Dropping MSVC 2015 (#3722)

* Changing `_MSC_VER` guard to `< 1910` (dropping MSVC 2015).

* Removing MSVC 2015 from ci.yml, and .appveyor.yml entirely.

* Bringing back .appveyor.yml from master.

* appveyor Visual Studio 2017

* 1st manual pass, builds & tests with unix_clang, before pre-commit.

* After clang-format (via pre-commit).

* Manual pass looking for "2015", builds & tests with unix_clang, before pre-commit.

* Backtracking for include/pybind11 changes in previous commit.

git checkout d07865846c include/pybind11/attr.h include/pybind11/detail/common.h include/pybind11/functional.h

--------------------

CI #4160 errors observed:

2a26873727
https://github.com/pybind/pybind11/runs/5168332130?check_suite_focus=true

$ grep ' error C' *.txt | sed 's/2022-02-12[^ ]*//' | sed 's/^[0-9][0-9]*//' | sed 's/^.*\.txt: //' | sort | uniqD:\a\pybind11\pybind11\include\pybind11\cast.h(1364,1): error C2752: 'pybind11::detail::type_caster<Eigen::Ref<Eigen::Vector3f,0,pybind11::EigenDStride>,void>': more than one partial specialization matches the template argument list [D:\a\pybind11\pybind11\build\tests\pybind11_tests.vcxproj]

d:\a\pybind11\pybind11\include\pybind11\detail/common.h(1023): error C2737: 'pybind11::overload_cast': 'constexpr' object must be initialized [D:\a\pybind11\pybind11\build\tests\cross_module_gil_utils.vcxproj]
d:\a\pybind11\pybind11\include\pybind11\detail/common.h(1023): error C2737: 'pybind11::overload_cast': 'constexpr' object must be initialized [D:\a\pybind11\pybind11\build\tests\pybind11_cross_module_tests.vcxproj]
d:\a\pybind11\pybind11\include\pybind11\detail/common.h(1023): error C2737: 'pybind11::overload_cast': 'constexpr' object must be initialized [D:\a\pybind11\pybind11\build\tests\pybind11_tests.vcxproj]
d:\a\pybind11\pybind11\include\pybind11\detail/common.h(1023): error C2737: 'pybind11::overload_cast': 'constexpr' object must be initialized [D:\a\pybind11\pybind11\build\tests\test_embed\external_module.vcxproj]
D:\a\pybind11\pybind11\include\pybind11\detail/common.h(624): fatal error C1001: Internal compiler error. [D:\a\pybind11\pybind11\build\tests\pybind11_tests.vcxproj]
D:\a\pybind11\pybind11\include\pybind11\detail/common.h(624): fatal error C1001: Internal compiler error. [D:\a\pybind11\pybind11\tests\pybind11_tests.vcxproj]

$ grep ': error C2737' *.txt | sed 's/^.*MSVC//' | sed 's/___.*//' | sort | uniq

_2017

$ grep ': error C2752' *.txt

3______3.8_____MSVC_2019_____x86_-DCMAKE_CXX_STANDARD=17.txt:2022-02-12T16:12:45.9921122Z D:\a\pybind11\pybind11\include\pybind11\cast.h(1364,1): error C2752: 'pybind11::detail::type_caster<Eigen::Ref<Eigen::Vector3f,0,pybind11::EigenDStride>,void>': more than one partial specialization matches the template argument list [D:\a\pybind11\pybind11\build\tests\pybind11_tests.vcxproj]

