https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57201
--- Comment #15 from Jonathan Wakely <redi at gcc dot gnu.org> --- PR 60100 seems to be the opposite case, where a warning disappears when -save-temps is used. Related, but maybe not a dup. Anyway, here's another example where -save-temps (or preprocessing and compiling separately) makes a warning disappear, from https://gcc.gnu.org/ml/gcc-help/2019-05/msg00073.html #include <stdio.h> int main() { return NULL; } g++ -Wconversion-null w.cc w.cc: In function ‘int main()’: w.cc:4:8: warning: converting to non-pointer type ‘int’ from NULL [-Wconversion-null] return NULL; ^~~~ g++ -Wconversion-null w.cc -save-temps [no warnings] g++ -Wconversion-null w.cc -save-temps -Wsystem-headers w.cc: In function ‘int main()’: w.cc:4:7: warning: converting to non-pointer type ‘int’ from NULL [-Wconversion-null] return NULL; ^~~~~~ g++ -E w.cc -o w.ii && g++ -Wconversion-null w.ii [no warnings] g++ -E w.cc -o w.ii && g++ -Wconversion-null w.ii -Wsystem-headers w.cc: In function ‘int main()’: w.cc:4:7: warning: converting to non-pointer type ‘int’ from NULL [-Wconversion-null] return NULL; ^~~~~~ So by default we warn, but when using -save-temps (or preprocessing and compiling separately) the warning is suppressed because GCC thinks it's from a system header. Our diagnostics related to macros in system headers are so broken.
