[MinGW] Do dllexport inline methods in template instantiation

[mstorsjo]

Summary

Normally, in MinGW mode, inline methods aren't dllexported.

However, in the case of a dllimported template instantiation, the inline methods aren't instantiated locally, but referenced from the instantiation. Therefore, those methods also need to be dllexported, in the case of an instantiation.

GCC suffers from the same issue, reported at [1], but the issue is still unresolved there.

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89088

[hans]

lgtm

lib/Sema/SemaDeclCXX.cpp

if (!VD && !MD)
       continue;
 
     if (MD) {
       // Don't process deleted methods.
       if (MD->isDeleted())
         continue;
 
       if (MD->isInlined()) {
-        // MinGW does not import or export inline methods.
+        // MinGW does not import or export inline methods. But do it for
+        // template instantiations.
         if (!Context.getTargetInfo().getCXXABI().isMicrosoft() &&
-            !Context.getTargetInfo().getTriple().isWindowsItaniumEnvironment())
+            !Context.getTargetInfo().getTriple().isWindowsItaniumEnvironment() &&
+            TSK != TSK_ExplicitInstantiationDeclaration &&
+            TSK != TSK_ExplicitInstantiationDefinition)
           continue;
 
         // MSVC versions before 2015 don't export the move assignment operators
         // and move constructor, so don't attempt to import/export them if
         // we have a definition.
         auto *Ctor = dyn_cast<CXXConstructorDecl>(MD);
         if ((MD->isMoveAssignmentOperator() ||
              (Ctor && Ctor->isMoveConstructor())) &&

test/CodeGenCXX/mingw-template-dllexport.cpp

 // RUN: %clang_cc1 -emit-llvm -triple i686-mingw32 %s -o - | FileCheck %s
 
 #define JOIN2(x, y) x##y
 #define JOIN(x, y) JOIN2(x, y)
 #define UNIQ(name) JOIN(name, __LINE__)
 #define USEMEMFUNC(class, func) void (class::*UNIQ(use)())() { return &class::func; }
 
 template <class T>
 class c {
-  void f();
+  void f() {}
 };
 
-template <class T> void c<T>::f() {}
-
 template class __declspec(dllexport) c<int>;
 
 // CHECK: define {{.*}} dllexport {{.*}} @_ZN1cIiE1fEv
 
 extern template class __declspec(dllexport) c<char>;
 template class c<char>;
 
 // CHECK: define {{.*}} dllexport {{.*}} @_ZN1cIcE1fEv