$ grep ': fatal error C1001:' *.txt

10______pypy-3.8-v7.3.7_____windows-2022_____x64.txt:2022-02-12T16:12:56.3163683Z D:\a\pybind11\pybind11\include\pybind11\detail/common.h(624): fatal error C1001: Internal compiler error. [D:\a\pybind11\pybind11\tests\pybind11_tests.vcxproj]
1______3.6_____MSVC_2019_____x86.txt:2022-02-12T16:12:47.6774625Z D:\a\pybind11\pybind11\include\pybind11\detail/common.h(624): fatal error C1001: Internal compiler error. [D:\a\pybind11\pybind11\build\tests\pybind11_tests.vcxproj]
16______3.6_____windows-latest_____x64_-DPYBIND11_FINDPYTHON=ON.txt:2022-02-12T16:12:27.0556151Z D:\a\pybind11\pybind11\include\pybind11\detail/common.h(624): fatal error C1001: Internal compiler error. [D:\a\pybind11\pybind11\tests\pybind11_tests.vcxproj]
17______3.9_____windows-2019_____x64.txt:2022-02-12T16:12:30.3822566Z D:\a\pybind11\pybind11\include\pybind11\detail/common.h(624): fatal error C1001: Internal compiler error. [D:\a\pybind11\pybind11\tests\pybind11_tests.vcxproj]
2______3.7_____MSVC_2019_____x86.txt:2022-02-12T16:12:38.7018911Z D:\a\pybind11\pybind11\include\pybind11\detail/common.h(624): fatal error C1001: Internal compiler error. [D:\a\pybind11\pybind11\build\tests\pybind11_tests.vcxproj]
6______3.6_____windows-2022_____x64.txt:2022-02-12T16:12:00.4513642Z D:\a\pybind11\pybind11\include\pybind11\detail/common.h(624): fatal error C1001: Internal compiler error. [D:\a\pybind11\pybind11\tests\pybind11_tests.vcxproj]
7______3.9_____windows-2022_____x64.txt:2022-02-12T16:11:43.6306160Z D:\a\pybind11\pybind11\include\pybind11\detail/common.h(624): fatal error C1001: Internal compiler error. [D:\a\pybind11\pybind11\tests\pybind11_tests.vcxproj]
8______3.10_____windows-2022_____x64.txt:2022-02-12T16:11:49.9589644Z D:\a\pybind11\pybind11\include\pybind11\detail/common.h(624): fatal error C1001: Internal compiler error. [D:\a\pybind11\pybind11\tests\pybind11_tests.vcxproj]
9______pypy-3.7-v7.3.7_____windows-2022_____x64.txt:2022-02-12T16:11:53.7912112Z D:\a\pybind11\pybind11\include\pybind11\detail/common.h(624): fatal error C1001: Internal compiler error. [D:\a\pybind11\pybind11\tests\pybind11_tests.vcxproj]

* common.h: is_template_base_of

* Re-applying 4 changes from 2a26873727 that work universally.

* `overload_cast = {};` only for MSVC 2017 and Clang 5

* Refining condition for using is_template_base_of workaround.

* Undoing MSVC 2015 workaround in test_constants_and_functions.cpp

* CentOS7: silence_unused_warnings

* Tweaks in response to reviews.

* Adding windows-2022 C++20

* Trying another way of adding windows-2022 C++20

.appveyor.yml

@@ -1,6 +1,6 @@
 version: 1.0.{build}
 image:
-- Visual Studio 2015
+- Visual Studio 2017
 test: off
 skip_branch_with_pr: true
 build:
@@ -13,7 +13,7 @@ environment:
     CONFIG: Debug
 install:
 - ps: |
-    $env:CMAKE_GENERATOR = "Visual Studio 14 2015"
+    $env:CMAKE_GENERATOR = "Visual Studio 15 2017"
     if ($env:PLATFORM -eq "x64") { $env:PYTHON = "$env:PYTHON-x64" }
     $env:PATH = "C:\Python$env:PYTHON\;C:\Python$env:PYTHON\Scripts\;$env:PATH"
     python -W ignore -m pip install --upgrade pip wheel

.github/workflows/ci.yml

@@ -755,53 +755,6 @@ jobs:
     - name: Python tests
       run: cmake --build build -t pytest
 
-  win32-msvc2015:
-    name: "🐍 ${{ matrix.python }} • MSVC 2015 • x64"
-    runs-on: windows-latest
-    strategy:
-      fail-fast: false
-      matrix:
-        python:
-          - 3.6
-          - 3.7
-          # todo: check/cpptest does not support 3.8+ yet
-
-    steps:
-    - uses: actions/checkout@v2
-
-    - name: Setup 🐍 ${{ matrix.python }}
-      uses: actions/setup-python@v2
-      with:
-        python-version: ${{ matrix.python }}
-
-    - name: Update CMake
-      uses: jwlawson/actions-setup-cmake@v1.12
-
-    - name: Prepare MSVC
-      uses: ilammy/msvc-dev-cmd@v1.10.0
-      with:
-        toolset: 14.0
-
-    - name: Prepare env
-      run: |
-        python -m pip install -r tests/requirements.txt
-
-    # First build - C++11 mode and inplace
-    - name: Configure
-      run: >
-        cmake -S . -B build
-        -G "Visual Studio 14 2015" -A x64
-        -DPYBIND11_WERROR=ON
-        -DDOWNLOAD_CATCH=ON
-        -DDOWNLOAD_EIGEN=ON
-
-    - name: Build C++14
-      run: cmake --build build -j 2
-
-    - name: Run all checks
-      run: cmake --build build -t check
-
-
   win32-msvc2017:
     name: "🐍 ${{ matrix.python }} • MSVC 2017 • x64"
     runs-on: windows-2016

