Fix #1994 Returning to err: Maybe(Error) raises compiler assert

src/llvm_backend_expr.cpp

@@ -1924,7 +1924,7 @@ lbValue lb_emit_conv(lbProcedure *p, lbValue value, Type *t) {
 		}
 		if (dst->Union.variants.count == 1) {
 			Type *vt = dst->Union.variants[0];
-			if (internal_check_is_assignable_to(src, vt)) {
+			if (internal_check_is_assignable_to(src_type, vt)) {
 				value = lb_emit_conv(p, value, vt);
 				lbAddr parent = lb_add_local_generated(p, t, true);
 				lb_emit_store_union_variant(p, parent.addr, value, vt);