case-lambda allows multiple cases with the same arity

[gfbee]

[I originally opened this as an issue for racket, but it more clearly violates the docs for typed/racket.]

Although for racket it doesn't seem explicitly forbidden, the typed/racket docs say:

Note that each formals must have a different arity.

Should this perhaps be an expansion-time error:

> (case-lambda [([z : Zero]) z]
               [([o : One]) o])
- : (-> Zero Zero)

This came up because a student encountering the case-> type constructor wondered whether case-lambda would provide overloading by type.

[samth]

I think this is a doc bug -- that case is allowed.

[bennn]

Note that the case-> formals must (currently) have a different arity if the type is being used to describe an untyped function:

#lang typed/racket/base

(require/typed racket/base
  (values
   (case-> (-> Integer Integer)
           (-> Symbol Symbol))))
;; Type Checker: Type .... could not be converted to a contract: function type has two cases of arity 1

[samth]

Right, but case-> is a type constructor and case-lambda, which this report is about, is a term constructor.

[spdegabrielle]

@gfbee is this still an issue?

[bennn]

Yes this is still an issue on Racket 7.4. The code runs the same and the docs make the same statement about formals and arity.