EXTERN_TEMPLATE_H
+#define CODEGEN_EXTERN_TEMPLATE_H
+
+template <typename T>
+inline T foo() { return 10; }
+
+extern template int foo<int>();
+
+inline int bar() { return foo<int>(); }
+
+#endif
diff --git a/clang/test/Modules/codegen-extern-template.modulemap b/clang/test/Modules/codegen-extern-template.modulemap
new file mode 100644
index 00000000000..8e050e52be6
--- /dev/null
+++ b/clang/test/Modules/codegen-extern-template.modulemap
@@ -0,0 +1 @@
+module foo { header "codegen-extern-template.h" }