Ok, upon going through the code, I found these two places:

https://github.com/golang/go/blob/master/src/cmd/go/build.go#L949
https://github.com/golang/go/blob/master/src/cmd/go/test.go#L951

_test.go files are not included in coverage. Fine but is it possible that go install/build can be trained to throw a more verbose error message? For example: Definition found in xxx_test.go, please change the file name to something other than xxx_test.go

Apparently this issue has the side effect of resulting in inconsistent test runs whether you run go test against a relative path (like ./...) or an absolute (import-like) path, like github.com/user/pkg.

Namely, if you run go test ./..., and your test files live in package foo_test, and they import foo, but they rely on something exported under *_test.go files (a common example is theexport_test.go, like the one in pkg fmt), go test will import the package from $GOPATH, not from wherever go test ./... is run (which makes sense). However, as @faizoff mentioned, the *_test.go files are not included in those builds (not even in tests builds, when the package being tested is other than the package that is imported, minus the _test suffix).

This usually never happens because people tend to run go test ./... while they're in the source tree. However, it does happen with the new Wercker Ewok, which runs Go tests in a different directory (/pipeline/source). This results in CI tests that use go test ./... failing, while CI tests that use go test github.com/user/pkg/... are passing.

/cc @hatchan

Working as intended. The *_test.go files are compiled into the package only when running go test for that package.