[v6] Fix parsing of @_ placeholder

v6/harness

@@ -27,6 +27,27 @@ use STD;
 
 augment class NieczaActions {
 sub node($M) { { line => $M.cursor.lineof($M.to) } }
+method get_placeholder_sig($/) {
+    # for some reason, STD wants to deparse this
+    my @things = $*CURLEX<$?SIGNATURE>.split(", ");
+    shift @things if @things[0] eq '';
+    my @parms;
+    for @things -> $t {
+        if substr($t, 0, 9) eq '$_ is ref' {
+            push @parms, ::Sig::Parameter.new(optional => True,
+                slot => '$_', name => '$_');
+        } elsif $t eq '*@_' {
+            push @parms, ::Sig::Parameter.new(slurpy => True, slot => '@_',
+                list => True, name => '*@_');
+        } elsif defined '$@%&'.index(substr($t,0,1)) {
+            push @parms, ::Sig::Parameter.new(slot => $t, name => $t,
+                list => (substr($t,0,1) eq '@'), hash => (substr($t,0,1) eq '%'));
+        } else {
+            $/.CURSOR.sorry('Named placeholder parameters NYI');
+        }
+    }
+    return Sig.new(params => @parms);
+}
 method FALLBACK($meth, $/) {
     if $meth eq '::($name)' { # XXX STD miscompilation
         if $<O><prec> eq 't=' { # additive
@@ -49,6 +70,55 @@ method FALLBACK($meth, $/) {
 }
 
 augment class STD { #OK exist
+sub _subst($M is rw, $text is rw, $regex, $repl) {
+    $text = $text.Str;
+    $M = ($text ~~ $regex);
+    if $M {
+        $text = $text.substr(0, $M.from) ~
+            (($repl ~~ Str) ?? $repl !! $repl()) ~
+                $text.substr($M.to, $text.chars - $M.to);
+    }
+    ?$M;
+}
+method add_placeholder($name) {
+    my $decl = $*CURLEX.<!IN_DECL> // '';
+    $decl = ' ' ~ $decl if $decl;
+    my $*IN_DECL = 'variable';
+
+    if $*SIGNUM {
+        return self.sorry("Placeholder variable $name is not allowed in the$decl signature");
+    }
+    elsif my $siggy = $*CURLEX.<$?SIGNATURE> {
+        return self.sorry("Placeholder variable $name cannot override existing signature $siggy");
+    }
+    if not $*CURLEX.<!NEEDSIG> {
+        if $*CURLEX === $*UNIT {
+            return self.sorry("Placeholder variable $name may not be used outside of a block");
+        }
+        return self.sorry("Placeholder variable $name may not be used here because the surrounding$decl block takes no signature");
+    }
+    if $name ~~ /\:\:/ {
+        return self.sorry("Placeholder variable $name may not be package qualified");
+    }
+
+    my $varname = $name;
+    my $twigil = '';
+    my $signame = $varname;
+    my $M;
+    if _subst($M, $varname, /<[ ^ : ]>/, "") {
+        $twigil = $M.Str;
+        $signame = ($twigil eq ':' ?? ':' !! '') ~ $varname;
+    }
+    return self if $*CURLEX.{'%?PLACEHOLDERS'}{$signame}++;
+
+    if $*CURLEX{$varname} {
+        return self.sorry("$varname has already been used as a non-placeholder in the surrounding$decl block,\n  so you will confuse the reader if you suddenly declare $name here");
+    }
+
+    self.add_my_name($varname);
+    $*CURLEX{$varname}<used> = 1;
+    self;
+}
 role herestop1 {
     token stopper { ^^ {} \h*? $*DELIM \h* <.unv>?? $$ \v? }
 }