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Typecore.force_delayed_checks does not run with -i option. #7620

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vicuna opened this Issue Sep 5, 2017 · 6 comments

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vicuna commented Sep 5, 2017

Original bug ID: 7620
Reporter: furuse
Assigned to: @garrigue
Status: resolved (set by @garrigue on 2017-09-12T23:38:15Z)
Resolution: fixed
Priority: normal
Severity: minor
Version: 4.05.0
Fixed in version: 4.06.0 +dev/beta1/beta2/rc1
Category: compiler driver

Bug description

The following program passes ocamlc -i, but not ocamlc:


let t =
(function A | B -> () : 'a) (A : [A]);
(failwith "dummy" : 'a) (* to know how 'a is unified *)

With ocamlc -i,


val t : [ `A ] -> unit

but with ocamlc


File "x.ml", line 2, characters 17-19:
Error: This pattern matches values of type [? B ] but a pattern was expected which matches values of type [ A ]
The second variant type does not allow tag(s) `B

This is because Typecore.force_delayed_checks never runs when -i option is given.

There is another issue of Typecore.force_delayed_checks: it runs only after printing the module signature in Compile.interface and Optcompile.interface.

I think they are bugs, but are they done intentionally? Even if they are intentional, Typecore.force_delayed_checks must run after printing the signatures for ocamlc -i.

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vicuna commented Sep 5, 2017

Comment author: @garrigue

This is clearly a bug. Going to fix it.

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vicuna commented Sep 5, 2017

Comment author: @garrigue

Actually, this is more complicated than I thought:
force_delayed_checks is also used for unused definition warning, which is dependent on the presence of an mli.
This is why it was moved outside of the -i path.
The answer is probably that we should disable unused definition warnings when -i is used...
Alain, any comment?

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vicuna commented Sep 5, 2017

Comment author: @garrigue

Tentative fix: #1317

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vicuna commented Sep 6, 2017

Comment author: furuse

BTW, why is it handled as a type error? Is there an example to break the type safety by changing it to a warning?

FYI, the example was found in a use of ppx_implicits, where, roughly speaking, pre-process a typed code, for example, from

(function A | B -> ()) (assert false)

to

(function A | B -> ()) (A : [A])

It relies heavily on the principal type property, but it is broken here...

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vicuna commented Sep 12, 2017

Comment author: @garrigue

Fixed in trunk by commit e286dbd.
Call Typecore.force_delayed_checks before printing the signature for ocamlc -i.

@vicuna vicuna closed this Sep 12, 2017

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vicuna commented Sep 12, 2017

Comment author: @garrigue

@furuse, is your question about the fix, or about the original behavior?

Note that the value restriction breaks the principal type property from the beginning (non-generalizable type variables are not allowed to remain in compilation units).

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