https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90829
--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> --- While the OpenACC standard has the specification far more vague than OpenMP, where the standard specifies only a subset of C/C++ for loops to be valid in OpenMP constructs, quick search of OpenACC 2.5 specification reveals e.g.: "A loop associated with a loop construct that does not have a seq clause must be written such that the loop iteration count is computable when entering the loop construct." That is definitely violated in this case.