README.rst

@@ -122,7 +122,7 @@ Supported compilers
 1. Clang/LLVM 3.3 or newer (for Apple Xcode's clang, this is 5.0.0 or
    newer)
 2. GCC 4.8 or newer
-3. Microsoft Visual Studio 2015 Update 3 or newer
+3. Microsoft Visual Studio 2017 or newer
 4. Intel classic C++ compiler 18 or newer (ICC 20.2 tested in CI)
 5. Cygwin/GCC (previously tested on 2.5.1)
 6. NVCC (CUDA 11.0 tested in CI)

docs/advanced/cast/stl.rst

@@ -87,8 +87,6 @@ included to tell pybind11 how to visit the variant.
 
     pybind11 only supports the modern implementation of ``boost::variant``
     which makes use of variadic templates. This requires Boost 1.56 or newer.
-    Additionally, on Windows, MSVC 2017 is required because ``boost::variant``
-    falls back to the old non-variadic implementation on MSVC 2015.
 
 .. _opaque:
 

docs/advanced/classes.rst

@@ -1118,13 +1118,6 @@ described trampoline:
     py::class_<A, Trampoline>(m, "A") // <-- `Trampoline` here
         .def("foo", &Publicist::foo); // <-- `Publicist` here, not `Trampoline`!
 
-.. note::
-
-    MSVC 2015 has a compiler bug (fixed in version 2017) which
-    requires a more explicit function binding in the form of
-    ``.def("foo", static_cast<int (A::*)() const>(&Publicist::foo));``
-    where ``int (A::*)() const`` is the type of ``A::foo``.
-
 Binding final classes
 =====================
 

docs/basics.rst

@@ -32,8 +32,7 @@ The last line will both compile and run the tests.
 Windows
 -------
 
-On Windows, only **Visual Studio 2015** and newer are supported since pybind11 relies
+On Windows, only **Visual Studio 2017** and newer are supported.
-on various C++11 language features that break older versions of Visual Studio.
 
 .. Note::
 

docs/classes.rst

@@ -434,8 +434,7 @@ you can use ``py::detail::overload_cast_impl`` with an additional set of parenth
         .def("set", overload_cast_<int>()(&Pet::set), "Set the pet's age")
         .def("set", overload_cast_<const std::string &>()(&Pet::set), "Set the pet's name");
 
-.. [#cpp14] A compiler which supports the ``-std=c++14`` flag
+.. [#cpp14] A compiler which supports the ``-std=c++14`` flag.
-            or Visual Studio 2015 Update 2 and newer.
 
 .. note::
 

docs/faq.rst

@@ -220,20 +220,6 @@ In addition to decreasing binary size, ``-fvisibility=hidden`` also avoids
 potential serious issues when loading multiple modules and is required for
 proper pybind operation.  See the previous FAQ entry for more details.
 
-Working with ancient Visual Studio 2008 builds on Windows
-=========================================================
-
-The official Windows distributions of Python are compiled using truly
-ancient versions of Visual Studio that lack good C++11 support. Some users
-implicitly assume that it would be impossible to load a plugin built with
-Visual Studio 2015 into a Python distribution that was compiled using Visual
-Studio 2008. However, no such issue exists: it's perfectly legitimate to
-interface DLLs that are built with different compilers and/or C libraries.
-Common gotchas to watch out for involve not ``free()``-ing memory region
-that that were ``malloc()``-ed in another shared library, using data
-structures with incompatible ABIs, and so on. pybind11 is very careful not
-to make these types of mistakes.
-
 How can I properly handle Ctrl-C in long-running functions?
 ===========================================================
 

include/pybind11/attr.h

@@ -61,7 +61,7 @@ struct base {
 
     PYBIND11_DEPRECATED(
         "base<T>() was deprecated in favor of specifying 'T' as a template argument to class_")
-    base() {} // NOLINT(modernize-use-equals-default): breaks MSVC 2015 when adding an attribute
+    base() = default;
 };
 
