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Warning 42: a case where a type constraint does not help a disambiguation #7239

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vicuna opened this Issue Apr 22, 2016 · 5 comments

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commented Apr 22, 2016

Original bug ID: 7239
Reporter: furuse
Assigned to: @gasche
Status: closed (set by @xavierleroy on 2017-09-24T15:32:51Z)
Resolution: fixed
Priority: low
Severity: text
Version: 4.02.3
Target version: 4.03.1+dev
Fixed in version: 4.03.1+dev
Category: ~DO NOT USE (was: OCaml general)
Monitored by: @gasche @hcarty

Bug description

The following (pretty artificial) code produces Warning 42, even with disambiguation by a type constraint:

type a = C
type b = C
let a = (C : a) (* Warning 42: this use of C required disambiguation. *)
let b = (C : b)

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commented Apr 23, 2016

Comment author: @garrigue

Maybe the problem is with the text of the warning.
I think that the intended meaning is: C was chosen based on its type information, in absence of type information, a different choice would have been done.
And this is the case here: the default would be b's C.
The goal of this warning is to check compatibility with previous versions of ocaml (and also know whether your code depends on type information).

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commented Apr 23, 2016

Comment author: @gasche

Note to self: the various disambiguation warnings would be excellent targets for the new documentation section on compiler Warnings.

I agree that changing the text to make it less confusing would be a good idea. What about:

Warning 42: this use of C relies on type-directed disambiguation,
it will not compile on OCaml 4.00 or earlier.

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commented Apr 25, 2016

Comment author: furuse

@gasche, it is great now I understand what Warning 42 is.

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commented Apr 25, 2016

Comment author: @gasche

I propose the change in wording in
#561

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commented Apr 26, 2016

Comment author: @gasche

The clarified wording has been merged in 4.04. Thanks for the report!

@vicuna vicuna closed this Sep 24, 2017

@vicuna vicuna added this to the 4.03.1 milestone Mar 14, 2019

@vicuna vicuna added the bug label Mar 20, 2019

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