mongo-cxx driver cannot find polyfill for unique_ptr

I'm trying to build mongo-cxx-driver on Windows; I'm following the instructions, but am getting errors:

C1189   #error:  "Cannot find a valid polyfill for make_unique"

Here's what I did:

Building mongo-c-driver

I cloned mongo-c-driver using the specified minimum release (1.15):

git clone https://github.com/mongodb/mongo-c-driver --branch r1.15

Using CMake-Gui (I'm using 3.18.2), I generated the .proj file using Visual Studio 16 2019 Generator. I used the default settings:

This builds successfully, and I can find the corresponding .dll and .lib files in C:\Program Files (x86)\mongo-c-driver.

Building mongo-cxx-driver

I cloned the project using my desired version (compatible with the above version of mongo-c-driver):

git clone https://github.com/mongodb/mongo-cxx-driver --branch releases/v3.5

I used Cmake-Gui to configure the CMake parameters, configuring the paths to where I installed mongo-c:

Of note:

• I did not fill out bson-1.0_DIR or mongoc-1.0_DIR because I can't figure out what they're looking for.
• I set BSONCXX_POLY_USE_STD=1 because I am using C++17 and do not want to use std::experimental or Boost.

I tried to compile this in Visual Studio, and got the above error about not finding a valid polyfill for make_unique. Where did I go wrong?

thank you!

The answer seems to be in the installation guide, I missed a step:

For building with Visual Studio 2017 (without a C++17 polyfill), it is necessary to configure with an additional option, /Zc:__cplusplus to opt into the correct definition of __cplusplus (problem described here):

'C:\Program Files (x86)\CMake\bin\cmake.exe' ..
-G "Visual Studio 15 2017 Win64"
-DCMAKE_CXX_STANDARD=17
-DCMAKE_CXX_FLAGS="/Zc:__cplusplus"
-DCMAKE_PREFIX_PATH=C:\mongo-c-driver
-DCMAKE_INSTALL_PREFIX=C:\mongo-cxx-driver \

I added the "/Zc:__cplusplus" flag as required and it compiled fine!