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Actually print the traits that we are missing.

    It makes for easier debugging.

--- a/parse/infer.c

+++ b/parse/infer.c

@@ -544,6 +544,10 @@

 /* Merges the constraints on types */

 static void mergetraits(Inferstate *st, Node *ctx, Type *a, Type *b)

 {

+    size_t i, n;

+    char *sep;

+    char buf[1024];

+

     if (b->type == Tyvar) {

         /* make sure that if a = b, both have same traits */

         if (a->traits && b->traits)

@@ -554,8 +558,14 @@

             a->traits = bsdup(b->traits);

     } else {

         if (!traitcheck(a, b)) {

-            /* FIXME: say WHICH constraints we're missing */

-            fatal(ctx->line, "%s missing constraints for %s near %s", tystr(b), tystr(a), ctxstr(st, ctx));

+            sep = "";

+            n = 0;

+            for (i = 0; bsiter(a->traits, &i); i++) {

+                if (!bshas(b->traits, i))

+                    n += snprintf(buf + n, sizeof(buf) - n, "%s%s", sep, namestr(traittab[i]->name));

+                sep = ",";

+            }

+            fatal(ctx->line, "%s missing constraints %s for %s near %s", tystr(b), buf, tystr(a), ctxstr(st, ctx));

         }

     }

 }