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Support the pseudo-member 'tag' for union types

Similar to `x.len` which reads the element count of a sequence,
`x.tag` reads the tag id of a union value.

--- a/parse/fold.c

+++ b/parse/fold.c

@@ -247,6 +247,9 @@

 		if (t->type == Tyarray && !strcmp(namestr(args[1]), "len")) {

 			r = t->asize;

 			r->expr.type = exprtype(n);

+		} else if (t->type == Tyunion && !strcmp(namestr(args[1]), "tag")) {

+			r = mkexpr(n->loc, Outag, args[0], NULL);

+			r->expr.type = mktype(n->loc, Tyint32);

 		}

 		break;

 	case Oarr:

--- a/parse/infer.c

+++ b/parse/infer.c

@@ -2293,6 +2293,12 @@

 			constrain(n, type(n), traittab[Tcint]);

 			found = 1;

 		}

+	} else if (ismemb && t->type == Tyunion) {

+		if (!strcmp(namestr(memb), "tag")) {

+			constrain(n, type(n), traittab[Tcnum]);

+			constrain(n, type(n), traittab[Tcint]);

+			found = 1;

+		}

 	} else {

 		if (tybase(t)->type == Typtr)

 			t = tybase(tf(t->sub[0]));