CL 10821 added generation of nil pointers to terminate recursive structures.

However, the following still does not terminate:

package main

import (
    "reflect"
    "testing/quick"
)

type R struct {
    N []*R
}

func main() {
    quick.Value(reflect.TypeOf(&R{}), nil)
}

The problem with slices (or maps) is the competition between expansion by a factor 'complexSize' and termination with probability '1/complexSize'. Some sort of automatic shrinking of the expansion factor would tip the weight in favour of termination.

To cover all problematic combinations of recursive types one should probably test a monster like the following:

type R struct {
    Ptr      *R
    SliceP   []*R
    Slice    []R
    Map      map[int]R
    MapP     map[int]*R
    MapR     map[*R]*R
    SliceMap []map[int]R
}