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Improve error messages with mismatched subtypes.

--- a/parse/infer.c

+++ b/parse/infer.c

@@ -573,14 +573,11 @@

     /* if the tyrank of a is 0 (ie, a raw tyvar), just unify.

      * Otherwise, match up subtypes. */

     if ((a->type == b->type || idxhacked(a, b)) && tyrank(a) != 0) {

-        for (i = 0; i < b->nsub; i++) {

-            /* types must have same arity */

-            if (i >= a->nsub)

-                fatal(ctx->line, "%s has wrong subtypes for %s near %s",

-                      tystr(a), tystr(b), ctxstr(st, ctx));

-

+        if (a->nsub != b->nsub)

+            fatal(ctx->line, "%s has wrong subtype count for %s (got %d, expected %d) near %s\n",

+                  tystr(a), tystr(b), a->nsub, b->nsub, ctxstr(st, ctx));

+        for (i = 0; i < b->nsub; i++)

             unify(st, ctx, a->sub[i], b->sub[i]);

-        }

         r = b;

     } else if (a->type != Tyvar) {

         fatal(ctx->line, "%s incompatible with %s near %s",