dll_app.cc

#define EXPORT
#define EXTERN_TEMPLATE_VIS
#include "header.h"

#include <stdio.h>
void dll_fn() {
    printf("dll:%p %p %p\n", &outer_template<int>::inner_class::static_var, outer_template<int>::fn_outer(), outer_template<int>::inner_class::fn_inner());
    printf("oth:%p %p %p\n", &outer_template<char>::inner_class::static_var, outer_template<char>::fn_outer(), outer_template<char>::inner_class::fn_inner());
}

dll_infra.cc

#define EXPORT __declspec(dllexport)
//#define EXPORT __attribute__((visibility("default")))
#define EXTERN_TEMPLATE_VIS EXPORT

#include "header.h"
template struct outer_template<int>;

exe.cc

#define EXPORT
#define EXTERN_TEMPLATE_VIS
#include "header.h"

#include <stdio.h>
void dll_fn();
int main() {
    printf("exe:%p %p %p\n", &outer_template<int>::inner_class::static_var, outer_template<int>::fn_outer(), outer_template<int>::inner_class::fn_inner());
    printf("oth:%p\n", outer_template<char>::fn_outer());
    dll_fn();
}

header.h

template <typename T>
struct outer_template {
    outer_template();
    virtual ~outer_template() = default;
    static T *fn_outer();
    struct inner_class {
        static T *fn_inner();
        static T static_var;
        inner_class();
        //inner_class(inner_class &&);
        virtual ~inner_class() = default;
    };
};
template <typename T>
outer_template<T>::outer_template() {}
template <typename T>
T *outer_template<T>::fn_outer() { return &inner_class::static_var; }
template <typename T>
T *outer_template<T>::inner_class::fn_inner() { return &static_var; }
template <typename T>
T outer_template<T>::inner_class::static_var;
template <typename T>
outer_template<T>::inner_class::inner_class() {}
//template <typename T>
//outer_template<T>::inner_class::inner_class(inner_class &&) {}
extern template struct EXTERN_TEMPLATE_VIS outer_template<int>;

SemaDeclCXX.cpp.diff

diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index 1bd9056cad81..cd84c306348d 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -6588,6 +6588,15 @@ void Sema::checkClassLevelDLLAttribute(CXXRecordDecl *Class) {
   // seem to be true in practice?

   for (Decl *Member : Class->decls()) {
+    // TESTING: inherit dllimport/dllexport to nested class in mingw
+    if (Context.getTargetInfo().getTriple().isOSCygMing() && !getDLLAttr(Member)) {
+      if (isa<RecordDecl>(Member)) {
+        InheritableAttr *NewAttr = cast<InheritableAttr>(ClassAttr->clone(getASTContext()));
+        NewAttr->setInherited(true);
+        Member->addAttr(NewAttr);
+        continue;
+      }
+    }
     VarDecl *VD = dyn_cast<VarDecl>(Member);
     CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(Member);