MSVC build failure with /Zc:__cplusplus and C++17

[djewsbury]

On develop branch (commit bde5712), MSVC will fail to compile if C++17 is disabled and the /Zc:__cplusplus is set.

The compile error seems reasonable:

json.hpp(6122): error C2416: attribute 'deprecated("Since 3.0.0")' cannot be applied in this context
json.hpp(8065): note: see reference to class template instantiation 'nlohmann::basic_json<ObjectType,ArrayType,StringType,BooleanType,NumberIntegerType,NumberUnsignedType,NumberFloatType,AllocatorType,JSONSerializer>' being compiled
json.hpp(6370): error C2416: attribute 'deprecated("Since 3.0.0")' cannot be applied in this context

Here, the "deprecated" attribute is begin applied to a "friend" declaration. There are free operators after the friend declaration (which presumably is what the attribute is supposed to bind to). But since this is technically a friend declaration and not really an operation declaration, MSVC doesn't seem to like it.

In a couple of other places, for static methods, JSON_HEDLEY_DEPRECATED() is compiled correctly.

Hedley seems to be hiding this issue somewhat, due to the behaviour of the __cplusplus define in MSVC. Quite oddly, __cplusplus is set to 199711L by default in MSVC, even when C++17 compilation is enabled. Hedley only reads the __cplusplus macro and interprets that as meaning that the compiler only supports C++98, and so disables the deprecated attribute entirely.

However, we can enable the /Zc:__cplusplus compiler option to make the __cplusplus macro behave in a more standard (or at least more clang/GCC-compatible) way. This causes Hedley to apply the attribute, and the error appears.

There's more info about this oddity in the compiler here: https://docs.microsoft.com/en-us/cpp/build/reference/zc-cplusplus?view=vs-2019

I've worked around it with this block:

#include <nlohmann/thirdparty/hedley/hedley.hpp>
#undef JSON_HEDLEY_DEPRECATED
#define JSON_HEDLEY_DEPRECATED(X)
#include <nlohmann/json.hpp>

Also, when not using the /Zc:__cplusplus option it compiles fine (even when C++17 is enabled). That's not convenient for me to do, though, because it would have to apply for every source file that includes json.hpp

[dconnet]

I've noticed the same thing, only it appears to compile properly in VS2019. Only VS2017 fails (only 2 versions I currently use). I have "/Zc:__cplusplus" set and __cplusplus is at 201402L for both compilers.

[t-b]

Duplicate of #1695.

[stale]

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