On Wed, 8 Jun 2016, Jason Merrill wrote: > On Wed, Jun 8, 2016 at 11:16 AM, Marc Glisse <marc.gli...@inria.fr> wrote: > > On Wed, 8 Jun 2016, Richard Biener wrote: > > > >> The following works around PR70992 but the issue came up repeatedly > >> that we are not very consistent in preserving the undefined behavior > >> of division or modulo by zero. Ok - the only inconsistency is > >> that we fold 0 % x to 0 but not 0 % 0 (with literal zero). > >> > >> After folding is now no longer done early in the C family FEs the > >> number of diagnostic regressions with the patch below is two. > >> > >> FAIL: g++.dg/cpp1y/constexpr-sfinae.C -std=c++14 (test for excess errors) > > Yep. We don't want to fold away undefined behavior in a constexpr > function, since constexpr evaluation wants to detect undefined > behavior and treat the expression as non-constant in that case.
Hmm. So 0 / x is not constant because x might be zero but 0 * x is constant because it can never invoke undefined behavior? Does this mean that 0 << n is not const because n might be too large (I suppose 0 << 12000 is not const already)? Is 0 * (-x) const? x might be INT_MIN. So can you clarify on "We don't want to fold away undefined behavior in a constexpr"? Thanks, Richard.
