refine comments and update test

src/V3Timing.cpp

@@ -892,8 +892,8 @@ class TimingControlVisitor final : public VNVisitor {
         m_underProcedure = true;
         // Workaround for killing `always` processes (doing that is pretty much UB)
         // TODO: Disallow killing `always` at runtime (throw an error)
-        // Combo blocks (always @*) must not become coroutines -- they have no
-        // suspend points and would spin forever.
+        // Skip for combinational blocks; if the body has timing controls
+        // iterateChildren will add T_SUSPENDEE, otherwise it would spin.
         if (hasFlags(nodep, T_HAS_PROC)
             && !(m_activep && m_activep->sentreep() && m_activep->sentreep()->hasCombo()))
             addFlags(nodep, T_SUSPENDEE);

test_regress/t/t_wait_iface_vif.v

@@ -54,11 +54,19 @@ endclass
 module t;
   my_if intf();
 
+  // Verify combinational always with timing controls still works as coroutine
+  int combo_timing_count = 0;
+  always @* begin
+    combo_timing_count = combo_timing_count + 1;
+    #1;
+  end
+
   initial begin
     automatic Driver d = new;
     d.vif = intf;
     d.run();
     repeat (4) @(posedge intf.clk);
+    if (combo_timing_count == 0) $stop;
     $write("*-* All Finished *-*\n");
     $finish;
   end