fix(#279): fix compilation with older msvc

by avoiding known SFINAE problem in msvc 2017

appveyor.yml

@@ -2,27 +2,40 @@ version: "{build}"
 
 environment:
   matrix:
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
+      MSVC_VERSION: 2017
+      CXX_VERSION: 11
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
+      MSVC_VERSION: 2017
+      CXX_VERSION: 14
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
+      MSVC_VERSION: 2017
+      CXX_VERSION: 17
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
-      generator: Visual Studio 16 2019
-      systemver: 10.0.18362.0
-    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2022
-      generator: Visual Studio 17 2022
-      systemver: 10.0.22621.0
+      MSVC_VERSION: 2019
+      CXX_VERSION: 11
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
+      MSVC_VERSION: 2019
+      CXX_VERSION: 14
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
+      MSVC_VERSION: 2019
+      CXX_VERSION: 17
 
 configuration:
-  - Release
   - Debug
 
 clone_depth: 10
 clone_folder: c:\toml11
 
 install:
-- git submodule update --init --recursive
+  - git submodule update --init --recursive
 
 build_script:
+  - '"C:\Program Files (x86)\Microsoft Visual Studio\%MSVC_VERSION%\Community\VC\Auxiliary\Build\vcvarsall.bat" x64'
   - cd C:\toml11
-  - cmake -B build -G"%generator%" -DCMAKE_SYSTEM_VERSION="%systemver%" -A x64 -DCMAKE_CXX_STANDARD=11 -DTOML11_BUILD_TESTS=ON -DTOML11_PRECOMPILE=ON
-  - cmake --build build --config "%configuration%"
+  - cmake -B build -G"NMake Makefiles" -DCMAKE_BUILD_TYPE=%configuration% -DCMAKE_CXX_STANDARD=%CXX_VERSION% -DTOML11_BUILD_TESTS=ON -DBUILD_TESTING=ON -DTOML11_PRECOMPILE=ON
+  - cmake --build build
 
 test_script:
-  - ctest --test-dir build --build-config "%configuration%" --timeout 300 --output-on-failure
+  - cd build\tests\
+  - ctest --timeout 300 --output-on-failure