go/build: MultiplePackageError values swapped.

[dmitshur]

This issue is present in the latest commit on master branch, f34964e.

MultiplePackageError is defined in src/go/build/build.go#L420-L426 with Dir, then Packages, and finally Files:

// MultiplePackageError describes a directory containing
// multiple buildable Go source files for multiple packages.
type MultiplePackageError struct {
    Dir      string   // directory containing files
    Packages []string // package names found
    Files    []string // corresponding files: Files[i] declares package Packages[i]
}

It is used in the following incorrect way in src/go/build/build.go#L690-L692:

} else if pkg != p.Name {
    return p, &MultiplePackageError{p.Dir, []string{firstFile, name}, []string{p.Name, pkg}}
}

If one rewrites that composite literal to use keyed fields, the mistake becomes obvious:

return p, &MultiplePackageError{
    Dir:      p.Dir,
    Packages: []string{firstFile, name}, // Packages being set to filenames.
    Files:    []string{p.Name, pkg},     // Files being set to package names.
}

Notably, the last 2 parameter values should be swapped.

[bradfitz]

Nice find. We should have a vet rule detect all unkeyed literals when the struct has any two fields with the same type.

/cc @adonovan

[dmitshur]

That sounds like a good idea.

For posterity, this issue was introduced in commit 4f80b50.

[alandonovan]

We should have a vet rule detect all unkeyed literals when the struct has any two fields with the same type.

Perhaps we should also consider the case when field value in a literal is assignable to more than one field. This is a much larger set. For example:

type T struct { uint; int; float64 }
var _ = T{0, 1, 2}

[bradfitz]

About two seconds after I wrote my comment I knew somebody would point that out. Yes, @alandonovan, assignable. :-)

[josharian]

Huh. Apparently github doesn't send emails from linked issues. Anyway, I filed #9421 to track this proposed vet rule.

[bradfitz]

Sent https://go-review.googlesource.com/2040 for this bug.