Accept hierarchical refs as cross members -- marked COVIGN for now

Signed-off-by: Matthew Ballance <matt.ballance@gmail.com>

src/verilog.y

@@ -7341,6 +7341,21 @@ cross_itemList<nodep>:  // IEEE: part of list_of_cross_items
 cross_item<nodep>:  // ==IEEE: cross_item
                 id/*cover_point_identifier*/
                         { $$ = new AstCoverpointRef{$<fl>1, *$1}; }
+        //                      // Verilator extension beyond strict IEEE (cross_item is a simple
+        //                      // identifier): some tools accept a hierarchical/dotted reference
+        //                      // here.  A dotted reference can never name a coverpoint (coverpoint
+        //                      // identifiers are flat, local names), so it is necessarily a plain
+        //                      // data reference, i.e. an implicit coverpoint over a variable.
+        //                      // Verilator does not support implicit coverpoints, so V3Covergroup
+        //                      // drops the whole cross with a COVERIGN warning; accept it here so
+        //                      // the file parses instead of erroring on the '.'.
+        |       id/*variable*/ crossItemHier
+                        { $$ = new AstCoverpointRef{$<fl>1, *$1 + *$2}; }
+        ;
+
+crossItemHier<strp>:  // Verilator extension: dotted suffix of a hierarchical cross_item
+                '.' idAny                               { $$ = PARSEP->newString("." + *$2); }
+        |       crossItemHier '.' idAny                 { $$ = PARSEP->newString(*$1 + "." + *$3); }
         ;
 
 cross_body<nodep>:  // ==IEEE: cross_body

test_regress/t/t_covergroup_cross_opt_unsup.out

@@ -7,4 +7,8 @@
                                                          : ... note: In instance 't'
    15 |     cross_implicit: cross cp_a, var_x;
       |                                 ^~~~~
+%Warning-COVERIGN: t/t_covergroup_cross_opt_unsup.v:18:29: Unsupported: cross of 's_cfg.m_p' which is not a coverpoint (implicit coverpoint)
+                                                         : ... note: In instance 't'
+   18 |     cross_hier: cross cp_a, s_cfg.m_p;
+      |                             ^~~~~
 %Error: Exiting due to

test_regress/t/t_covergroup_cross_opt_unsup.v

@@ -13,7 +13,12 @@ module t;
       option.per_instance = 1;  // unsupported for cross; triggers COVERIGN
     }
     cross_implicit: cross cp_a, var_x;
+    // Hierarchical/dotted cross item: can only be a data reference (implicit
+    // coverpoint), never a coverpoint; treated as unsupported (COVERIGN)
+    cross_hier: cross cp_a, s_cfg.m_p;
   endgroup
+  typedef struct packed {logic m_p; logic h_mode;} cfg_t;
+  cfg_t s_cfg = '0;
   logic var_x = 1'b0;
   cg cg_i = new;
   initial begin