Was SLFing on my then employer's A-310 in '89 on a fine Summer day and noticed frost on the upper surface over the wing tanks.
The area was quite limited, about 20 sq. ft. and towards the rear of the wing; so, choose not to bring it to the attention of the crew. Takeoff was fine.
I sent an inquiry up through channels and got a reply back from the CP that the FOM allowed takeoffs with frost in that location.
After the Dryden report, Transport Canada changed the regs to exclude all upper surface frost.