Bad error message when struct initializer inside a map is missing an argument

Originator:pharkas
Number:rdar://42338973 Date Originated:7/18/2018
Status: Resolved:
Product:Swift Compiler Product Version:
Classification:Serious Bug Reproducible:Always
 
I ran into an odd compiler error when I was trying to return the results of a map() in a function that used a generic type from a class. The error message was:
  error: 'map' produces '[T]', not the expected contextual result type '[Person]'
        return data.map({ Person(firstName: $0) })

The code causing this error is:
class MyPersonClass : MyGenericSuperclass<[(String, String)]> {
    override func mapPeople(from data: [(String, String)]) -> [Person] {
==>     return data.map({ Person(firstName: $0) })
    }
}

Person is defined as
struct Person {
    public var firstName : String
    public var lastName : String?
}

I incorrectly assumed that since lastName was optional I didn't need to pass it in the initializer for Person, but I got sent down a weird path because this error message has nothing to do with the initializer for Person. I assumed I was doing something wrong with map() or with the arguments or return type of that function.

I would expect that Swift would have the same error for this bad initializer inside of a map as it would when I try to do this outside of a map call:

error: missing argument for parameter 'lastName' in call
Person(firstName: "Jacob") 
                         ^
                         , lastName: <#String?#>

Comments

Sample code:

import Foundation

struct Person { public var firstName : String public var lastName : String? 

func description() -> String {
    if let lastName = lastName {
        return firstName + " " + lastName
    } else {
        return firstName
    }
}

}

class MyGenericSuperclass { public func mapPeople(from data: T) -> [Person] { fatalError("Must be overridden by subclass") } }

class MyPersonClass : MyGenericSuperclass<[(String, String)]> { override func mapPeople(from data: [(String, String)]) -> [Person] { // This line produces an odd error about map not producing the correct type return data.map({ Person(firstName: $0) }) 

    // This is the proper initializer for Person
    //return data.map({ Person(firstName: $0, lastName: $1) })

    // This line produces the error that I would expect on line 33 above
    Person(firstName: "Jacob")
}

}

let fullNames = [("Johnny", "Appleseed"), ("Kate", "Bell"), ("Anna", "Haro")] let personClass = MyPersonClass() let people = personClass.mapPeople(from: fullNames) for person in people { print("Person: (person.description())") }

By pharkas at July 18, 2018, 5 p.m. (reply...)

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