Panics on struct arbitraries

The adjusted example from the doc:

package main

import (
    "errors"
    "math/cmplx"

    "github.com/leanovate/gopter"
    "github.com/leanovate/gopter/arbitrary"
    "github.com/leanovate/gopter/gen"
)

type QudraticEquation struct {
    A, B, C complex128
}

func (q QudraticEquation) Eval(x complex128) complex128 {
    return q.A*x*x + q.B*x + q.C
}

func (q QudraticEquation) Solve() (complex128, complex128, error) {
    if q.A == 0 {
        return 0, 0, errors.New("No solution")
    }
    v := q.B*q.B - 4*q.A*q.C
    v = cmplx.Sqrt(v)
    return (-q.B + v) / 2 / q.A, (-q.B - v) / 2 / q.A, nil
}

func main() {
    parameters := gopter.DefaultTestParameters()
    parameters.Rng.Seed(1234) // Just for this example to generate reproducable results

    arbitraries := arbitrary.DefaultArbitraries()
    arbitraries.RegisterGen(gen.Complex128Box(-1e8-1e8i, 1e8+1e8i)) // Only use complex values within a range

    properties := gopter.NewProperties(parameters)

    properties.Property("Quadratic equations can be solved", arbitraries.ForAll(
        func(quadratic QudraticEquation) bool {
            x1, x2, err := quadratic.Solve()
            if err != nil {
                return true
            }

            return cmplx.Abs(quadratic.Eval(x1)) < 1e-5 && cmplx.Abs(quadratic.Eval(x2)) < 1e-5
        }))

    properties.Property("Quadratic equations can be solved alternative", arbitraries.ForAll(
        func(a, b, c complex128) bool {
            quadratic := &QudraticEquation{
                A: a,
                B: b,
                C: c,
            }
            x1, x2, err := quadratic.Solve()
            if err != nil {
                return true
            }

            return cmplx.Abs(quadratic.Eval(x1)) < 1e-5 && cmplx.Abs(quadratic.Eval(x2)) < 1e-5
        }))

    // When using testing.T you might just use: properties.TestingRun(t)
    properties.Run(gopter.ConsoleReporter(false))
}

Output:

! Quadratic equations can be solved: Error on property evaluation after 0
   passed tests: Check paniced: runtime error: invalid memory address or nil
   pointer dereference goroutine 1 [running]:
runtime/debug.Stack(0xc42004ba90, 0x4ffa40, 0x5d0ca0)
    /usr/x86_64-pc-linux-gnu/lib/go/src/runtime/debug/stack.go:24 +0xa7
github.com/hasufell/gopter-examples/vendor/github.com/leanovate/gopter.Save
  Prop.func1.1(0xc42004bd20)
    /home/julian/go/src/github.com/hasufell/gopter-examples/vendor/github.com/
  leanovate/gopter/prop.go:19 +0x6e
panic(0x4ffa40, 0x5d0ca0)
    /usr/x86_64-pc-linux-gnu/lib/go/src/runtime/panic.go:505 +0x229
github.com/hasufell/gopter-examples/vendor/github.com/leanovate/gopter/prop
  .ForAll.func1(0xc420098460, 0x52aaf8)
    /home/julian/go/src/github.com/hasufell/gopter-examples/vendor/github.com/
  leanovate/gopter/prop/forall.go:31 +0x13b
github.com/hasufell/gopter-examples/vendor/github.com/leanovate/gopter.Save
  Prop.func1(0xc420098460, 0x0)
    /home/julian/go/src/github.com/hasufell/gopter-examples/vendor/github.com/
  leanovate/gopter/prop.go:24 +0x6c
github.com/hasufell/gopter-examples/vendor/github.com/leanovate/gopter.Prop
  .Check.func1(0x0, 0xc42007e3d0, 0x5d7f60)
    /home/julian/go/src/github.com/hasufell/gopter-examples/vendor/github.com/
  leanovate/gopter/prop.go:52 +0x15c
github.com/hasufell/gopter-examples/vendor/github.com/leanovate/gopter.(*ru
  nner).runWorkers(0xc420082510, 0x0)
    /home/julian/go/src/github.com/hasufell/gopter-examples/vendor/github.com/
  leanovate/gopter/runner.go:45 +0x2d9
github.com/hasufell/gopter-examples/vendor/github.com/leanovate/gopter.Prop
  .Check(0xc42007e3a0, 0xc4200a05c0, 0x52741e)
    /home/julian/go/src/github.com/hasufell/gopter-examples/vendor/github.com/
  leanovate/gopter/prop.go:110 +0x1af
github.com/hasufell/gopter-examples/vendor/github.com/leanovate/gopter.(*Pr
  operties).Run(0xc420082480, 0x53a700, 0xc420098420, 0x2d)
    /home/julian/go/src/github.com/hasufell/gopter-examples/vendor/github.com/
  leanovate/gopter/properties.go:37 +0xc9
main.main()
    /home/julian/go/src/github.com/hasufell/gopter-examples/gopterexamples.go:
  64 +0x2af

+ Quadratic equations can be solved alternative: OK, passed 100 tests.

leanovate / gopter

Panics on struct arbitraries #27