Conditional Default Constructor compiles in GCC but not MSVC

Question

I'm working on creating a template class where I have the following constraints:

• If the type of the class is generic, it should have normal constructors
• If the type of the class is NOT generic, it should have explicit constructors.

The solution I came up with looks like this:

#include<type_traits>
#include<vector>

enum class point_type {
    generic, center, corner
};

template<point_type type, typename T>
struct point2_base {
    T x, y;

    template<point_type _type = type, typename std::enable_if_t<_type == point_type::generic, int> = 0>
    point2_base() :
        x(0), y(0) {
    }
    template<point_type _type = type, typename std::enable_if_t<_type != point_type::generic, int> = 0>
    explicit point2_base() :
        x(0), y(0) {
    }
};

using point2f = point2_base<point_type::generic, float>;
using corner2f = point2_base<point_type::corner, float>;
using center2f = point2_base<point_type::center, float>;

This seemed to work just fine (I can instantiate the type with point2f point; or corner2f corner;, etc.), but some esoteric code that I wrote fails to compile in MSVC:

struct line {
    point2f baseline;
    std::vector<int> characters; //The type of the vector doesn't seem to matter
};

int main() {
    std::vector<line> lines;
    lines.emplace_back();
}

Godbolt reports these errors in MSVC:

example.cpp
/opt/compiler-explorer/windows/19.10.25017/lib/native/include/vector(927): warning C4530: C++ exception handler used, but unwind semantics are not enabled. Specify /EHsc
33 : <source>(33): note: see reference to function template instantiation 'void std::vector<line,std::allocator<_Ty>>::emplace_back<>(void)' being compiled
        with
        [
            _Ty=line
        ]
33 : <source>(33): note: see reference to function template instantiation 'void std::vector<line,std::allocator<_Ty>>::emplace_back<>(void)' being compiled
        with
        [
            _Ty=line
        ]
/opt/compiler-explorer/windows/19.10.25017/lib/native/include/xmemory0(840): error C2783: 'point2_base<point_type::corner,float>::point2_base(void)': could not deduce template argument for '__formal'
13 : <source>(13): note: see declaration of 'point2_base<point_type::corner,float>::point2_base'
/opt/compiler-explorer/windows/19.10.25017/lib/native/include/xmemory0(959): note: see reference to function template instantiation 'void std::allocator<_Ty>::construct<_Objty,>(_Objty *)' being compiled
        with
        [
            _Ty=line,
            _Objty=line
        ]
/opt/compiler-explorer/windows/19.10.25017/lib/native/include/xmemory0(959): note: see reference to function template instantiation 'void std::allocator<_Ty>::construct<_Objty,>(_Objty *)' being compiled
        with
        [
            _Ty=line,
            _Objty=line
        ]
/opt/compiler-explorer/windows/19.10.25017/lib/native/include/xmemory0(1097): note: see reference to function template instantiation 'void std::allocator_traits<_Alloc>::construct<_Ty,>(std::allocator<_Ty> &,_Objty *)' being compiled
        with
        [
            _Alloc=std::allocator<line>,
            _Ty=line,
            _Objty=line
        ]
/opt/compiler-explorer/windows/19.10.25017/lib/native/include/xmemory0(1096): note: see reference to function template instantiation 'void std::allocator_traits<_Alloc>::construct<_Ty,>(std::allocator<_Ty> &,_Objty *)' being compiled
        with
        [
            _Alloc=std::allocator<line>,
            _Ty=line,
            _Objty=line
        ]
/opt/compiler-explorer/windows/19.10.25017/lib/native/include/vector(928): note: see reference to function template instantiation 'void std::_Wrap_alloc<std::allocator<_Ty>>::construct<_Ty,>(_Ty *)' being compiled
        with
        [
            _Ty=line
        ]
/opt/compiler-explorer/windows/19.10.25017/lib/native/include/vector(928): note: see reference to function template instantiation 'void std::_Wrap_alloc<std::allocator<_Ty>>::construct<_Ty,>(_Ty *)' being compiled
        with
        [
            _Ty=line
        ]
33 : <source>(33): note: see reference to function template instantiation 'void std::vector<line,std::allocator<_Ty>>::emplace_back<>(void)' being compiled
        with
        [
            _Ty=line
        ]
33 : <source>(33): note: see reference to function template instantiation 'void std::vector<line,std::allocator<_Ty>>::emplace_back<>(void)' being compiled
        with
        [
            _Ty=line
        ]
Microsoft (R) C/C++ Optimizing Compiler Version 19.10.25017 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.
Compiler exited with result code 2

What's strange is that GCC compiles this code with no problems.

I've also found that there are a few things that allow MSVC to compile:

struct line {
    point2f baseline;
    //If I remove the vector, the code compiles
};

//////////

struct line {
    point2f baseline;
    std::vector<int> characters; //The type of the vector doesn't seem to matter
    line() {} //Adding a trivial, explicitly declared constructor allows the code to compile
};

The following does NOT compile in MSVC:

struct line {
    point2f baseline;
    std::vector<int> characters; //The type of the vector doesn't seem to matter
    line() = default; //Same errors as before.
};

Is this a flaw in how I've written the constructors for my point2_base class? Is there a different mechanism I should be using to conditionally set those constructors explicit? Is this just a bug in MSVC?

Answer 1

This appears to be a bug in Visual Studio's compiler, which is fixed as of version 19.14. It is not clear to me which version of Visual Studio fixed this bug.