What did you do?

Write Go programs corresponding to the interfaces illustrated in Fig. 5 and Fig. 7 of Featherweight Go:

Figure 5: https://go.dev/play/p/pkXtATz0zJW?v=gotip
Figure 7: https://go.dev/play/p/w1ULe7lazaT?v=gotip

What did you expect to see?

Behavior per the design described in https://go.dev/design/43651-type-parameters and accepted in #43651:

A generic type can refer to itself in cases where a type can ordinarily refer to itself, but when it does so the type arguments must be the type parameters, listed in the same order. This restriction prevents infinite recursion of type instantiation.

Or, an accepted proposal modifying the design, including a discussion of the tradeoffs involved.

What did you see instead?

invalid recursive type errors.

It appears that what was actually implemented (and what is described in the current spec) does not match what was accepted in the proposal. The spec text was changed to:

Within a type parameter list of a generic type T, a type constraint may not (directly, or indirectly through the type parameter list of another generic type) refer to T.

That breaks the examples from Figure 5 and Figure 7 of the paper, and results in an implementation of type parameters weaker than what was proposed.

For comparison, if the type parameters are removed, both figures compile:
Figure 5 (parameter removed from Eq): https://go.dev/play/p/DKHGVraxMRX?v=gotip
Figure 7 (parameters removed from Edge and Vertex): https://go.dev/play/p/YVxG0BOQgqm?v=gotip

(CC @ianlancetaylor @griesemer)