I also ran into this today. Here is a repro as small as I can easily make it (it requires an imported package):

$ mkdir p
$ cat << EOF > p/p.go
> package p
> type X []interface{}
> EOF
$ cat << EOF > vet.go
> package main
> import "./p"
> func main() {
>   var x p.X
>   x = append(x, p.X{1})
> }
> EOF
$ go vet .
vet.go:5: p.X composite literal uses unkeyed fields
exit status 1

To be clear, this seems like a totally bogus warning.

The vet check needs to know about types. cc @josharian @robpike

I believe that this is yet another case of using gcimporter when what we really want to do is to load from source. Note that if you put @cespare's example into GOPATH and use an absolute import rather than a relative import, vet does the right thing.

This is a perennial problem at this point. The correct, deep fix is probably to stabilize the go/loader API (cc @alandonovan), put go/loader in the stdlib, and use it in cmd/vet.

The superficial fix is to not complain when we are missing type information, which seems like a better default anyway. I can send a CL for that if @robpike agrees.

Note that if you put @cespare's example into GOPATH and use an absolute import rather than a relative import, vet does the right thing.

I don't think this is true. I originally found this problem with real packages and only used a relative path for my repro. Here's another GOPATH-based repro:

$ mkdir -p src/p
$ cat << EOF > src/p/p.go
package p
type X []interface{}
EOF
$ mkdir src/vet
$ cat << EOF > src/vet/vet.go
> package main
> import "p"
> func main() {
>   var x p.X
>   x = append(x, p.X{1})
> }
> EOF
$ GOPATH=`pwd` go vet vet
src/vet/vet.go:5: p.X composite literal uses unkeyed fields
exit status 1

@cespare if you go install p vet and then try again, does it still reproduce?

@josharian ah, it does not.

Yep. :) Loading from source would fix that.

Josh: Maybe open an issue for goimporter and link it to here?

This appears to have been fixed in Go 1.7. It's extra-fixed now because in Go 1.10 go vet makes sure that vet has full type information 100% of the time.