Support method calls on a sub-interface via a virtual interface

src/V3Width.cpp

@@ -3789,7 +3789,7 @@ class WidthVisitor final : public VNVisitor {
                     if (!varp->didWidth()) userIterate(varp, nullptr);
                     nodep->dtypep(foundp->dtypep());
                     nodep->varp(varp);
-                    AstIface* const ifacep = adtypep->ifacep();
+                    AstIface* const ifacep = adtypep->ifaceViaCellp();
                     varp->sensIfacep(ifacep);
                     nodep->didWidth(true);
                     return;
@@ -3830,7 +3830,26 @@ class WidthVisitor final : public VNVisitor {
                         UASSERT_OBJ(viftopVarp, nodep,
                                     "No __Viftop variable for sub-interface cell");
                         if (!viftopVarp->didWidth()) userIterate(viftopVarp, nullptr);
-                        nodep->dtypep(viftopVarp->dtypep());
+                        AstNodeDType* subDtypep = viftopVarp->dtypep();
+                        if (adtypep->isVirtual()) {
+                            // A sub-interface selected through a virtual interface
+                            // handle is itself a virtual reference (a runtime
+                            // instance pointer), not a static instance. Mark the
+                            // dtype virtual so a method call on it dispatches per
+                            // instance instead of inlining to one fixed scope.
+                            AstIfaceRefDType* const subRefp
+                                = VN_CAST(subDtypep->skipRefp(), IfaceRefDType);
+                            if (subRefp) {
+                                AstIfaceRefDType* const newDtypep = new AstIfaceRefDType{
+                                    nodep->fileline(), subRefp->cellName(), subRefp->ifaceName()};
+                                newDtypep->ifacep(subRefp->ifacep());
+                                newDtypep->cellp(subRefp->cellp());
+                                newDtypep->isVirtual(true);
+                                v3Global.rootp()->typeTablep()->addTypesp(newDtypep);
+                                subDtypep = newDtypep;
+                            }
+                        }
+                        nodep->dtypep(subDtypep);
                         nodep->varp(viftopVarp);
                         viftopVarp->sensIfacep(VN_AS(cellp->modp(), Iface));
                         nodep->didWidth(true);
@@ -4781,16 +4800,9 @@ class WidthVisitor final : public VNVisitor {
         }
     }
     void methodCallIfaceRef(AstMethodCall* nodep, AstIfaceRefDType* adtypep) {
-        AstIface* const ifacep = adtypep->ifacep();
-        if (!ifacep) {
-            // A sub-interface reached through a virtual interface handle has a
-            // ref dtype with cellp set but ifacep null; the per-instance binding
-            // is not modelled, so the call cannot be resolved to one instance.
-            nodep->v3warn(E_UNSUPPORTED, "Unsupported: method call on a sub-interface "
-                                         "accessed through a virtual interface");
-            nodep->dtypeSetVoid();
-            return;
-        }
+        // ifaceViaCellp() resolves the interface via cellp when ifacep is null,
+        // as for a sub-interface selected through a virtual interface handle.
+        AstIface* const ifacep = adtypep->ifaceViaCellp();
         UINFO(5, __FUNCTION__ << ":" << nodep);
         if (AstNodeFTask* const ftaskp
             = VN_CAST(m_memberMap.findMember(ifacep, nodep->name()), NodeFTask)) {

test_regress/t/t_interface_virtual_sub_iface_method.py

@@ -9,8 +9,10 @@
 
 import vltest_bootstrap
 
-test.scenarios('linter')
+test.scenarios('simulator')
 
-test.lint(fails=True, expect_filename=test.golden_filename)
+test.compile()
+
+test.execute()
 
 test.passes()

test_regress/t/t_interface_virtual_sub_iface_method.v

@@ -0,0 +1,68 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain.
+// SPDX-FileCopyrightText: 2026 PlanV GmbH
+// SPDX-License-Identifier: CC0-1.0
+// verilog_format: off
+`define checkd(gotv,expv) do if ((gotv) !== (expv)) begin $write("%%Error: %s:%0d:  got=%0d exp=%0d\n", `__FILE__,`__LINE__, (gotv), (expv)); $stop; end while(0);
+// verilog_format: on
+
+interface inner_if;
+  logic [7:0] cnt = 0;
+  function automatic void bump();
+    cnt = cnt + 1;
+  endfunction
+  function automatic logic [7:0] get();
+    return cnt;
+  endfunction
+  task automatic addn(input logic [7:0] n);
+    cnt = cnt + n;
+  endtask
+endinterface
+
+interface mid_if;
+  inner_if sub ();
+endinterface
+
+interface data_if;
+  logic [7:0] val = 0;
+endinterface
+
+interface outer_if;
+  inner_if a ();
+  inner_if b ();
+  mid_if m ();
+  data_if d ();
+endinterface
+
+class driver_c;
+  virtual outer_if vif;
+  task run();
+    vif.a.bump();  // sibling a: void function, +1
+    vif.b.bump();  // sibling b: void function, +1
+    vif.b.addn(8'd5);  // sibling b: task with arg, +5
+    vif.m.sub.bump();  // two-level nesting, +1
+    vif.d.val = 8'd99;  // sub-interface variable write via vif (no method)
+  endtask
+  function logic [7:0] read_a();
+    return vif.a.get();  // function returning value via vif sub-interface
+  endfunction
+endclass
+
+module t;
+  outer_if oif ();
+  driver_c drv;
+  initial begin
+    drv = new();
+    drv.vif = oif;
+    drv.run();
+    // Per-instance dispatch: a, b and m.sub are distinct instances.
+    `checkd(oif.a.cnt, 8'd1)
+    `checkd(oif.b.cnt, 8'd6)
+    `checkd(oif.m.sub.cnt, 8'd1)
+    `checkd(drv.read_a(), 8'd1)
+    `checkd(oif.d.val, 8'd99)
+    $write("*-* All Finished *-*\n");
+    $finish;
+  end
+endmodule

test_regress/t/t_interface_virtual_sub_iface_method_bad.out

@@ -1,6 +0,0 @@
-%Error-UNSUPPORTED: t/t_interface_virtual_sub_iface_method_bad.v:21:16: Unsupported: method call on a sub-interface accessed through a virtual interface
-                                                                      : ... note: In instance 't'
-   21 |     vif.sub_if.set_active();
-      |                ^~~~~~~~~~
-                    ... For error description see https://verilator.org/warn/UNSUPPORTED?v=latest
-%Error: Exiting due to

test_regress/t/t_interface_virtual_sub_iface_method_bad.v

@@ -1,33 +0,0 @@
-// DESCRIPTION: Verilator: Verilog Test module
-//
-// This file ONLY is placed under the Creative Commons Public Domain.
-// SPDX-FileCopyrightText: 2026 PlanV GmbH
-// SPDX-License-Identifier: CC0-1.0
-
-interface inner_if;
-  logic active = 1'b0;
-  function automatic void set_active();
-    active = 1'b1;
-  endfunction
-endinterface
-
-interface outer_if;
-  inner_if sub_if ();
-endinterface
-
-class cfg_c;
-  virtual outer_if vif;
-  function void doit();
-    vif.sub_if.set_active();
-  endfunction
-endclass
-
-module t;
-  outer_if oif ();
-  cfg_c cfg;
-  initial begin
-    cfg = new();
-    cfg.vif = oif;
-    cfg.doit();
-  end
-endmodule