Consider the following code (playground):

package main

import (
	"io/ioutil"
)

func F(type T, U)(func(T) (U, error)) {}

func main() {
	F(ioutil.ReadAll) // F(io.Reader, []byte)(ioutil.ReadAll)
}

Note that in the call to F, the type parameter T ends up being inferred as io.Reader, which is not explicitly imported (although of course it is transitively). This results in the following confusing errors:

./prog.go2:9: cannot use ioutil.ReadAll (type func(io.Reader) ([]byte, error)) as type func(<T>) ([]byte, error) in argument to instantiate୦୦F୦io୮aReader୦୮6୮7byte
./prog.go2:7: undefined: io in io.Reader

Adding a blank import _ "io" works around the problem; presumably this shouldn't be necessary.