https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81855
Bug ID: 81855
Summary: [mingw-w64] dllexport-ed explicit template
instantiation has no symbol in DLL when the compile
unit has non-template symbol
Product: gcc
Version: 7.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: lanxingcan at gmail dot com
Target Milestone: ---
I'm cross building a library for Windows, but encounterd an issue about
dllexport and explicit template instantiation.
I tested with MinGW-W64 GCC/7.1.0 cross compiler on either macOS serria (from
homebrew/core/mingw-w64) as well as Fedora 26 (from stock repository).
Here is a small reproducible.
test.cpp:
```cpp
template<typename T>
struct __declspec(dllexport) Template
{
~Template() noexcept(false) { throw 1; }
};
extern template struct __declspec(dllexport) Template<void>;
template struct Template<void>;
#ifdef WITH_NON_TEMPLATE
struct __declspec(dllexport) NonTemplate
{
~NonTemplate();
};
NonTemplate::~NonTemplate() = default;
#endif
```
Compiles with:
x86_64-w64-mingw32-g++ test.cpp -shared -o libtest1.dll
-Wl,--out-implib,libtest1.dll.a
x86_64-w64-mingw32-g++ test.cpp -DWITH_NON_TEMPLATE -shared -o libtest2.dll
-Wl,--out-implib,libtest2.dll.a
Check exported symbol in libtest1.dll :
```
$ x86_64-w64-mingw32-objdump -d libtest1.dll.a | x86_64-w64-mingw32-c++filt
In archive libtest1.dll.a:
d000005.o: file format pe-x86-64
d000002.o: file format pe-x86-64
d000004.o: file format pe-x86-64
Disassembly of section .text:
0000000000000000 <Template<void>::~Template()>:
0: ff 25 00 00 00 00 jmpq *0x0(%rip) # 6
<Template<void>::~Template()+0x6>
6: 90 nop
7: 90 nop
d000003.o: file format pe-x86-64
Disassembly of section .text:
0000000000000000 <Template<void>::~Template()>:
0: ff 25 00 00 00 00 jmpq *0x0(%rip) # 6
<Template<void>::~Template()+0x6>
6: 90 nop
7: 90 nop
```
Check exported symbol in libtest2.dll:
```
$ x86_64-w64-mingw32-objdump -d libtest2.dll.a | x86_64-w64-mingw32-c++filt
In archive libtest2.dll.a:
d000005.o: file format pe-x86-64
d000002.o: file format pe-x86-64
d000004.o: file format pe-x86-64
Disassembly of section .text:
0000000000000000 <NonTemplate::~NonTemplate()>:
0: ff 25 00 00 00 00 jmpq *0x0(%rip) # 6
<NonTemplate::~NonTemplate()+0x6>
6: 90 nop
7: 90 nop
d000003.o: file format pe-x86-64
Disassembly of section .text:
0000000000000000 <NonTemplate::~NonTemplate()>:
0: ff 25 00 00 00 00 jmpq *0x0(%rip) # 6
<NonTemplate::~NonTemplate()+0x6>
6: 90 nop
7: 90 nop
```
the symbols for `Template<void>` is anyway missing in libtest2.dll.
I'm confusing its behavior and believe it's a bug.
