Extended Description

Macros that expand to "defined" are undefined behavior according to the C standard, but clang, as a language extension, is implementing it by always treating "defined" as the preprocessor operator.

While it is a good idea to add a separate warning flag for -Wexpansion-to-defined, it should only be enabled by default if -pedantic or perhaps -Wextra (and of course -Weverything) is provided. It should not be enabled by -Wall.