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Incorrect warning about unused match cases with first-class modules in patterns #6194

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vicuna opened this issue Sep 26, 2013 · 2 comments

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commented Sep 26, 2013

Original bug ID: 6194
Reporter: @mmottl
Assigned to: @garrigue
Status: closed (set by @xavierleroy on 2015-12-11T18:24:13Z)
Resolution: fixed
Priority: normal
Severity: minor
Version: 4.01.0
Fixed in version: 4.01.1+dev
Category: ~DO NOT USE (was: OCaml general)
Monitored by: @mmottl

Bug description

Lets consider the following code snippet:


module type S = sig val x : bool end

let f = function
| Some (module M : S) when M.x -> ()
| Some _ -> ()
| None -> ()

The compiler will warn that the second match case is unused. This is incorrect, because the first case is guarded. It seems that the warning mistakenly assumes that matching first-class modules in patterns makes them irrefutable even in the presence of guards.

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commented Sep 26, 2013

Comment author: @lpw25

The following fatal error is probably related:

(* match.ml *)
module type S = sig val x : bool end

let f = function
| Some (module M : S) when M.x -> ()
| None -> ();;

$ ocamlc match.ml

Fatal error: Matching.comp_exit
Fatal error: exception Misc.Fatal_error

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commented Sep 26, 2013

Comment author: @garrigue

Fixed in 4.01 branch revision 14189, file parmatch.ml.
Apparently already fixed in trunk (but should check).

@vicuna vicuna closed this Dec 11, 2015

@vicuna vicuna added the bug label Mar 20, 2019

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