Fix errors on some $past cases (#4425)

src/verilog.y

@@ -4254,11 +4254,13 @@ system_f_call_or_t<nodeExprp>:      // IEEE: part of system_tf_call (can be task
         |       yD_ONEHOT '(' expr ')'                  { $$ = new AstOneHot{$1, $3}; }
         |       yD_ONEHOT0 '(' expr ')'                 { $$ = new AstOneHot0{$1, $3}; }
         |       yD_PAST '(' expr ')'                    { $$ = new AstPast{$1, $3, nullptr}; }
-        |       yD_PAST '(' expr ',' expr ')'           { $$ = new AstPast{$1, $3, $5}; }
+        |       yD_PAST '(' expr ',' exprE ')'          { $$ = new AstPast{$1, $3, $5}; }
-        |       yD_PAST '(' expr ',' expr ',' expr ')'
+        |       yD_PAST '(' expr ',' exprE ',' exprE ')'
-                        { $$ = $3; BBUNSUP($1, "Unsupported: $past expr2 and clock arguments"); }
+                        { if ($7) BBUNSUP($1, "Unsupported: $past expr2 and/or clock arguments");
-        |       yD_PAST '(' expr ',' expr ',' expr ',' expr')'
+                          $$ = new AstPast{$1, $3, $5}; }
-                        { $$ = $3; BBUNSUP($1, "Unsupported: $past expr2 and clock arguments"); }
+        |       yD_PAST '(' expr ',' exprE ',' exprE ',' clocking_eventE ')'
+                        { if ($7 || $9) BBUNSUP($1, "Unsupported: $past expr2 and/or clock arguments");
+                          $$ = new AstPast{$1, $3, $5}; }
         |       yD_POW '(' expr ',' expr ')'            { $$ = new AstPowD{$1, $3, $5}; }
         |       yD_RANDOM '(' expr ')'                  { $$ = new AstRand{$1, $3, false}; }
         |       yD_RANDOM parenE                        { $$ = new AstRand{$1, nullptr, false}; }
@@ -4683,6 +4685,11 @@ constExpr<nodeExprp>:
                 expr                                    { $$ = $1; }
         ;
 
+exprE<nodep>:                   // IEEE: optional expression
+                /*empty*/                               { $$ = nullptr; }
+        |       expr                                    { $$ = $1; }
+        ;
+
 expr<nodeExprp>:                // IEEE: part of expression/constant_expression/primary
         // *SEE BELOW*          // IEEE: primary/constant_primary
         //
@@ -5665,6 +5672,11 @@ clocking_declaration<nodep>:            // IEEE: clocking_declaration
                         { $$ = new AstClocking{$<fl>3, *$3, $4, $6, false, true}; }
         ;
 
+clocking_eventE<senItemp>:      // IEEE: optional clocking_event
+                /* empty */                             { $$ = nullptr; }
+        |       clocking_event                          { $$ = $1; }
+        ;
+
 clocking_event<senItemp>:       // IEEE: clocking_event
                 '@' id
                         { $$ = new AstSenItem{$<fl>2, VEdgeType::ET_CHANGED, new AstParseRef{$<fl>2, VParseRefExp::PX_TEXT, *$2, nullptr, nullptr}}; }

test_regress/t/t_past.v

@@ -74,8 +74,10 @@ module Test (/*AUTOARG*/
       // $past(expression, ticks, expression, clocking)
       // In clock expression
       if (dly0 != $past(in)) $stop;
-      if (dly0 != $past(in,1)) $stop;
+      if (dly0 != $past(in,)) $stop;
       if (dly1 != $past(in, 2)) $stop;
+      if (dly1 != $past(in, 2, )) $stop;
+      if (dly1 != $past(in, 2, , )) $stop;
       // $sampled(expression) -> expression
       if (in != $sampled(in)) $stop;
    end

test_regress/t/t_past_unsup.out

@@ -0,0 +1,11 @@
+%Error-UNSUPPORTED: t/t_past_unsup.v:16:11: Unsupported: $past expr2 and/or clock arguments
+   16 |       if ($past(d, 1, 1)) $stop;   
+      |           ^~~~~
+                    ... For error description see https://verilator.org/warn/UNSUPPORTED?v=latest
+%Error-UNSUPPORTED: t/t_past_unsup.v:17:11: Unsupported: $past expr2 and/or clock arguments
+   17 |       if ($past(d, 1, 1, )) $stop;   
+      |           ^~~~~
+%Error-UNSUPPORTED: t/t_past_unsup.v:18:11: Unsupported: $past expr2 and/or clock arguments
+   18 |       if ($past(d, 1, 1, @(posedge clk))) $stop;   
+      |           ^~~~~
+%Error: Exiting due to

test_regress/t/t_past_unsup.pl

@@ -0,0 +1,19 @@
+#!/usr/bin/env perl
+if (!$::Driver) { use FindBin; exec("$FindBin::Bin/bootstrap.pl", @ARGV, $0); die; }
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2003 by Wilson Snyder. This program is free software; you
+# can redistribute it and/or modify it under the terms of either the GNU
+# Lesser General Public License Version 3 or the Perl Artistic License
+# Version 2.0.
+# SPDX-License-Identifier: LGPL-3.0-only OR Artistic-2.0
+
+scenarios(vlt => 1);
+
+lint(
+    fails => 1,
+    expect_filename => $Self->{golden_filename},
+    );
+
+ok(1);
+1;

test_regress/t/t_past_unsup.v

@@ -0,0 +1,20 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain, for
+// any use, without warranty, 2018 by Wilson Snyder.
+// SPDX-License-Identifier: CC0-1.0
+
+module t (/*AUTOARG*/
+   // Inputs
+   d, clk, num
+   );
+   input d;
+   input clk;
+   input int num;
+
+   always @ (posedge clk) begin
+      if ($past(d, 1, 1)) $stop;  // Unsup
+      if ($past(d, 1, 1, )) $stop;  // Unsup
+      if ($past(d, 1, 1, @(posedge clk))) $stop;  // Unsup
+   end
+endmodule