Can a player wait to see which unit is targeted by Hextech Ray before using Retreat to bounce that unit, or must targets be declared when the spell is played?

Ruling: When Hextech Ray is played and put on the chain, the target must be declared immediately. Therefore, the defending player knows which unit is targeted before they have the opportunity to respond with Retreat.

Sequence:

• Player B plays Hextech Ray and declares the target when placing it on the chain
• Player A then has the opportunity to respond (e.g., with Retreat) knowing which unit was targeted
• If Player A uses Retreat to bounce a different unit, Hextech Ray will still resolve targeting the originally declared unit

Nuances:

• Some spells with Reflexive Triggers (containing a "do this" clause) work differently - these spells don't target when played, but the triggered abilities they create do require targets when added to the chain after the spell resolves