V2015. Identifiers that start with '__' or '_[A-Z]' are reserved.

This diagnostic was added at the request of our users.

As defined by the C++ standard, identifiers that have a double underscore anywhere or begin with an underscore followed by an uppercase letter are reserved for use in the language and standard library implementation. In C, this rule is formulated a bit differently: identifiers that begin with two underscores or an underscore followed by a capital letter are reserved.

Such identifiers may cause problems because of possible clashes with the compiler's reserved identifiers. For example, the compiler may fail to properly optimize the following code:

#include <string.h>

int my_strcmp(const char *s1, const char *s2);
#define __strcmp my_strcmp // <=

void foo(const char *a, const char *b)
  if (!__strcmp(a, b))
    // do something

'__strcmp' could be declared as the built-in (intrinsic) function, which the compiler can optimize better than the direct call of the 'strcmp' function.

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