Confusing error message when mixing polymorphic variant and type variable name starting with a capital letter

[vicuna]

Original bug ID: 7012
Reporter: @Octachron
Assigned to: @garrigue
Status: closed (set by @xavierleroy on 2017-02-16T14:14:47Z)
Resolution: fixed
Priority: low
Severity: minor
Version: 4.02.3
Fixed in version: 4.03.0+dev / +beta1
Category: ~DO NOT USE (was: OCaml general)
Monitored by: @gasche @hcarty

Bug description

The problem appears with

#type t = [ 'A_name | `Hi ]
Error: The type 'a is not a polymorphic variant type.

For some reasons, the type variable name 'A_name seems to be forgotten and replaced by the generic and confusing 'a.

Moreover, this happens only when the type variable name starts with a capital letter, more standard type variable names are not subject to this erasure:

type t = [ 'a_name | `Hi ] ;;

Error: The type 'a_name is not a polymorphic variant type

[vicuna]

Comment author: @Octachron

Correction:
The compiler simply forgets the name of type variable starting with a capital letter.

Compare

let f (x:'id_arg) = x;;

val f : 'id_arg -> 'id_arg =

with

let f (x:'Id_arg) = x;;

val f : 'a -> 'a =

[vicuna]

Comment author: @garrigue

Fixed in trunk at revision 16489.

Didn't know that variable names could start with a capital letter...

[vicuna]

Comment author: talex

Might be worth extending the error message in this case with a "- did you mean `A_name"?

[vicuna]

Comment author: @gasche

I extended the error message with a hint as you suggest.

Note that while

type t = [ 'Foo | 'Bar ]

(or any combination with a ' and a `)
will get you the hint, just writing

type t = [ 'Foo ]

is caught as a syntax error at an earlier stage, and does not benefit from the same treatment. We're thinking about general improvements on syntax error reporting, so I won't spend time on this error specifically.

[vicuna]

Comment author: talex

Thanks!