How to assert values in golang unit tests

main_test.go

package main

import (
  "fmt"
  "testing"
)

func assertEqual(t *testing.T, a interface{}, b interface{}, message string) {
  if a == b {
    return
	}
	if len(message) == 0 {
		message = fmt.Sprintf("%v != %v", a, b)
	}
	t.Fatal(message)
}

func TestSimple(t *testing.T) {
  a := 42
  assertEqual(t, a, 42, "")
  assertEqual(t, a, 43, "This message is displayed in place of the default one")
}

This function can be simplified to:

package main

import "testing"

func assertEqual(t *testing.T, a interface{}, b interface{}) {
	if a != b {
		t.Fatalf("%s != %s", a, b)
	}
}

You don't need the return and the testing module provides a Fatalf function to format messages

Additional type info

package main

import (
	"reflect"
	"testing"
)

// AssertEqual checks if values are equal
func AssertEqual(t *testing.T, a interface{}, b interface{}) {
	if a == b {
		return
	}
	// debug.PrintStack()
	t.Errorf("Received %v (type %v), expected %v (type %v)", a, reflect.TypeOf(a), b, reflect.TypeOf(b))
}