https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97965
Bug ID: 97965 Summary: constexpr evaluation vs. NaNs inconsistency Product: gcc Version: 11.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: jakub at gcc dot gnu.org Target Milestone: --- constexpr bool a = __builtin_nan ("") > 0.0; constexpr bool b = __builtin_nans ("") > 0.0; constexpr bool c = __builtin_nan ("") < 0.0; constexpr bool d = __builtin_nans ("") < 0.0; strangely accepts the < 0.0 comparisons and rejects the > 0.0 comparisons. clang++ accepts all of them. IMHO either we should accept all of them, or just the ones not involving SNaNs, or reject all of them, it is unclear what exceptions appart from division by zero (and does that apply to floating point?) should cause constexpr evaluation to fail (I'd hope inexact exception doesn't count, another question is underflow/overflow, another one is invalid operations that from non-NaN operands create NaN, another one are operations with NaNs, another one are operations with SNaNs). Seems the reason why < 0.0 is accepted is fold_binary_loc uses tree_expr_nonnegative_warnv_p on the NaN REAL_CST which in the end uses tree_single_nonnegative_warnv_p which uses !REAL_VALUE_NEGATIVE. While NaNs have a sign in the representation, it shouldn't affect behavior of the comparisons, so I think we should never treat NaNs with the sign bit clear as non-negative.