Long noses and tongues are an important part of most dogs cooling system. They pant and with air, cool the fluids in their mouths which helps cool the blood vessels in their heads. In addition, panting helps release heat from their bodies.
Because pugs have virtually no noses, they have inefficient internal air conditioning. They need to be protected when it gets too hot outside. Anything above 80 or 85 degrees can challenge a pug. In summer months, don't leave your pug outside for too long and make sure they have plenty of water.
Your dog may love soaking up the sun, but they can't always be trusted. It is up to you to be smart about the heat.