From b9d36540a85b859ff1189c6c2c3acbc59033ae49 Mon Sep 17 00:00:00 2001
From: Hans Wennborg <hans@chromium.org>
Date: Thu, 5 Nov 2020 13:44:50 +0100
Subject: [PATCH] [dllexport] Avoid assert for explicitly defaulted methods in
 explicit instantiation definitions (PR47683)

Clang was asserting due to attempting to codegen such methods twice.

Differential revision: https://reviews.llvm.org/D90849
---
 clang/lib/Sema/SemaDeclCXX.cpp      | 19 ++++++++++++++-----
 clang/test/CodeGenCXX/dllexport.cpp | 30 ++++++++++++++++++++++++++++++
 2 files changed, 44 insertions(+), 5 deletions(-)

diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index ebda013..c49e9ca 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -5895,13 +5895,22 @@ static void ReferenceDllExportedMembers(Sema &S, CXXRecordDecl *Class) {
 
         // The function will be passed to the consumer when its definition is
         // encountered.
-      } else if (!MD->isTrivial() || MD->isExplicitlyDefaulted() ||
+      } else if (MD->isExplicitlyDefaulted()) {
+        // Synthesize and instantiate explicitly defaulted methods.
+        S.MarkFunctionReferenced(Class->getLocation(), MD);
+
+        if (TSK != TSK_ExplicitInstantiationDefinition) {
+          // Except for explicit instantiation defs, we will not see the
+          // definition again later, so pass it to the consumer now.
+          S.Consumer.HandleTopLevelDecl(DeclGroupRef(MD));
+        }
+      } else if (!MD->isTrivial() ||
                  MD->isCopyAssignmentOperator() ||
                  MD->isMoveAssignmentOperator()) {
-        // Synthesize and instantiate non-trivial implicit methods, explicitly
-        // defaulted methods, and the copy and move assignment operators. The
-        // latter are exported even if they are trivial, because the address of
-        // an operator can be taken and should compare equal across libraries.
+        // Synthesize and instantiate non-trivial implicit methods, and the copy
+        // and move assignment operators. The latter are exported even if they
+        // are trivial, because the address of an operator can be taken and
+        // should compare equal across libraries.
         S.MarkFunctionReferenced(Class->getLocation(), MD);
 
         // There is no later point when we will see the definition of this
diff --git a/clang/test/CodeGenCXX/dllexport.cpp b/clang/test/CodeGenCXX/dllexport.cpp
index 3371db3..f8dc2ab 100644
--- a/clang/test/CodeGenCXX/dllexport.cpp
+++ b/clang/test/CodeGenCXX/dllexport.cpp
@@ -915,6 +915,36 @@ template<> template<> void ExportedClassTemplate2<int>::baz<int>() {}
 // M32-DAG: define dso_local x86_thiscallcc void @"??$baz@H@?$ExportedClassTemplate2@H@pr34849@@QAEXXZ"
 }
 
+namespace pr47683 {
+struct X { X() {} };
+
+template <typename> struct S {
+  S() = default;
+  X x;
+};
+template struct __declspec(dllexport) S<int>;
+// M32-DAG: define weak_odr dso_local dllexport x86_thiscallcc %"struct.pr47683::S"* @"??0?$S@H@pr47683@@QAE@XZ"
+
+template <typename> struct T {
+  T() = default;
+  X x;
+};
+extern template struct T<int>;
+template struct __declspec(dllexport) T<int>;
+// Don't assert about multiple codegen for explicitly defaulted method in explicit instantiation def.
+// M32-DAG: define weak_odr dso_local dllexport x86_thiscallcc %"struct.pr47683::T"* @"??0?$T@H@pr47683@@QAE@XZ"
+
+template <typename> struct U {
+  U();
+  X x;
+};
+template <typename T> U<T>::U() = default;
+extern template struct U<int>;
+template struct __declspec(dllexport) U<int>;
+// Same as T, but with out-of-line ctor.
+// M32-DAG: define weak_odr dso_local dllexport x86_thiscallcc %"struct.pr47683::U"* @"??0?$U@H@pr47683@@QAE@XZ"
+}
+
 //===----------------------------------------------------------------------===//
 // Classes with template base classes
 //===----------------------------------------------------------------------===//
-- 
2.7.4