I've started looking into this.

The underlying issue happens in cmd/go/pkg.go around line 1570. The code attempts to turn something like "./ioutil" into "io/ioutil". The code is problematic because when specifying a path inside GOROOT or GOPATH, go/build will set ImportPath to a plausible import path. Whether the package actually exists isn't checked.

Thus, with GOPATH=/go, ImportDir("/go/src/foo", 1) will unconditionally set ImportPath to "foo".

I am wondering whether the ./ioutil -> io/ioutil rewrite is even a useful feature and whether the logic should be fixed, or removed entirely.

It's useful, it just shouldn't trigger a search. When you're in src/math and run go test ./rand you want to get math/rand not a relative import.

CL https://golang.org/cl/16944 mentions this issue.

CL 16944 didn't work and was abandoned. (Writing this because I click on it every time I view this issue.) The actual fix is not obvious.

This is fixed in module mode, but not in GOPATH mode.

_gopath/src/example.com$ go env GOPATH
/tmp/tmp.VrFzeLPnb6/_gopath:/tmp/tmp.Nz3gatY2ZI:/tmp/tmp.MZ8FiGUB8Q

_gopath/src/example.com$ GO111MODULE=off go install ./aoeuhtns
can't load package: package example.com/aoeuhtns: cannot find package "example.com/aoeuhtns" in any of:
        /usr/local/google/home/bcmills/go/src/example.com/aoeuhtns (from $GOROOT)
        /tmp/tmp.VrFzeLPnb6/_gopath/src/example.com/aoeuhtns (from $GOPATH)
        /tmp/tmp.Nz3gatY2ZI/src/example.com/aoeuhtns
        /tmp/tmp.MZ8FiGUB8Q/src/example.com/aoeuhtns

_gopath/src/example.com$ GO111MODULE=on go install ./aoeuhtns
can't load package: package example.com/aoeuhtns: unknown import path "example.com/aoeuhtns": cannot find package