Describe the bug

The unexported-return rule produces non-deterministic and incorrect results when a directory contains both source files and external test files (package foo + package foo_test):

1. Wrong package qualifier — the return type is reported as *foo_test.T instead of *foo.T.
2. Flaky detection — the issue is sometimes not reported at all (~15% of runs in the reproduction below).

Root cause: dots.ResolvePackages returns GoFiles + TestGoFiles + XTestGoFiles collapsed into a single []string. lint.Linter.lintPackage then puts all of them into one lint.Package whose TypeCheck picks anyFile by iterating a Go map (non-deterministic order) and passes that file's package name to go/types.Config.Check. When anyFile is an XTestGoFile (package foo_test) the whole package is type-checked under the foo_test name, and mixing files from two different Go packages in a single types.Config.Check call also causes some types to fail to resolve (yielding nil from TypeOf), which silently drops the issue.

To Reproduce

Steps to reproduce the behavior:

1. I updated revive go install github.com/mgechev/revive@v1.15.0
2. Minimal reproduction: https://gist.github.com/far4599/937fb8f53d644c54cddbb07d769ba98d

git clone https://gist.github.com/far4599/937fb8f53d644c54cddbb07d769ba98d revive-repro
cd revive-repro
go mod tidy
bash reproduce.sh

revive-repro/
  go.mod                  # module revive-repro
  pub.go                  # generic function returning unexported type
  pub_internal_test.go    # package pub       (internal test)
  pub_external_test.go    # package pub_test  (external test)
  reproduce.sh            # runs revive 20 times and classifies results

pub.go:

package pub

type impl[T any] struct{ val T }

func New[T any](v T) *impl[T] { return &impl[T]{val: v} }

pub_internal_test.go (package pub):

package pub
import "testing"
func TestInternal(t *testing.T) { _ = New[int](1) }

pub_external_test.go (package pub_test):

package pub_test
import (
    "testing"
    "revive-repro"
)
func TestExternal(t *testing.T) { _ = pub.New[string]("x") }

No custom config file is required — the default rules are enough to trigger the bug.

Expected behavior

Every run produces exactly one issue with the correct package qualifier:

pub.go:6:22: exported func New returns unexported type *pub.impl[T], which can be annoying to use

Logs

Typical bash reproduce.sh output over 20 runs:

Run 1:  WRONG   — pub.go:6:22: ... *pub_test.impl[T] ...
Run 8:  MISSING — issue not reported at all
Run 18: OK      — pub.go:6:22: ... *pub.impl[T] ...

Results:
  OK (correct package name):  1 / 20
  WRONG (pub_test.impl[T]):  16 / 20
  MISSING (race, lost):       3 / 20

Desktop (please complete the following information):

• OS: macOS 26.4
• Go: 1.26.0

Additional context

Non-generic unexported return types are not affected — only generic functions trigger the bug, because the go/types package qualifies generic type names with the enclosing package path while non-generic ones often match regardless of which test variant was used.

Downstream impact: this race surfaces in golangci-lint as nolintlint flagging //nolint:revive directives as "unused" on the runs where revive drops the issue, producing intermittent CI failures.

A fix with RED→GREEN test is available at https://github.com/far4599/revive — the repro test fails without the fix and passes with it, and all existing tests continue to pass.