Inherit default clocking for a sequence used as an event control

src/V3AssertNfa.cpp

@@ -2762,7 +2762,12 @@ class AssertNfaVisitor final : public VNVisitor {
             VL_DO_DANGLING(pushDeletep(specp), specp);
             return;
         }
-        // A clockless sequence has no sampling edge; require an explicit clock.
+        // Inherit the module default clocking (IEEE 14.12, 16.15) when the
+        // sequence has none of its own.
+        if (!specp->sensesp() && m_defaultClockingp) {
+            specp->sensesp(m_defaultClockingp->sensesp()->cloneTree(true));
+        }
+        // A clockless sequence with no default clocking has no sampling edge.
         if (!specp->sensesp()) {
             specp->v3warn(E_UNSUPPORTED,
                           "Unsupported: '@' event control on a sequence without a clocking event");

test_regress/t/t_assert_seq_event.v

@@ -16,7 +16,7 @@ module t (
   bit a, b, c;
   bit a1, a2, b1;
   int cyc = 0;
-  int seq_hits = 0, seq_hits2 = 0, ref_hits = 0, one_hits = 0;
+  int seq_hits = 0, seq_hits2 = 0, ref_hits = 0, one_hits = 0, dc_hits = 0;
 
   // verilog_format: off  // verible does not support clocking events inside sequence declarations
   sequence seq;
@@ -28,6 +28,15 @@ module t (
   endsequence
   // verilog_format: on
 
+  // seq_dc has no clocking event, so it inherits the default clocking and must
+  // behave identically to the explicitly-clocked seq above.
+  default clocking @(posedge clk);
+  endclocking
+
+  sequence seq_dc;
+    a ##1 b ##1 c;
+  endsequence
+
   // A sequence used as an `@` event control resumes once per sequence end point
   // (IEEE 1800-2023 9.4.2.4). seq_hits/seq_hits2 are two waiters on the same
   // multi-cycle sequence; ref_hits is an independent shift-register oracle (end
@@ -44,6 +53,10 @@ module t (
     @seq_one;
     one_hits = one_hits + 1;
   end
+  initial forever begin
+    @seq_dc;
+    dc_hits = dc_hits + 1;
+  end
 
   always @(posedge clk) begin
     if (a2 && b1 && c) ref_hits = ref_hits + 1;
@@ -66,10 +79,11 @@ module t (
 
   // Counts read in final (Postponed) to avoid same-timestep races.
   final begin
-    `checkd(seq_hits, 14);  // Questa: 14
-    `checkd(seq_hits2, 14);  // Questa: 14
-    `checkd(ref_hits, 14);  // Questa: 14
-    `checkd(one_hits, 30);  // Questa: 30
+    `checkd(seq_hits, 14);
+    `checkd(seq_hits2, 14);
+    `checkd(ref_hits, 14);
+    `checkd(one_hits, 30);
+    `checkd(dc_hits, 14);
     $write("*-* All Finished *-*\n");
   end
 endmodule

test_regress/t/t_assert_seq_event_unsup.out

@@ -1,11 +1,11 @@
-%Error-UNSUPPORTED: t/t_assert_seq_event_unsup.v:33:6: Unsupported: '@' event control on a sequence without a clocking event
-   33 |     @s_unclocked;
+%Error-UNSUPPORTED: t/t_assert_seq_event_unsup.v:28:6: Unsupported: '@' event control on a sequence without a clocking event
+   28 |     @s_clockless;
       |      ^~~~~~~~~~~
                     ... For error description see https://verilator.org/warn/UNSUPPORTED?v=latest
-%Error-UNSUPPORTED: t/t_assert_seq_event_unsup.v:24:5: Unsupported: non-edge clocking event on a sequence; use an edge such as @(posedge clk)
-   24 |     @(g) a ##1 b;
+%Error-UNSUPPORTED: t/t_assert_seq_event_unsup.v:19:5: Unsupported: non-edge clocking event on a sequence; use an edge such as @(posedge clk)
+   19 |     @(g) a ##1 b;
       |     ^
-%Error-UNSUPPORTED: t/t_assert_seq_event_unsup.v:35:6: Unsupported: this sequence form referenced by an '@' event control
-   35 |     @s_noncons;
+%Error-UNSUPPORTED: t/t_assert_seq_event_unsup.v:30:6: Unsupported: this sequence form referenced by an '@' event control
+   30 |     @s_noncons;
       |      ^~~~~~~~~
 %Error: Exiting due to

test_regress/t/t_assert_seq_event_unsup.v

@@ -10,13 +10,8 @@ module t (
   bit a, b;
   logic g = 0;
 
-  // Clockless `@seq` stays E_UNSUPPORTED even under a default clocking, matching
-  // Questa; whether 9.4.2.4 should inherit it here is an open PR question.
-  default clocking @(posedge clk);
-  endclocking
-
   // verilog_format: off
-  sequence s_unclocked;
+  sequence s_clockless;
     a ##1 b;
   endsequence
 
@@ -30,7 +25,7 @@ module t (
   // verilog_format: on
 
   initial begin
-    @s_unclocked;
+    @s_clockless;
     @s_nonedge;
     @s_noncons;
   end