Drop C++ standard tests for MSVC versions which do not implement requ…

…ired features.

Signed-off-by: Richard Musil <[email protected]>

.github/external_ci/appveyor.yml

@@ -41,7 +41,7 @@ environment:
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
       configuration: Release
       platform: x86
-      CXX_FLAGS: "/permissive- /std:c++latest /utf-8 /W4 /WX"
+      CXX_FLAGS: "/permissive- /std:c++17 /utf-8 /W4 /WX"
       CMAKE_OPTIONS: ""
       GENERATOR: Visual Studio 15 2017
 
@@ -62,7 +62,7 @@ environment:
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
       configuration: Release
       platform: x64
-      CXX_FLAGS: "/permissive- /std:c++latest /Zc:__cplusplus /utf-8 /W4 /WX"
+      CXX_FLAGS: "/permissive- /std:c++17 /Zc:__cplusplus /utf-8 /W4 /WX"
       CMAKE_OPTIONS: ""
       GENERATOR: Visual Studio 15 2017
 

tests/CMakeLists.txt

@@ -31,7 +31,7 @@ if (CMAKE_CXX_COMPILER_ID STREQUAL "Clang" AND CMAKE_CXX_COMPILER_VERSION VERSIO
     unset(compiler_supports_cpp_17)
 endif()
 # MSVC 2015 (14.0) does not support C++17
-if (CMAKE_CXX_COMPILER_ID STREQUAL "MSVC" AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 19.1)
+if (CMAKE_CXX_COMPILER_ID STREQUAL "MSVC" AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 19.10)
     unset(compiler_supports_cpp_17)
 endif()
 # GCC 5 and 6 do claim experimental support for C++17, but do not implement <optional>
@@ -43,6 +43,10 @@ endif()
 if (CMAKE_CXX_COMPILER_ID STREQUAL "Clang" AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 9.0)
     unset(compiler_supports_cpp_20)
 endif()
+# MSVC 2017 (15.x) does not support C++20
+if (CMAKE_CXX_COMPILER_ID STREQUAL "MSVC" AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 19.20)
+    unset(compiler_supports_cpp_20)
+endif()
 # GCC started supporting C++20 features in 8.0 but a test for #3070 segfaults prior to 9.0
 if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 9.0)
     unset(compiler_supports_cpp_20)