Using testing.T as anonymous struct field: "too many arguments in call to this.T.common.Fail"

I am trying to solve the Karate Chop kata in Go as an exercise and stuck with this compiler error from my test case:

> too many arguments in call to this.T.common.Fail

I wrapped testing.T into a struct with additional methods, as an anonymous struct field:

package main

import (
	"fmt"
	"testing"
)

type assertions struct {
	*testing.T
}
func (this assertions) assert_equal(expected int, actual int) {
	if (expected != actual) {
		this.Fail(fmt.Sprintf("Failed asserting that %v is %v", actual, expected));
	}
}

func TestChop(t *testing.T) {
  test := assertions{t}
  
  test.assert_equal(-1, Chop(3, []int{}))
  test.assert_equal(-1, Chop(3, []int{1}))
  ...
}

I expect this.Fail to call Fail() on the anonymous testing.T struct field, which takes a string parameter. Why isn't this the case and where does this.T.common.Fail come from? I cannot find any reference to common in the testing package documentation.

> Source file src/testing/testing.go
>
>
> // Fail marks the function as having failed but continues execution.
> func (c *common) Fail() {
> c.mu.Lock()
> defer c.mu.Unlock()
> c.failed = true
> }
>
> // common holds the elements common between T and B and
> // captures common methods such as Errorf.
> type common struct {
> mu sync.RWMutex // guards output and failed
> output []byte // Output generated by test or benchmark.
> failed bool // Test or benchmark has failed.
> skipped bool // Test of benchmark has been skipped.
> finished bool
>
> start time.Time // Time test or benchmark started
> duration time.Duration
> self interface{} // To be sent on signal channel when done.
> signal chan interface{} // Output for serial tests.
> }
>
> // T is a type passed to Test functions to manage test state and support formatted test logs.
> // Logs are accumulated during execution and dumped to standard error when done.
> type T struct {
> common
> name string // Name of test.
> startParallel chan bool // Parallel tests will wait on this.
> }
>
> func (*T) Fail
>
> func (c *T) Fail()
>
> Fail marks the function as having failed but continues execution.

There are no arguments to T.common.Fail().

Try Errorf:

> func (*T) Errorf
>
> func (c *T) Errorf(format string, args ...interface{})
>
> Errorf is equivalent to Logf followed by Fail.

For example,

this.Errorf("Failed asserting that %v is %v", actual, expected)