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Quick answer is, nothing! HDPE is EXTREMELY hard to glue since nothing sticks to it well. Fortunately, HDPE bell-and-spigot pipe makes a pretty good press joint, not quite water tight, but darn close.
For actual waterproof joints, I suppose you could use a latex based caulk between the flare and straight pipe, and get a pretty tight, somewhat flexible joint.
For a mechanically strong joint, you'd need a mechanical/hardware solution, like a self-drilling, self tapping stainless steel sheet metal screw.
While there are glues that are supposed to join HDPE, they're expensive, finicky, and generally not very satisfactory.
The other solution is heat welding, which requires accurate heat control, by a tool built for that purpose. Expensive investment, but worth it if you're going to do a lot of pipe joining. The weld is as strong as the pipe.
