The bare-flag check asked pytest's parser which tokens it does not know,
but wrapped parse_known_args in the same blanket except that guards
parser construction. A known flag with a missing value (`--tb` alone)
raises pytest.UsageError there, which the except turned into "nothing
unknown", so discovery ran and every per-file pytest died with
"argument --tb: expected one argument".
Keep the fallback around building the parser only; let parse_known_args
run outside it and surface UsageError (and the unknown-token list) as
this runner's own usage error before discovery. One invariant test.
49397cf2b4