go/types, types2: missing error for invalid cycle in function declaration

[findleyr]

The following invalid cycle is currently detected by the compiler with the error "typechecking loop involving f".

func f([unsafe.Sizeof(f)]int) {}

https://play.golang.org/p/cfAFJHgsAkX

However no error is detected by go/types, both at master and in the dev.typeparams branch (where there have been some changes to cycle detection in function declarations).

CC @griesemer