unused: false positive involving unnamed struct types, named struct types and implicit conversions

[kylrth]

I discovered this while using the unused linter in golangci-lint. I hope I found the right place to report this issue. 🤞

Here's some code that reproduces it:

package unused_test

import "fmt"

type Thing struct {
	stuff int
	has   struct {
		a bool
		b bool
	}
}

func ExampleThing() {
	type temp struct {
		a bool
		b bool
	}

	stuff := map[string]Thing{
		"hello": {has: temp{true, false}},
	}

	fmt.Println(stuff["hello"].stuff, stuff["hello"].has)
	// Output: 0 {true false}
}

Running ./golangci-lint run unused_test.go results in unused_test.go:14:7: type `temp` is unused (unused).

[dgryski]

Does this reproduce with the latest staticcheck?

[kylrth]

Yes, I get the following output with staticcheck built from master on this repo:

unused_test.go:14:7: type temp is unused (U1000)

Run './staticcheck -explain <check>' or visit https://staticcheck.io/docs/checks for documentation on checks.

[dominikh]

Simpler reproducer:

package foo

type Thing struct {
	has struct {
		a bool
	}
}

func Fn() {
	type temp struct {
		a bool
	}

	x := Thing{
		has: temp{true},
	}
	_ = x
}

the issue seems to be caused by the implicit conversion to the unnamed struct type in Thing.

[qdm12]

I can confirm (found using golangci-lint v1.44.2 built using Go 1.18):

package main

import (
	"fmt"
)

type someStruct struct {
	parentField struct {
		n int
	}
}

type someStruct2 struct {
	n int
}

func main() {
	x := someStruct{
		parentField: someStruct2{n: 1},
	}
	fmt.Println(x)
}

Error:

type `someStruct2` is unused (unused)