The root cause is the following:
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Erlang/OTP 26 added new linting errors to detect previously invalid (but not warned) typespecs: dialyzer: fix singleton type var in union types by kikofernandez · Pull Request #6864 · erlang/otp · GitHub
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cowlib was affected by this. A new version (2.9.1) was released but Cowboy depends on cowlib (2.9.0)
For now, you can either forcibly bump cowlib by declaring 2.9.1 with override or wait until there is a new cowboy version out. I have also asked the Erlang/OTP team if it can be a warning for now.
