Incorrectly flags last switch case

[vfaronov]

Given the following toy.go:

package toy

import "math/rand"

func Foo() string {
	switch rand.Intn(2) {
	case 0:
		return "zero"
	case 1:
		return "one"
	}
	panic("impossible")
}

and the following toy_test.go:

package toy

import (
	"math/rand"
	"testing"
)

func TestFoo_zero(t *testing.T) {
	rand.Seed(0)
	if s := Foo(); s != "zero" {
		t.Fatalf(`%q != "zero"`, s)
	}
}

func TestFoo_one(t *testing.T) {
	rand.Seed(1)
	if s := Foo(); s != "one" {
		t.Fatalf(`%q != "one"`, s)
	}
}

gobco reports:

Branch coverage: 3/4
toy.go:9:7: condition "rand.Intn(2) == 1" was once true but never false

although both cases are actually covered.

[rillig]

That's a tricky situation, and you need to read the message very carefully.

toy.go:9:7: condition "rand.Intn(2) == 1" was once true but never false

The "once true" refers to TestFoo_one. The "never false" says that the cases rand.Intn(2) == 0 and rand.Intn(2) == 1 are covered, but other values for rand.Intn(2) were never seen. This is not surprising since rand.Intn(2) is guaranteed to only ever return 0 or 1, never a higher number.

In other words, gobco is trying to tell you that you don't need the case 1 and could simply replace it with default:.

If you have an idea how to improve gobco's message to make it easier to understand, please tell me. Otherwise you can just close the issue since gobco is correct here.

[vfaronov]

Ah, right, thank you.

If you have an idea how to improve gobco's message to make it easier to understand, please tell me.

This would help:

toy.go:6:2: "switch rand.Intn(2)" never took the implicit default branch

[rillig]

I thought about the improved diagnostic again, and I'm not going to implement it.

• If the new message were to replace the current one, it wouldn't be beneficial to omit the text of the condition from the message.
• Adding a separate message for uncovered cases would generate two separate messages for the same underlying issue.
• The suggested message, although nicely worded, doesn't fit well into the set of existing messages, as all these messages pinpoint to a specific condition.