Fix for #83 (improve constraint error message)

[Martin521]

This fix for #83 includes

• An addition of field "source" to TraitConstraintInfo / TTrait in TypedTree.fs/fsi
• Code in CompilerImports.fs which, after import, adds the source of the constraint to all constraints
• A new error message in FSComp.txt
• Application of the error message, including the constraint source, in ConstraintSolver.fs
• 4 new tests in MemberConstraints.fs
• A few updated test expectations in AbsTests.fs, neg61.bsl and E_NoMethodsOnEnums01.fs
• 29 adaptations in 9 files because of the new field in TTrait

[Martin521]

/run xlf

[github-actions]

Ran xlf: https://github.com/dotnet/fsharp/actions/runs/6921785319

[psfinaki]

Hey @Martin521 - thanks for taking this long-standing issue! :)

I know there is a discussion in the related ticket - my opinion is that whatever we do will be better than the current situation. That said, for me even having DivideByInt notion in the error message is weird, feels like a leaking technical detail. The code in the new test can very well be written by some F# newcomer and that message would just explode their mind. I would prefer something like "Hey, the average function expects int result which cannot be guaranteed by division".

I see that the message in FSComp is generic for types and operators so for now I would appreciate just adding a few more test cases that are expected to produce the same error, so that we can see a bigger picture.

[Martin521]

@psfinaki - thanks for your feedback!

I see that the message in FSComp is generic for types and operators so for now I would appreciate just adding a few more test cases that are expected to produce the same error, so that we can see a bigger picture.

I will do that.

I will also respond (tomorrow) to your other remarks, but in the ticket.

[smoothdeveloper]

@psfinaki,

the average function expects int result which cannot be guaranteed by division

AFAIU, this would require infrastructure for API authors, and adjusting FSharp.Core to use it; the infrastructure would allow to give customized error message on members with constraints, given predicate on the passed / infered type arguments, to deliver a custom message.

IMO, this is out of scope in just trying to improve the constraint not matching error, which should, unless the infrastructure & special condition that it would detect in API, always be displayed, in order to give the full picture.

[vzarytovskii]

That said, for me even having DivideByInt notion in the error message is weird, feels like a leaking technical detail. The code in the new test can very well be written by some F# newcomer and that message would just explode their mind. I would prefer something like "Hey, the average function expects int result which cannot be guaranteed by division".

I personally disagree with that. It doesn't have to be either/or. It should be both - a simple message as well as bunch of details.

We don't need to simplify messages in sake of simplifying them. But rather extend them.

@DedSec256 and @auduchinok may have some opinions here.

[psfinaki]

No yeah for sure we shouldn't overhaul the compiler just in order to make this usecase clear.

We don't need to simplify messages in sake of simplifying them. But rather extend them.

... is definitely a valid approach.

[Martin521]

Thanks @psfinaki for asking for more examples and the broader picture. This made me realize that the picture is broader indeed. I will continue with implementation (including test cases) once the discussion on the ticket is converging.

[Martin521]

This is ready.
This fixes #83 for the limited scope mentioned here.

Also note that the extended error message only appears when the constraint source (the operator or method or function that has the constraint typar) is imported. Doing the same while that source is in your own project is more difficult, but also deemed to be not needed, because both the language guide and the spec say that creating member constraints is "not intended for common use".

[Martin521]

/azp run

[azure-pipelines]

Commenter does not have sufficient privileges for PR 16304 in repo dotnet/fsharp

[T-Gro]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[vzarytovskii]

Thanks, let's see how it looks in IDEs, and get feedback from people.