boot.user=> (ns foo (:refer-clojure :as [rand-int]))

clojure.lang.Compiler$CompilerException: clojure.lang.ExceptionInfo: Call to clojure.core/refer-clojure did not conform to spec:
                                         In: [0 1] val: :as fails at: [:args :exclude :op :quoted-spec :spec] predicate: #{:exclude}
                                         In: [0 1] val: :as fails at: [:args :only :op :quoted-spec :spec] predicate: #{:only}
                                         In: [0 1] val: :as fails at: [:args :rename :op :quoted-spec :spec] predicate: #{:rename}
                                         In: [0] val: (quote :as) fails at: [:args :exclude :op :spec] predicate: #{:exclude}
                                         In: [0] val: (quote :as) fails at: [:args :only :op :spec] predicate: #{:only}
                                         In: [0] val: (quote :as) fails at: [:args :rename :op :spec] predicate: #{:rename}
                                         :clojure.spec.alpha/spec  #object[clojure.spec.alpha$regex_spec_impl$reify__1200 0x7b34e158 "clojure.spec.alpha$regex_spec_impl$reify__1200@7b34e158"]
                                         :clojure.spec.alpha/value  ((quote :as) (quote [rand-int]))
                                         :clojure.spec.alpha/args  ((quote :as) (quote [rand-int]))
                                          #:clojure.spec.alpha{:problems ({:path [:args :exclude :op :spec], :pred #{:exclude}, :val (quote :as), :via [], :in [0]} {:path [:args :exclude :op :quoted-spec :spec], :pred #{:exclude}, :val :as, :via [], :in [0 1]} {:path [:args :only :op :spec], :pred #{:only}, :val (quote :as), :via [], :in [0]} {:path [:args :only :op :quoted-spec :spec], :pred #{:only}, :val :as, :via [], :in [0 1]} {:path [:args :rename :op :spec], :pred #{:rename}, :val (quote :as), :via [], :in [0]} {:path [:args :rename :op :quoted-spec :spec], :pred #{:rename}, :val :as, :via [], :in [0 1]}), :spec #object[clojure.spec.alpha$regex_spec_impl$reify__1200 0x7b34e158 "clojure.spec.alpha$regex_spec_impl$reify__1200@7b34e158"], :value ((quote :as) (quote [rand-int])), :args ((quote :as) (quote [rand-int]))}, compiling:(/var/folders/42/zcfb2hds38l3shl7lqnjw2gh0000gp/T/boot.user8397969512473900026.clj:1:1)

with expound

boot.user=> (require '[expound.alpha :as expound])
nil
boot.user=> (require '[expound.alpha :as expound])
nil
boot.user=> (require '[clojure.spec.alpha :as s])
nil
boot.user=> (set! s/*explain-out* expound/printer)
#object[expound.alpha$printer 0x694ea272 "expound.alpha$printer@694ea272"]
boot.user=> (ns foo (:refer-clojure :as [rand-int]))

clojure.lang.Compiler$CompilerException: clojure.lang.ExceptionInfo: Call to clojure.core/refer-clojure did not conform to spec:
                                         -- Spec failed --------------------

                                           ((... :as) ...)
                                                 ^^^

                                         should be: `:exclude`