It might be cleaner to write a custom parser that avoids the implicit conversion (basically parsing an APInt and doing the necessary checks before converting it to uint64_t).

Agreed that would be nicer - I'll merge this for now (once CI has passed) so we have a fix in but I'll try and come back and fix it up properly at a later date. I'll make a tracking issue to remind me once this is merged (edit: #8064).

I assume something like #8062 can still happen after this change (just the bitwidth to trigger it would be smaller)?

Yes, after this fix circt-opt spits out:

hw.module @ImplicitOverflow() {
  %0 = smt.bv.constant #smt.bv<1> : !smt.bv<9223372036854775803>
}

as:

module {
  hw.module @ImplicitOverflow() {
    %c1_bv4294967291 = smt.bv.constant #smt.bv<1> : !smt.bv<4294967291>
    hw.output
  }
}