https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108742
--- Comment #7 from Michael Matz <matz at gcc dot gnu.org> --- (In reply to Jakub Jelinek from comment #5) > https://eel.is/c++draft/cfloat.syn points to the C standard for > FLT_EVAL_METHOD > (plus https://eel.is/c++draft/expr#pre-6 talks about excess precision too) > and e.g. C17 > 5.2.4.2.2/9): > "2 evaluate all operations and constants to the range and precision of the > long double type." > > Note the " and constants" above. Yes. But that leaves unspecified exactly to what bit pattern the string "4.2" should be converted to. Both values used (0x8.666666666666666p-1 and 0x8.6666666666668p-1) are in "the range and precision of the long doubel type". For that: 6.4.4.2 Floating constants (sorry, only c11 here, I hope wording to same effect is still in c17): 4 An unsuffixed floating constant has type double. If suffixed by the letter f or F... 5 Floating constants are converted to internal format as if at translation-time. ... So, my interpretation is that unsuffixed "4.2" has to be the double constant 4.2 (in IEEE double aka 0x1.0cccccccccccdp+2), which is then, because of FLT_EVAL_METHOD, evaluated to "range and precision of long double" leading to 0x8.6666666666668p-1, not to 0x8.666666666666666p-1 .