 /// Keep patient alive while nurse lives
@@ -82,8 +82,7 @@ struct metaclass {
     handle value;
 
     PYBIND11_DEPRECATED("py::metaclass() is no longer required. It's turned on by default now.")
-    // NOLINTNEXTLINE(modernize-use-equals-default): breaks MSVC 2015 when adding an attribute
+    metaclass() = default;
-    metaclass() {}
 
     /// Override pybind11's default metaclass
     explicit metaclass(handle value) : value(value) {}

include/pybind11/detail/common.h

@@ -47,7 +47,7 @@
 // or newer.
 #    if _MSVC_LANG >= 201402L
 #        define PYBIND11_CPP14
-#        if _MSVC_LANG > 201402L && _MSC_VER >= 1910
+#        if _MSVC_LANG > 201402L
 #            define PYBIND11_CPP17
 #            if _MSVC_LANG >= 202002L
 #                define PYBIND11_CPP20
@@ -81,10 +81,8 @@
 #        error pybind11 requires gcc 4.8 or newer
 #    endif
 #elif defined(_MSC_VER)
-// Pybind hits various compiler bugs in 2015u2 and earlier, and also makes use of some stl features
+#    if _MSC_VER < 1910
-// (e.g. std::negation) added in 2015u3:
+#        error pybind11 2.10+ requires MSVC 2017 or newer
-#    if _MSC_FULL_VER < 190024210
-#        error pybind11 requires MSVC 2015 update 3 or newer
 #    endif
 #endif
 
@@ -149,7 +147,7 @@
 
 /* Don't let Python.h #define (v)snprintf as macro because they are implemented
    properly in Visual Studio since 2015. */
-#if defined(_MSC_VER) && _MSC_VER >= 1900
+#if defined(_MSC_VER)
 #    define HAVE_SNPRINTF 1
 #endif
 
@@ -561,7 +559,7 @@ static_assert(std::is_standard_layout<instance>::value,
               "Internal error: `pybind11::detail::instance` is not standard layout!");
 
 /// from __cpp_future__ import (convenient aliases from C++14/17)
-#if defined(PYBIND11_CPP14) && (!defined(_MSC_VER) || _MSC_VER >= 1910)
+#if defined(PYBIND11_CPP14)
 using std::conditional_t;
 using std::enable_if_t;
 using std::remove_cv_t;
@@ -824,10 +822,10 @@ struct is_template_base_of_impl {
 /// Check if a template is the base of a type. For example:
 /// `is_template_base_of<Base, T>` is true if `struct T : Base<U> {}` where U can be anything
 template <template <typename...> class Base, typename T>
-#if !defined(_MSC_VER)
+#if defined(PYBIND11_CPP20) || !defined(_MSC_VER) // Sadly, all MSVC versions incl. 2022 need this.
 using is_template_base_of
     = decltype(is_template_base_of_impl<Base>::check((intrinsic_t<T> *) nullptr));
-#else // MSVC2015 has trouble with decltype in template aliases
+#else
 struct is_template_base_of
     : decltype(is_template_base_of_impl<Base>::check((intrinsic_t<T> *) nullptr)) {
 };
@@ -1002,9 +1000,6 @@ struct nodelete {
 PYBIND11_NAMESPACE_BEGIN(detail)
 template <typename... Args>
 struct overload_cast_impl {
-    // NOLINTNEXTLINE(modernize-use-equals-default):  MSVC 2015 needs this
-    constexpr overload_cast_impl() {}
-
     template <typename Return>
     constexpr auto operator()(Return (*pf)(Args...)) const noexcept -> decltype(pf) {
         return pf;
@@ -1031,8 +1026,12 @@ PYBIND11_NAMESPACE_END(detail)
 ///  - regular: static_cast<Return (Class::*)(Arg0, Arg1, Arg2)>(&Class::func)
 ///  - sweet:   overload_cast<Arg0, Arg1, Arg2>(&Class::func)
 template <typename... Args>
+#    if (defined(_MSC_VER) && _MSC_VER < 1920) /* MSVC 2017 */                                    \
+        || (defined(__clang__) && __clang_major__ == 5)
 static constexpr detail::overload_cast_impl<Args...> overload_cast = {};
-// MSVC 2015 only accepts this particular initialization syntax for this variable template.
+#    else
+static constexpr detail::overload_cast_impl<Args...> overload_cast;
+#    endif
 #endif
 
