reports the correct __cplusplus macro for MSVC IntelliSense Mode

[elahehrashedi]

feature-request: #2595
the language-server side is also altered to accommodate this feature.
The reason I am adding a default value for _Cplusplus is that if the IntelliSense mode is MSVC and the std version is c++17, the default value for _cplusplus is 201703L.

[sean-mcmanus]

How does this work? I wouldn't think "/Zc" would be a valid define. It would think it would have be passed as a compilerArg, although then cpptools would have to be updated to process that.

[bobbrow]

Does cl.exe set this by default in newer versions? You have to opt in. I didn't expect to see any change to the TypeScript for this issue.

EDIT: From: https://devblogs.microsoft.com/cppblog/msvc-now-correctly-reports-__cplusplus/
You need to compile with the /Zc:__cplusplus switch to see the updated value of the __cplusplus macro. We tried updating the macro by default and discovered that a lot of code doesn’t compile correctly when we change the value of __cplusplus. We’ll continue to require use of the /Zc:__cplusplus switch for all minor versions of MSVC in the 19.xx family.

[sean-mcmanus]

Does cl.exe set this by default in newer versions? You have to opt in. I didn't expect to see any change to the TypeScript for this issue.

EDIT: From: https://devblogs.microsoft.com/cppblog/msvc-now-correctly-reports-__cplusplus/
You need to compile with the /Zc:__cplusplus switch to see the updated value of the __cplusplus macro. We tried updating the macro by default and discovered that a lot of code doesn’t compile correctly when we change the value of __cplusplus. We’ll continue to require use of the /Zc:__cplusplus switch for all minor versions of MSVC in the 19.xx family.

That seems to imply we shouldn't change the defaults?

[bobbrow]

We shouldn't change the defaults for now. When the compiler updates the version to 20.xx we may have to make some changes, but we're still on 19.xx now.