https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80756
--- Comment #4 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> --- (In reply to nsz from comment #3) > fabs and fma identifiers are reserved for the implementation and it is valid > to treat them as constant expression in initializers based on c99 6.6p10 Well, if <math.h> is included, perhaps, but this would need to be documented as an implementation-defined behavior. The GCC manual Section 4 "C Implementation-Defined Behavior" doesn't seem to document such constant expressions and when they are available. If <math.h> is not included, there should be at least a warning because fabs() or fma() is used without including the header first, thus potentially yielding unexpected results and obscure bugs.
