2 files changed, 28 insertions, 0 deletions
diff --git a/clang/test/CodeGenCXX/dllexport.cpp b/clang/test/CodeGenCXX/dllexport.cpp
index 5e2b3921c14..cfbc7e10078 100644
--- a/clang/test/CodeGenCXX/dllexport.cpp
+++ b/clang/test/CodeGenCXX/dllexport.cpp
@@ -615,6 +615,21 @@ NonExportedBaseClass::~NonExportedBaseClass() {}
 struct __declspec(dllexport) ExportedDerivedClass : NonExportedBaseClass {};
 // M32-DAG: weak_odr dllexport x86_thiscallcc void @"\01??1ExportedDerivedClass@@UAE@XZ"
 
+// Do not assert about generating code for constexpr functions twice during explicit instantiation (PR21718).
+template <typename T> struct ExplicitInstConstexprMembers {
+  // Copy assignment operator
+  // M32-DAG: define weak_odr dllexport x86_thiscallcc dereferenceable(1) %struct.ExplicitInstConstexprMembers* @"\01??4?$ExplicitInstConstexprMembers@X@@QAEAAU0@ABU0@@Z"
+
+  constexpr ExplicitInstConstexprMembers() {}
+  // M32-DAG: define weak_odr dllexport x86_thiscallcc %struct.ExplicitInstConstexprMembers* @"\01??0?$ExplicitInstConstexprMembers@X@@QAE@XZ"
+
+  ExplicitInstConstexprMembers(const ExplicitInstConstexprMembers&) = default;
+  // M32-DAG: define weak_odr dllexport x86_thiscallcc %struct.ExplicitInstConstexprMembers* @"\01??0?$ExplicitInstConstexprMembers@X@@QAE@ABU0@@Z"
+
+  constexpr int f() const { return 42; }
+  // M32-DAG: define weak_odr dllexport x86_thiscallcc i32 @"\01?f@?$ExplicitInstConstexprMembers@X@@QBEHXZ"
+};
+template struct __declspec(dllexport) ExplicitInstConstexprMembers<void>;
 
 //===----------------------------------------------------------------------===//
 // Classes with template base classes
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 {