[cfe-dev] [libc++] #if defined(_MSC_VER) && !defined(__clang__)

[Shoaib Meenai via cfe-dev]

Clang can operate in Visual Studio compatibility mode, in which case it will define both _MSC_VER and __clang__. The check you posted ensures that the compiler is actually MSVC and not clang in MSVC compatibility mode. https://clang.llvm.org/docs/MSVCCompatibility.html has more information on the MSVC compatibility.

From: cfe-dev <cfe-dev-bounces at lists.llvm.org> on behalf of cfe-dev <cfe-dev at lists.llvm.org>
Reply-To: Christiano SA <christiano at engineer.com>
Date: Sunday, April 15, 2018 at 6:39 PM
To: cfe-dev <cfe-dev at lists.llvm.org>
Subject: [cfe-dev] [libc++] #if defined(_MSC_VER) && !defined(__clang__)

The line 14 of include/__config has the following code: #if defined(_MSC_VER) &&  !defined(__clang__) . I know that the compiler clang defines __clang__ and Visual Studio defines _MSC_VER, however, why should it test the __clang__ if it would only be tested if it is being compiled on Visual Studio, which doesn't have __clang__ anyway?
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