Don't emit implicit template instantiations eagerly (PR21718)

Their linkage can change if they are later explicitly instantiated. We would
previously emit such functions eagerly (as opposed to lazily on first use) if
they have a 'dllexport' or 'used' attribute, and fail an assert when hitting the
explicit instantiation.

This is achieved by replacing the old CodeGenModule::MayDeferGeneration() method
with two new ones: MustBeEmitted() and MayBeEmittedEagerly().

Differential Revision: http://reviews.llvm.org/D6674

llvm-svn: 225570

1 files changed, 13 insertions, 0 deletions

diff --git a/clang/test/CodeGenCXX/explicit-instantiation.cpp b/clang/test/CodeGenCXX/explicit-instantiation.cpp
index 5bd06784cfa..6076444c25b 100644
--- a/clang/test/CodeGenCXX/explicit-instantiation.cpp
+++ b/clang/test/CodeGenCXX/explicit-instantiation.cpp
@@ -90,6 +90,19 @@ namespace LateInstantiation {
   // CHECK-OPT: define available_externally i32 @_ZN17LateInstantiation1fIiEEiv(
 }
 
+namespace PR21718 {
+// The linkage of a used constexpr member function can change from linkonce_odr
+// to weak_odr after explicit instantiation without errors about defining the
+// same function twice.
+template <typename T>
+struct S {
+// CHECK-LABEL: define weak_odr i32 @_ZN7PR217181SIiE1fEv
+  __attribute__((used)) constexpr int f() { return 0; }
+};
+int g() { return S<int>().f(); }
+template struct S<int>;
+}
+
 // Check that we emit definitions from explicit instantiations even when they
 // occur prior to the definition itself.
 template <typename T> struct S {