diff --git a/src/core/Main.pm6 b/src/core/Main.pm6
index 3bf55a00106..388bdb34a51 100644
--- a/src/core/Main.pm6
+++ b/src/core/Main.pm6
@@ -125,14 +125,30 @@ my sub RUN-MAIN(&main, $mainline, :$in-as-argsfiles) {
           ?? "-e '...'"
           !! strip_path_prefix($prog-name);
 
+        # return the Cool constant if the post_constraints of a Parameter is
+        # a single Cool constant, else Nil
+        sub cool_constant(Parameter:D $p) {
+            nqp::not_i(
+              nqp::isnull(
+                (my \post_constraints :=
+                  nqp::getattr($p,Parameter,'@!post_constraints'))
+              )
+            ) && nqp::elems(post_constraints) == 1
+              && nqp::istype((my \value := nqp::atpos(post_constraints,0)),Cool)
+              ?? value
+              !! Nil
+        }
+
         # Select candidates for which to create USAGE string
         sub usage-candidates($capture) {
             my @candidates = &main.candidates.grep: { !.?is-hidden-from-USAGE }
             if $capture.list -> @positionals {
                 my $first := @positionals[0];
                 if @candidates.grep: -> $sub {
-                    if $sub.signature.params[0].cool_constant -> $literal {
-                       $literal.ACCEPTS($first)
+                    if $sub.signature.params[0] -> $param {
+                        if cool_constant($param) -> $literal {
+                            $literal.ACCEPTS($first)
+                        }
                     }
                 } -> @candos {
                     return @candos;
diff --git a/src/core/Parameter.pm6 b/src/core/Parameter.pm6
index 544d4dd6741..8477cd1c6d9 100644
--- a/src/core/Parameter.pm6
+++ b/src/core/Parameter.pm6
@@ -630,15 +630,6 @@ my class Parameter { # declared in BOOTSTRAP
         $perl ~ $rest;
     }
 
-    # return the cool constant if the post_constraint is a single Cool constant
-    method cool_constant() {
-        nqp::not_i(nqp::isnull(@!post_constraints))
-          && nqp::elems(@!post_constraints) == 1
-          && nqp::istype((my \value := nqp::atpos(@!post_constraints,0)),Cool)
-          ?? value
-          !! Nil
-    }
-
     method sub_signature(Parameter:D:) {
         nqp::isnull($!sub_signature) ?? Any !! $!sub_signature
     }