 /// Const member function selector for overload_cast
@@ -1118,7 +1117,7 @@ try_get_shared_from_this(std::enable_shared_from_this<T> *holder_value_ptr) {
 
 // For silencing "unused" compiler warnings in special situations.
 template <typename... Args>
-#if defined(_MSC_VER) && _MSC_VER >= 1910 && _MSC_VER < 1920 // MSVC 2017
+#if defined(_MSC_VER) && _MSC_VER < 1920 // MSVC 2017
 constexpr
 #endif
     inline void

include/pybind11/functional.h

@@ -99,8 +99,7 @@ public:
             Return operator()(Args... args) const {
                 gil_scoped_acquire acq;
                 object retval(hfunc.f(std::forward<Args>(args)...));
-                /* Visual studio 2015 parser issue: need parentheses around this expression */
+                return retval.template cast<Return>();
-                return (retval.template cast<Return>());
             }
         };
 

tests/pybind11_tests.h

@@ -3,12 +3,6 @@
 #include <pybind11/eval.h>
 #include <pybind11/pybind11.h>
 
-#if defined(_MSC_VER) && _MSC_VER < 1910
-// We get some really long type names here which causes MSVC 2015 to emit warnings
-#    pragma warning(                                                                              \
-        disable : 4503) // NOLINT: warning C4503: decorated name length exceeded, name was truncated
-#endif
-
 namespace py = pybind11;
 using namespace pybind11::literals;
 

tests/test_class.cpp

@@ -354,13 +354,7 @@ TEST_SUBMODULE(class_, m) {
         using ProtectedA::foo;
     };
 
-    py::class_<ProtectedA>(m, "ProtectedA")
+    py::class_<ProtectedA>(m, "ProtectedA").def(py::init<>()).def("foo", &PublicistA::foo);
-        .def(py::init<>())
-#if !defined(_MSC_VER) || _MSC_VER >= 1910
-        .def("foo", &PublicistA::foo);
-#else
-        .def("foo", static_cast<int (ProtectedA::*)() const>(&PublicistA::foo));
-#endif
 
     class ProtectedB {
     public:
@@ -391,11 +385,7 @@ TEST_SUBMODULE(class_, m) {
 
     py::class_<ProtectedB, TrampolineB>(m, "ProtectedB")
         .def(py::init<>())
-#if !defined(_MSC_VER) || _MSC_VER >= 1910
         .def("foo", &PublicistB::foo);
-#else
-        .def("foo", static_cast<int (ProtectedB::*)() const>(&PublicistB::foo));
-#endif
 
     // test_brace_initialization
     struct BraceInitialization {

tests/test_const_name.cpp

@@ -4,13 +4,6 @@
 
 #include "pybind11_tests.h"
 
-#if defined(_MSC_VER) && _MSC_VER < 1910
-
-// MSVC 2015 fails in bizarre ways.
-#    define PYBIND11_SKIP_TEST_CONST_NAME
-
-#else // Only test with MSVC 2017 or newer.
-
 // IUT = Implementation Under Test
 #define CONST_NAME_TESTS(TEST_FUNC, IUT)                                                          \
     std::string TEST_FUNC(int selector) {                                                         \
@@ -51,18 +44,10 @@ CONST_NAME_TESTS(const_name_tests, py::detail::const_name)
 CONST_NAME_TESTS(underscore_tests, py::detail::_)
 #endif
 
-#endif // MSVC >= 2017
-
 TEST_SUBMODULE(const_name, m) {
-#ifdef PYBIND11_SKIP_TEST_CONST_NAME
-    m.attr("const_name_tests") = "PYBIND11_SKIP_TEST_CONST_NAME";
-#else
     m.def("const_name_tests", const_name_tests);
-#endif
 
-#ifdef PYBIND11_SKIP_TEST_CONST_NAME
+#if defined(PYBIND11_DETAIL_UNDERSCORE_BACKWARD_COMPATIBILITY)
-    m.attr("underscore_tests") = "PYBIND11_SKIP_TEST_CONST_NAME";
-#elif defined(PYBIND11_DETAIL_UNDERSCORE_BACKWARD_COMPATIBILITY)
     m.def("underscore_tests", underscore_tests);
 #else
     m.attr("underscore_tests") = "PYBIND11_DETAIL_UNDERSCORE_BACKWARD_COMPATIBILITY not defined.";

tests/test_constants_and_functions.cpp

@@ -137,18 +137,15 @@ TEST_SUBMODULE(constants_and_functions, m) {
     m.def("f4", f4);
 
