This line in type.go:

    tInterface = bootstrapType("interface", interface{}(nil), 8)

passes a nil to bootstrapType, which calls reflect.NewValue,
which returns a nil reflect.Type.  This manages to pass all the
way through validUserType without causing a crash, because the
type assertion comma-ok form simply fails if an interface is nil.
So the code ends up filling in userTypeCache[nil] with apparently
no ill effect.

Still it's probably not the intent.  One way to address this would
be to pass pointers to bootstrapType:


var (
    // Primordial types, needed during initialization.
    tBool      = bootstrapType("bool", new(bool), 1)
    tInt       = bootstrapType("int", new(int), 2)
    tUint      = bootstrapType("uint", new(uint), 3)
    tFloat     = bootstrapType("float", new(float64), 4)
    tBytes     = bootstrapType("bytes", new([]byte), 5)
    tString    = bootstrapType("string", new(string), 6)
    tComplex   = bootstrapType("complex", new(complex128), 7)
    tInterface = bootstrapType("interface", new(interface{}), 8)
    // Reserve some Ids for compatible expansion
    tReserved7 = bootstrapType("_reserved1", new(struct{ r7 int }), 9)
    tReserved6 = bootstrapType("_reserved1", new(struct{ r6 int }), 10)
    tReserved5 = bootstrapType("_reserved1", new(struct{ r5 int }), 11)
    tReserved4 = bootstrapType("_reserved1", new(struct{ r4 int }), 12)
    tReserved3 = bootstrapType("_reserved1", new(struct{ r3 int }), 13)
    tReserved2 = bootstrapType("_reserved1", new(struct{ r2 int }), 14)
    tReserved1 = bootstrapType("_reserved1", new(struct{ r1 int }), 15)
)

and then the first line of bootstrapType would be

    rt := reflect.Typeof(e).(*reflect.PtrType).Elem()

I tried this and it still passes the unit tests but I don't know
if there is some subtle effect that is not being caught.