diff.diff

diff --git a/src/core/traits.pm b/src/core/traits.pm
index ff15d09..ca550d4 100644
--- a/src/core/traits.pm
+++ b/src/core/traits.pm
@@ -144,6 +144,35 @@ multi sub trait_mod:<is>(Routine:D $r, :$onlystar!) {
     $r.set_onlystar();
 }
 multi sub trait_mod:<is>(Routine:D $r, :prec(%spec)!) {
+    # "die if %spec<assoc> is "list" and $r is not an infix op"
+    # the nqp dance is because it kept failing to compile with normal Perl 6;
+    # XXX TODO: someone with more patience could figure out how to simplify it? 
+    nqp::if(
+        nqp::if(
+            nqp::iseq_s(
+                'list',
+                nqp::if(
+                    nqp::istype(%spec, Pair),
+                    nqp::if( # %spec is a Pair
+                        nqp::iseq_s(
+                            'assoc',
+                            nqp::if(
+                                nqp::istype(($_ := %spec.key), Str),
+                                $_, ''
+                            ),
+                        ),
+                        nqp::istype(($_ := %spec.value), Str), $_, ''),
+                    ),
+                    nqp::if( # %spec is a hash
+                        nqp::istype(($_ := nqp::atkey(%spec, 'assoc'), Str),
+                        $_, ''
+                    ),
+                ),
+            ),
+            nqp::isne_s('infix:', nqp::substr($r.name, 0, 6)),
+        ),
+        die("Cannot use `list` assoc with non-infix routines"),
+    );
     my role Precedence {
         has %!prec;
         proto method prec(|) { * }
@@ -169,27 +198,24 @@ multi sub trait_mod:<is>(Routine:D $r, :prec(%spec)!) {
 # three other trait_mod sub for equiv/tighter/looser in operators.pm
 multi sub trait_mod:<is>(Routine $r, :&equiv!) {
     nqp::can(&equiv, 'prec')
-        ?? trait_mod:<is>($r, :prec(&equiv.prec))
+        ?? trait_mod:<is>($r, :prec(:prec(&equiv.prec('prec'))))
         !! die "Routine given to equiv does not appear to be an operator";
-    $r.prec<assoc>:delete;
 }
 multi sub trait_mod:<is>(Routine $r, :&tighter!) {
     die "Routine given to tighter does not appear to be an operator"
         unless nqp::can(&tighter, 'prec');
     if !nqp::can($r, 'prec') || ($r.prec<prec> // "") !~~ /<[@:]>/ {
-        trait_mod:<is>($r, :prec(&tighter.prec))
+        trait_mod:<is>($r, :prec(:prec(&tighter.prec('prec'))))
     }
     $r.prec<prec> := $r.prec<prec>.subst(/\=/, '@=');
-    $r.prec<assoc>:delete;
 }
 multi sub trait_mod:<is>(Routine $r, :&looser!) {
     die "Routine given to looser does not appear to be an operator"
         unless nqp::can(&looser, 'prec');
     if !nqp::can($r, 'prec') || ($r.prec<prec> // "") !~~ /<[@:]>/ {
-        trait_mod:<is>($r, :prec(&looser.prec))
+        trait_mod:<is>($r, :prec(:prec(&looser.prec('prec'))))
     }
     $r.prec<prec> := $r.prec<prec>.subst(/\=/, ':=');
-    $r.prec<assoc>:delete;
 }
 multi sub trait_mod:<is>(Routine $r, :$assoc!) {
     trait_mod:<is>($r, :prec({ :$assoc }))