[cfe-dev] Why does Xcode define _LIBCPP_HAS_NO_ASAN when enabling address sanitizer?

[Jonathan Coe via cfe-dev]

Turning on address sanitizer in Xcode 7 passes the compile and linker flag
-fsanitize=address and also defines _LIBCPP_HAS_NO_ASAN.

When building my library from the command line and running tests on a
sanitized build without defining _LIBCPP_HAS_NO_ASAN I see non-repeatable
address-sanitizer-reported memory access issues. Defining
_LIBCPP_HAS_NO_ASAN, as Xcode does, gets rid of the sanitizer issues but
I'm curious as to why it needs doing.

Why do I need to define _LIBCPP_HAS_NO_ASAN with AppleClang7 to avoid
getting memory access issues in libcxx?

Regards,

Jon
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