Inliner chokes on sum

[julianhyde]

The Inliner throws on a program with sum in the group clause. This program fails:

fun mapReduce2 mapper reducer list =
  from e in list,
      (k, v) in mapper e
    group k compute c = sum of v;

with

java.lang.ClassCastException: class net.hydromatic.morel.type.TypeVar cannot be cast to class net.hydromatic.morel.type.PrimitiveType (net.hydromatic.morel.type.TypeVar and net.hydromatic.morel.type.PrimitiveType are in unnamed module of loader 'app')
 at net.hydromatic.morel.eval.Codes.lambda$static$6(Codes.java:1454)
 at net.hydromatic.morel.compile.Inliner.visit(Inliner.java:90)
 at net.hydromatic.morel.ast.Core$Id.accept(Core.java:459)
 at net.hydromatic.morel.ast.Shuttle.visit(Shuttle.java:369)
 at net.hydromatic.morel.ast.Core$Aggregate.accept(Core.java:1126)

while this similar program succeeds:

fun int_sum [] = 0
  | int_sum (x :: xs) = x + (int_sum xs);
val int_sum = fn : int list -> int

fun mapReduce2 mapper reducer list =
  from e in list,
      (k, v) in mapper e
    group k compute c = int_sum of v;
val mapReduce2 = fn
   : ('a -> ('b * int) list) -> 'c -> 'a list -> {c:int, k:'b} list

I think the problem is that Relational.sum has a polymorphic type (which we convert to either int or real during type inference).