     // test_function_record_leaks
-    struct LargeCapture {
+    m.def("register_large_capture_with_invalid_arguments", [](py::module_ m) {
         // This should always be enough to trigger the alternative branch
         // where `sizeof(capture) > sizeof(rec->data)`
-        uint64_t zeros[10] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
+        uint64_t capture[10] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
-    };
+#if defined(__GNUC__) && __GNUC__ == 4 // CentOS7
-    m.def("register_large_capture_with_invalid_arguments", [](py::module_ m) {
+        py::detail::silence_unused_warnings(capture);
-        LargeCapture capture; // VS 2015's MSVC is acting up if we create the array here
+#endif
         m.def(
-            "should_raise",
+            "should_raise", [capture](int) { return capture[9] + 33; }, py::kw_only(), py::arg());
-            [capture](int) { return capture.zeros[9] + 33; },
-            py::kw_only(),
-            py::arg());
     });
     m.def("register_with_raising_repr", [](py::module_ m, const py::object &default_value) {
         m.def(

tests/test_eigen.cpp

@@ -14,9 +14,6 @@
 #include "pybind11_tests.h"
 
 #if defined(_MSC_VER)
-#    if _MSC_VER < 1910                 // VS 2015's MSVC
-#        pragma warning(disable : 4127) // C4127: conditional expression is constant
-#    endif
 #    pragma warning(disable : 4996) // C4996: std::unary_negation is deprecated
 #endif
 

tests/test_factory_constructors.cpp

@@ -381,13 +381,6 @@ TEST_SUBMODULE(factory_constructors, m) {
             ::operator delete(p);
         }
         static void operator delete(void *, void *) { py::print("noisy placement delete"); }
-#if defined(_MSC_VER) && _MSC_VER < 1910
-        // MSVC 2015 bug: the above "noisy delete" isn't invoked (fixed in MSVC 2017)
-        static void operator delete(void *p) {
-            py::print("noisy delete");
-            ::operator delete(p);
-        }
-#endif
     };
 
     py::class_<NoisyAlloc> pyNoisyAlloc(m, "NoisyAlloc");

tests/test_iostream.cpp

@@ -7,10 +7,6 @@
     BSD-style license that can be found in the LICENSE file.
 */
 
-#if defined(_MSC_VER) && _MSC_VER < 1910 // VS 2015's MSVC
-#    pragma warning(disable : 4702)      // unreachable code in system header (xatomic.h(382))
-#endif
-
 #include <pybind11/iostream.h>
 
 #include "pybind11_tests.h"

tests/test_smart_ptr.cpp

@@ -8,10 +8,6 @@
     BSD-style license that can be found in the LICENSE file.
 */
 
-#if defined(_MSC_VER) && _MSC_VER < 1910 // VS 2015's MSVC
-#    pragma warning(disable : 4702)      // unreachable code in system header (xatomic.h(382))
-#endif
-
 #include "object.h"
 #include "pybind11_tests.h"
 

tests/test_stl.cpp

@@ -37,7 +37,7 @@ struct type_caster<boost::none_t> : void_caster<boost::none_t> {};
 // Test with `std::variant` in C++17 mode, or with `boost::variant` in C++11/14
 #if defined(PYBIND11_HAS_VARIANT)
 using std::variant;
-#elif defined(PYBIND11_TEST_BOOST) && (!defined(_MSC_VER) || _MSC_VER >= 1910)
+#elif defined(PYBIND11_TEST_BOOST)
 #    include <boost/variant.hpp>
 #    define PYBIND11_HAS_VARIANT 1
 using boost::variant;

tests/test_virtual_functions.cpp

@@ -154,8 +154,7 @@ class NCVirtTrampoline : public NCVirt {
 };
 
 struct Base {
-    /* for some reason MSVC2015 can't compile this if the function is pure virtual */
+    virtual std::string dispatch() const = 0;
-    virtual std::string dispatch() const { return {}; };
     virtual ~Base() = default;
     Base() = default;
     Base(const Base &) = delete;