* (setf sb-ext:*on-package-variance* '())
NIL

* (defpackage #:foo (:export #:bar))
#<PACKAGE "FOO">

* (defpackage #:foo (:export))
WARNING: FOO also exports the following symbols:
  (FOO:BAR)
See also:
  The ANSI Standard, Macro DEFPACKAGE
  The SBCL Manual, Variable *ON-PACKAGE-VARIANCE*
#<PACKAGE "FOO">

* (setf sb-ext:*on-package-variance* '(:warn ()))
(:WARN NIL)

* (defpackage #:foo (:export))
WARNING: FOO also exports the following symbols:
  (FOO:BAR)
See also:
  The ANSI Standard, Macro DEFPACKAGE
  The SBCL Manual, Variable *ON-PACKAGE-VARIANCE*
#<PACKAGE "FOO">