Wrong type inference

[Birowsky]

Given:

apiUrl : ApiEndpoint
apiUrl = global.config.apiUrl

Compiler fails with:

Something is off with the body of the `apiUrl` definition:  
	156|                     global.config.apiUrl                          
	                         ^^^^^^^^^^^^^^^^^^^^ 
The value at .apiUrl is a:
      String  
But the type annotation on `apiUrl` says it should be:
      ApiEndpoint

Which is wrong because global.config.apiUrl is indeed of type ApiEndpoint.

Some more context:

type ApiEndpoint = ApiEndpoint String
type alias Config = ConfigWithTaggedApiUrl ConfigArgs
type alias ConfigWithTaggedApiUrl a = { a | apiUrl : ApiEndpoint }
type alias ConfigArgs =
    { googleMapApiKey : String
    , iosGoogleClientId : String
    , androidGoogleClientId : String
    , sentryId : String
    , gtmId : String
    , facebookId : String
    , appUrl : String
    , apiUrl : String
    , staticUrl : String
    , googleAppNumber : String
    , privacyPolicyUrl : String
    , termsOfUseUrl : String
    , cognito : CognitoConfig
    , appleAppStoreUrl : String
    , googlePlayStoreUrl : String
    }

Potentially useful finding:
This issue occurs when the code looks like this:

update global msg state =
    case msg of
        SomeMsg ->
            let
                apiUrl2 : ApiEndpoint
                apiUrl2 =
                    global.config.apiUrl

But it doesn't when it looks like so:

update global msg state =
    let
        apiUrl2 : ApiEndpoint
        apiUrl2 =
            global.config.apiUrl
    in
    case msg of
        SomeMsg ->

I am unfortunately unable to provide an SSCCE at this time because this is too big of an application. Please ask if I can provide further useful info.

• Elm: 19.1
• Operating System: Mac 11.0.1

[github-actions]

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[avh4]

If my recollection is correct, Elm 0.19.x no longer supports changing the type of a field when extending a record type. So ConfigWithTaggedApiUrl ConfigArgs will still have the field apiUrl : String because ConfigArgs has that (the apiUrl : ApiEndpoint in the record extension cannot change the type of that field; it can only add the field to a base record type that doesn't yet have a field of that name). Though I am surprised that the compiler doesn't give an error about that.

[Birowsky]

@avh4 Not only does it not report that error, but everything works nicely with that tagged type member. If it weren't by design, wouldn't we expect a runtime exception?

[evancz]

I think I would need an SSCCE to understand what is going on here.

[Birowsky]

Ah.. sorry, I won't be able to provide one in a reasonable amount of time.