Panic: Equal on type aliases causes panic

[bfosberry]

Current behaviour:

With go 1.9.3 and testify 1.2.2 or latest master (f35b8ab) if I require/assert.Equal two aliased types, it causes testify to panic:

goroutine 5 [running]:
testing.tRunner.func1(0xc42010a0f0)
	/Users/brendan.fosberry/.gvm/gos/go1.9.3/src/testing/testing.go:711 +0x2d2
panic(0x1289000, 0xc420066780)
	/Users/brendan.fosberry/.gvm/gos/go1.9.3/src/runtime/panic.go:491 +0x283
github.com/stretchr/testify/assert.diff(0x126fae0, 0xc420010d10, 0x126fae0, 0x130a0b0, 0x0, 0x0)
	/Users/brendan.fosberry/go/src/github.com/stretchr/testify/assert/assertions.go:1352 +0x494
github.com/stretchr/testify/assert.Equal(0x141c480, 0xc42010a0f0, 0x126fae0, 0xc420010d10, 0x126fae0, 0x130a0b0, 0x0, 0x0, 0x0, 0xc420039f01)
	/Users/brendan.fosberry/go/src/github.com/stretchr/testify/assert/assertions.go:338 +0x271
_/Users/brendan.fosberry/workspace/derp.TestMyTypeRequire(0xc42010a0f0)
	/Users/brendan.fosberry/workspace/derp/derp_test.go:19 +0x11f
testing.tRunner(0xc42010a0f0, 0x12e9c88)
	/Users/brendan.fosberry/.gvm/gos/go1.9.3/src/testing/testing.go:746 +0xd0
created by testing.(*T).Run
	/Users/brendan.fosberry/.gvm/gos/go1.9.3/src/testing/testing.go:789 +0x2de
FAIL	_/Users/brendan.fosberry/workspace/derp	0.038s

Expected behaviour: testify should determine if the two types are equal, or fail gracefully.

Here is a test case to replicate the issue

package main

import (
        "testing"

        "github.com/stretchr/testify/assert"
)

type MyType string

const (
        MyTypeA MyType = "a"
        MyTypeB MyType = "b"
)

func TestMyTypeRequire(t *testing.T) {
        mt := MyTypeA
        assert.Equal(t, mt, MyTypeA)
        assert.Equal(t, mt, MyTypeB)
}

I don't know much about reflect, but I think it comes down to this:

	var e, a string
	if ek != reflect.String {
		e = spewConfig.Sdump(expected)
		a = spewConfig.Sdump(actual)
	} else {
		e = expected.(string)
		a = actual.(string)
	}

ek == String comes back as true for string type aliases since they are of kind string, however a type assertion causes a panic since they are not of type string.

My workaround for now is not to use Equal with type aliases, however this is something that should be handled internally.

[ryanmcnamara]

👍 just hit this took a while to debug

[HaraldNordgren]

Created a PR here which should solve the problem 😊

#661

[bfosberry]

Nice, thanks Harald!

[georgelesica-wf]

@HaraldNordgren could you comment on the difference between this solution and the one referenced above (#651)?

[HaraldNordgren]

@georgelesica-wf I'm not sure, both solutions might be equivalent. But the other solution is currently untested which is a problem for a testing library.

[georgelesica-wf]

@HaraldNordgren agreed, thanks for looking at it. I just want to make sure that we resolve all the issues people have seen with this bit of code.