Polymorphic record contracts should know which fields their row variables cannot contain

[matthew-healy]

Consider the following program:

let f | forall r. { ; r } -> { x: Num; r } = fun r => %record_insert% "x" r 1 in f { x = 0 }

We know statically that the r row variable cannot contain the field x, and we return an error to that effect if we typecheck this code (by changing the end to (f { x = 0 } : _)).

However, after merging #949, the dynamic error for this is:

error: non mergeable terms
  ┌─ repl-input-1:1:78
  │
1 │ let f | forall r. { ; r } -> { x : Num; r } = fun r => %record_insert% "x" r 1 in (f { x = 0 })
  │                                                                              ^             ^ with this expression
  │                                                                              │              
  │                                                                              cannot merge this expression
  │
  = Both values have the same merge priority but they can't be combined

it should be possible to give a more detailed error message (e.g. "arguments to f cannot contain the field x"), by providing this information to the polymorphic record contract.

[Radvendii]

Fixed in #1323