Does a creature enchanted with an aura that says 'enchanted creature has haste' still have summoning sickness if the aura was attached before it entered the battlefield?

Summoning sickness (the rule preventing a creature from attacking or using tap/untap abilities) applies to creatures that haven't been under their controller's control since the beginning of that controller's most recent turn. This is governed by CR 302.6. Haste overrides this restriction entirely — a creature with haste can attack and use those abilities the turn it enters the battlefield, regardless of how long it has been under your control.

The key question here is whether the creature has haste as it enters the battlefield. If an aura is placed onto a creature as it enters (for example, via a bestow creature entering attached, or an effect that lets the aura enter attached), the creature has haste from the very first moment it is on the battlefield. CR 611.3 establishes that a continuous effect from an aura applies as long as the aura is attached, which includes the moment of entry.

However, if the aura was already on the battlefield and the creature entered afterward — the most common scenario — the aura attaches to the creature as it enters (CR 303.4f), so the creature still has haste from the moment it enters. Either way, the creature is never in a state where it lacks haste while on the battlefield.

Concrete example: You control an Aura enchantment with 'enchanted creature has haste' attached to nothing (normally impossible, but imagine it enters simultaneously with the creature via a special effect). Your 2/2 creature enters the battlefield already enchanted. You can immediately attack with it that same turn — summoning sickness is bypassed because it has haste from the moment it entered.

Note: If the aura is somehow attached after the creature has already entered without haste, haste will apply going forward, but summoning sickness would have already applied in the window before the aura attached. In practice, auras attach simultaneously or immediately upon the creature entering (CR 303.4f), so this edge case rarely matters.