"Overloaded" records with constructors in patterns fail instead of getting a proper warning.

[j14159]

This came up while exploring the native AST in a little more depth for a future version.

type renameable = Named string | Renamed { name: string, original: string }

type symbol = Symbol { line: int, name: renameable }

let symbol_rename (Symbol {line=l, name=Named n}) new_name =
  Symbol {line=l, name=Renamed {name=new_name, original=n}}
-- Interesting!  This is not a type error, but rather requires that the record
-- coming in simply has `orig` in addition to `name` and `original`.  Didn't think of
-- this at all as a possible library-writer-side problem.
let symbol_rename (Symbol {line=l, name=Renamed {orig=o}}) new_name =
  Symbol {line=l, name=Renamed {name=new_name, original=o}}

Two problems:

1. Serious: instead of warning that symbol_rename performs an incomplete match (no wildcard for Renamed, no checks for original or name), we get a {badkey, orig} error.
2. Less immediate, if still serious: while Renamed { orig=o } is completely legitimate, the type system right now would actually forget the orig: String member per Records lose information in type constructors #125 without additional work to define renameable with an explicit row variable (and this is a hypothetical solution that I'm not sure would work properly), meaning that a caller could never satisfy this pattern.

Full stack trace from eunit:

     Failure/Error: {error,
                        {badkey,orig},
                        [{maps,get,[orig,#{name => '_',original => '_'}],[]},
                         {alpaca_exhaustiveness,'-matches_record/2-fun-0-',2,
                             [{file,
                                  "/home/jeremy/code/erlang/alpaca/_build/test/lib/alpaca/src/alpaca_exhaustiveness.erl"},
                              {line,280}]},
                         {lists,all,2,[{file,"lists.erl"},{line,1213}]},
                         {alpaca_exhaustiveness,matches,2,
                             [{file,
                                  "/home/jeremy/code/erlang/alpaca/_build/test/lib/alpaca/src/alpaca_exhaustiveness.erl"},
                              {line,292}]},
                         {lists,any,2,[{file,"lists.erl"},{line,1225}]},
                         {alpaca_exhaustiveness,'-missing_patterns/4-fun-1-',
                             2,
                             [{file,
                                  "/home/jeremy/code/erlang/alpaca/_build/test/lib/alpaca/src/alpaca_exhaustiveness.erl"},
                              {line,86}]},
                         {lists,flatmap,2,[{file,"lists.erl"},{line,1250}]}]}