That's not a vet error message. That file is failing the type-checking phase that precedes the vet checks.

The reason the file fails typecheking is that this:

var parentPersPreArgs string

PersistentPreRun: func(_ *Command, args []string) {
    parentPersPreArgs = strings.Join(args, " ")
},

doesn't count as using the parentPersPreArgs variable. The program should not compile, but it does because of #3059. But go/types, which is used by vet to type-check, correctly marks the variable as unused.

Root cause is the decision taken on #21287 (cmd/vet: decide how to handle type checking failure). I quote:

So the decision here is "go vet" will be fixed so that it can always typecheck, and then failure to typecheck will be a fatal error.

This happened in go1.10, but AFAIK it wasn't documented in the release notes. I don't see it here:

https://golang.org/doc/go1.10#vet

We could document this in the 1.11 release notes(?)

Is the error message misleading?

vet: typecheck failures

anyway, regardless of where the origin of this change is, the changed behavior should be documented clearly. It's a newly observed behavior from 1.11.

Is the error message misleading?

I think it's correct, but too brief. Maybe it should be changed to something like "vet: the package did not typecheck"

the changed behavior should be documented clearly

Agree, but what changed? vet typechecks since 1.10, and test runs vet since 1.10. Why is this failing now on tip and not on 1.10?

Ah, my bad. This is false:

vet typechecks since 1.10

it was put in, disabled to fix a go/types issue, and then re-enabled again during the 1.11 cycle, here:

https://go-review.googlesource.com/c/go/+/108555

So it should be documented.

Change https://golang.org/cl/121455 mentions this issue: doc: document new vet behaviour for typechecking failures

Sorry, I'm experiencing this error and I'd like to know what I'm supposed to be doing instead of

var parentPersPreArgs string

PersistentPreRun: func(_ *Command, args []string) {
    parentPersPreArgs = strings.Join(args, " ")
},

Why is this considered a failure?

Sorry, I'm experiencing this error

Where? When?

I'd like to know what I'm supposed to be doing

Use the parentPersPreArgs variable in some way, or remove it from the code.

Why is this considered a failure?

Because unused variables are not allowed in Go (at least in gc and in go/types), and parentPersPreArgs is an unused variable.

@ALTree I'm experiencing it with the same closure usecase as you posted. By my reading, parentPersPreArgs is "used" when strings.Join assigns its result to it. I don't understand what makes it "unused."
Until I can refactor the code to improve the poor scoping, I have worked around this "typecheck" issue with _ = parentPersPreArgs.

A value that is only ever written to but never read from is effectively unused. go/types (which vet uses) is more strict about this than the compiler.