cmd/go: no error reported for duplicate module paths among modules in go.work

[findleyr]

Spin off from #53933, where gopls did not present a workspace error for a problematic go.work file because the go command was happy.

The go command does not return an error for duplicate module paths in a go.work file. Instead, it happily lets the last directory win:

$ ls -R
.:
go.work  mod1  mod2

./mod1:
a.go  go.mod

./mod2:
a.go  go.mod
$ cat go.work
go 1.19

use (
        ./mod1
        ./mod2
)
$ cat mod1/go.mod
module golang.org/foo

go 1.18
$ cat mod2/go.mod
module golang.org/foo

go 1.18
$ go list -m -json all
{
        "Path": "golang.org/foo",
        "Main": true,
        "Dir": "/home/rfindley/tmp/duplmodpath/mod2",
        "GoMod": "/home/rfindley/tmp/duplmodpath/mod2/go.mod",
        "GoVersion": "1.18"
}

CC @bcmills

[bcmills]

We do check that the same directory doesn't appear multiple times:
https://cs.opensource.google/go/go/+/master:src/cmd/go/internal/modload/init.go;l=583-585;drc=86b5f6a7be707b9d84108df6f459c7e84bf9dbac

However, we haven't parsed the actual go.mod files at that point and can't check for duplicate module directives.
Probably the right place to do this check is in the makeMainModules helper-function here:
https://cs.opensource.google/go/go/+/master:src/cmd/go/internal/modload/init.go;l=1011;drc=2ff563a00e45308fad74f1f86d816f4f636584a0

[ianwoolf]

i'm working on it. i will send a cl.

[gopherbot]

Change https://go.dev/cl/419557 mentions this issue: cmd/go/internal/modload: return error when duplicate module paths among modules in go.work

[ianwoolf]

please feel free to review, thanks