I've found very inconclusive and inconsistent results through searches about how Overheal and Shield Bash work together.

Here's the Rune description for reference:

Shield bash

Passive: While shielded, gain 1 − 10 (based on level) bonus armor and bonus magic resistance.

Whenever you gain a shield, your next basic attack against a champion deals 5 − 30 (based on level) (+ 1.5% bonus health) (+ 8.5% shield amount) bonus Adaptive damage. The empowered attack lasts up to 2 seconds after the shield expires.

Adaptive Damage: Deals either physical or magic damage depending on your bonus statistics, defaulting to physical damage.

Overheal

Passive: Converts 20% − 100% (based on level) of the Heal power healing in excess of your maximum health into a Hybrid resistances icon shield for an amount of up to 10 (+ 10% of maximum health).

After 6 seconds, the shield decays by 2.5% of its maximum value each second. Receiving healing, dealing or taking damage will refresh the shield duration to 6 seconds.