Duplicate fields with record extensions

[martinmodrak]

This is related to #1344 and #1323, but as the following bug is not fixed in a build made after those bugs were closed in #1374, I think it is relevant (tested against build taken from https://groups.google.com/forum/#!topic/elm-dev/c_BDwK5UXFs)

The point is that I can make duplicate fields by using record extension. The following code compiles without errors:

type alias A a =
  {a | x : Int}

type alias B =
  {x : String}

test : A B
test = 
  { x = 1}

In all cases, the actual type is taken from the outermost type (A here). I think this should be a compile-time error, for the same reasons that field duplication is forbidden in other contexts.

Furthermore, having fields of the same name is also the only case that breaks a simple assumption about type aliases with record extension:

type alias T a =
  {a | .... }

type alias U a =
 {a | ... }

Now, T (U a) and U (T a) are exactly the same type for all a (unless there are duplicate field names).

[process-bot]

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