http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55602
Bug #: 55602 Summary: Does not generate Error message for redefined macros Classification: Unclassified Product: gcc Version: unknown Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ AssignedTo: unassig...@gcc.gnu.org ReportedBy: guoji...@gmail.com gcc and g++ generate warning message(do not generate any information if there is '#pragma GCC system_header' is the header) instead an error message for redefined macros. While in standard it seems require an error. Second paragraph of section "Macro replacement" in C standard: -------- An identifier currently defined as an object-like macro shall not be redefined by another #define preprocessing directive unless the second definition is an object-like macro definition and the two replacement lists are identical. Likewise, an identifier currently defined as a function-like macro shall not be redefined by another #define preprocessing directive unless the second definition is a function-like macro definition that has the same number and spelling of parameters, and the two replacement lists are identical. ---------- C++ standard: Second paragraph of section "Macro replacement" --------- An identifier currently defined as an object-like macro may be redefined by another #define preprocessing directive provided that the second definition is an object-like macro definition and the two replacement lists are identical, otherwise the program is ill-formed. Likewise, an identifier currently defined as a function-like macro may be redefined by another #define preprocessing directive provided that the second definition is a function-like macro definition that has the same number and spelling of parameters, and the two replacement lists are identical, otherwise the program is ill-formed. ------------------ case: a.h ---------------- #define A 1 #define A 2 --------------- a.c --------------- #include "a.h" int main(){ return A; } --------------