cmd/compile: confusing error messages when package paths mismatch

[rogpeppe]

Example error message:

./fakeapi_test.go:29: cannot use names.NewModelTag("dead-beef") (type names.ModelTag) as type names.ModelTag in field value

The error was actually that I'd imported the names package from the wrong path.
It would be nice if the error message could make that clearer - as it is, it
it confusing because both types look identical.

For example:

./fakeapi_test.go:29: cannot use names.NewModelTag("dead-beef") (type names.ModelTag) as type names.ModelTag in field value (package github.com/juju/names, not gopkg.in/juju/names.v2)

Another possibility might be to do general disambiguation when more than one
identical package name referring to a different path is used in the same error
message.

For example:

./fakeapi_test.go:52: cannot use "github.com/juju/names".NewModelTag("dead-beef") (type "github.com/juju/names".ModelTag) as type "gopkg.in/juju/names.v2".ModelTag in field value

[ianlancetaylor]

There is actually code in symfmt in cmd/compile/internal/gc/fmt.go that is supposed to handle this (look for If the name was used by multiple packages). I don't know why it is not working for your example. What is your test case?

[rogpeppe]

Here's a test case that can probably be reduced further.

==> ./b/b.go <==
package b
import "a2"

type T struct {
    X a.X
}

==> ./a1/a.go <==
package a

type X string

func NewX() X {
    return ""
}

==> ./a2/a.go <==
package a

type X string

func NewX() X {
    return ""
}

==> ./p/tst.go <==
package main
import (
    "a1"
    "b"
)

var _ = b.T{
    X: a.NewX(),
}

func main() {
}

Sample run:

% pwd
/tmp/d/src/p
% go install
# p
./tst.go:8: cannot use a.NewX() (type a.X) as type a.X in field value

[griesemer]

It looks like the respective code in the compiler doesn't trigger because numImport[s.Pkg.Name] is always 0 in this case. Possibly an error in the new importer. Investigating.

[griesemer]

Yes, this got lost in the move from the old textual importer to the new binary one. This is a 1.7 regression. We should probably fix it, the fix is simple; with it, the error message is:

tst.go:8: cannot use "/Users/gri/tmp/a".NewX() (type "/Users/gri/tmp/a".X) as type "/Users/gri/tmp/a2".X in field value

(I've compiled all packages in the tmp directory.)
CL coming later today.

[ianlancetaylor]

@rogpeppe Thanks for the test case. This is in fact a regression since 1.6, so setting milestone to 1.7maybe.

1.6.2:

# issue16133
./tst.go:9: cannot use "issue16133/a1".NewX() (type "issue16133/a1".X) as type "issue16133/a2".X in field value

1.7beta2:

# issue16133
./tst.go:9: cannot use a.NewX() (type a.X) as type a.X in field value

[gopherbot]

CL https://golang.org/cl/24312 mentions this issue.