Resolution of inference variables, 18.4, gives special treatment to variables appearing in 'throws' clauses that don't have "extra" bounds. Specifically:

"Otherwise, if the bound set contains throws αi, and the proper upper bounds of αi are, at most, Exception, Throwable, and Object, then Ti = RuntimeException."

The idea here is that RuntimeException satisfies any of these upper bounds, and is also less intrusive, since it isn't a checked exception type.

The type Serializable is also a supertype of RuntimeException, and there's no particular reason such a bound couldn't also be ignored. Proposed new rule:

"Otherwise, if the bound set contains throws αi, and ***each*** proper upper ***bound*** of αi ***is a supertype of RuntimeException***, then Ti = RuntimeException."

This rule is also more future-proof -- if, say, a new interface is added as a supertype of Throwable; and it supports intersection types and type variables as upper bounds, should that situation arise.