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The following definition for static_assert() from DCL03-A. Use a static assertion to test the value of a constant expression uses the JOIN() macro to concatenate the token assertion_failed_at_line_ with the value of __LINE__.

#define static_assert(e) \
  typedef char JOIN(assertion_failed_at_line_, __LINE__) \
    [(e) ? 1 : -1]

{{\_\_LINE\_\_}} is a predefined macro names which expands to an integer constant representing the presumed line number of the current source line within the current source file \[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\].

If the intention is to expand the __LINE__ macro, which is likely the case here, the following definition for JOIN() is non-compliant:

#define JOIN(x, y) x ## y

because the __LINE__ is not expanded, and the character array is subsequently named assertion_failed_at_line___LINE__.

Compliant Solution

To get the macro to expand, a second level of indirection is required, as shown by this compliant solution:

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