Type-checking a call of the built-in function cap or len records the signature of the respective function for the actual arguments provided; the type can be found in in the Info.Types map supplied to the the type checker through its API.

The type recorded for these two functions uses the argument's underlying type rather than it's actual type. For instance, given

type List []int
var a List
var n = len(a)

records the signature func ([]int) int. It should record func(List) int instead.