[llvm-bugs] [Bug 51414] New: Exporting nested classes doesn't compile and link with Clang on Windows

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=51414

            Bug ID: 51414
           Summary: Exporting nested classes doesn't compile and link with
                    Clang on Windows
           Product: clang
           Version: 12.0
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: peterghost86 at gmail.com
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org,
                    neeilans at live.com, richard-llvm at metafoo.co.uk

The following code won't compile and link using `clang-cl /std:c++17 test.cpp`.

#include <optional>
#include <vector>
#include <string>
#include <iostream>

#define EXPORT_OUTER __declspec(dllexport)
#define EXPORT_INNER __declspec(dllexport)

class EXPORT_OUTER A
{
public:
    class EXPORT_INNER B
    {
    public:
        B(
            const std::optional<std::vector<std::string>> &opt =
                std::nullopt)
        {
        }
    };
};

int main()
{
    auto b = A::B();
    return 0;
}

Error:
test-e1dae6.obj: error LNK2019: Unresolved external symbol "public: __cdecl
std::optional<class std::vector<class std::basic_string<char,struct
std::char_traits<char>,class std:: allocator<char> >,class std::allocator<class
std::basic_string<char,struct std::char_traits<char>,class
std::allocator<char>>>> >::~optional<class std:: vector<class
std::basic_string<char,struct std::char_traits<char>,class std::allocator<char>
>,class std::allocator<class std::basic_string<char,struct
std::char_traits<char >,class std::allocator<char>>>> >(void)"
(??1?$optional at V?$vector at V?$basic_string at DU?$char_traits at D@std@@V?$allocator
@D at 2@@std@@V?$allocator at V?$basic_string at DU?$char_traits at D@std@@V?$allocator at D@2@@std@@@2@@std@@@
std@@QEAA at XZ), the symbol is referenced in the function main
test.exe: fatal error LNK1120: 1 unresolved external command
clang-cl: error: linker command failed with exit code 1120 (use -v to see
invocation) 

However, the same code compiles and links if
1. using cl.exe from MSVC
2. using `#define EXPORT_INNER` instead
3. removing the default value of `opt` in the ctor of the class B and passing
in `std::nullopt` explicitly

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