throw typed exception on illegal use of placeholder parameters

also catches  class A { $^x }, which used to live errornously

src/Perl6/Actions.pm

@@ -620,7 +620,7 @@ class Perl6::Actions is HLL::Actions {
     method block($/) {
         my $block := $<blockoid>.ast;
         if $block<placeholder_sig> {
-            $/.CURSOR.panic("Cannot use placeholder parameters in this kind of block");
+            $*W.throw($/, ['X', 'Placeholder', 'Block']);
         }
         ($*W.cur_lexpad())[0].push(my $uninst := PAST::Stmts.new($block));
         my $code := $*W.create_code_object($block, 'Block', $*W.create_signature([]));
@@ -1197,6 +1197,10 @@ class Perl6::Actions is HLL::Actions {
         }
         $block.blocktype('immediate');
 
+        if $*PKGDECL ne 'role' && $block<placeholder_sig> {
+            $*W.throw($/, ['X', 'Placeholder', 'Block']);
+        }
+
         # If it's a stub, add it to the "must compose at some point" list,
         # then just evaluate to the type object. Don't need to do any more
         # just yet.

src/core/Exceptions.pm

@@ -50,6 +50,12 @@ my class X::Signature::Placeholder is X::Base does X::Comp {
     }
 }
 
+my class X::Placeholder::Block is X::Base does X::Comp {
+    method message() {
+        'Cannot use placeholder parameters in this kind of block';
+    }
+}
+
 my class X::Attribute::Undeclared is X::Base does X::Comp {
     has $.name;
     has $.package-type;