Fix virtual interface function calls binding to wrong instance (#7363)

src/V3Task.cpp

@@ -251,6 +251,13 @@ private:
         if (nodep->dpiImport()) m_curVxp->noInline(true);
         if (nodep->classMethod()) m_curVxp->noInline(true);  // Until V3Task supports it
         if (nodep->recursive()) m_curVxp->noInline(true);
+        // V3Scope resolves virtual-interface MethodCalls via user2p (last-wins),
+        // so inlining would bake in the wrong instance's VarScope refs.
+        if (v3Global.hasVirtIfaces()) {
+            if (const AstScope* const scopep = VN_CAST(nodep->user3p(), Scope)) {
+                if (VN_IS(scopep->modp(), Iface)) m_curVxp->noInline(true);
+            }
+        }
         if (nodep->isConstructor()) {
             m_curVxp->noInline(true);
             m_ctorp = nodep;
@@ -1690,10 +1697,12 @@ class TaskVisitor final : public VNVisitor {
             }
 
             const bool noInline = m_statep->ftaskNoInline(nodep);
-            // Warn if not inlining an impure ftask (unless method or recursvie).
+            // Warn if not inlining an impure ftask (unless method, recursive,
+            // or interface function -- interface member access is not truly external).
             // Will likely not schedule correctly.
             // TODO: Why not if recursive? It will not work ...
-            if (noInline && !nodep->classMethod() && !nodep->recursive()) {
+            if (noInline && !nodep->classMethod() && !nodep->recursive()
+                && !VN_IS(m_modp, Iface)) {
                 if (AstNode* const impurep = m_statep->checkImpure(nodep)) {
                     nodep->v3warn(
                         IMPURE,

test_regress/t/t_interface_virtual_func_wait.py

@@ -0,0 +1,18 @@
+#!/usr/bin/env python3
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# 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-FileCopyrightText: 2026 Wilson Snyder
+# SPDX-License-Identifier: LGPL-3.0-only OR Artistic-2.0
+
+import vltest_bootstrap
+
+test.scenarios('simulator')
+
+test.compile(verilator_flags2=["--binary"])
+
+test.execute()
+
+test.passes()

test_regress/t/t_interface_virtual_func_wait.v

@@ -0,0 +1,41 @@
+// 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 my_if;
+  logic clk = 0;
+  bit   clk_active = 0;
+
+  initial begin
+    wait (clk_active);
+    forever #5 clk = ~clk;
+  end
+
+  function void start_clk();
+    clk_active = 1;
+  endfunction
+endinterface
+
+class Driver;
+  virtual my_if vif;
+  task run();
+    #10;
+    vif.start_clk();
+  endtask
+endclass
+
+module t;
+  my_if intf();
+  my_if intf_unused();  // Second instance triggered the bug
+
+  initial begin
+    automatic Driver d = new;
+    d.vif = intf;
+    d.run();
+    repeat (4) @(posedge intf.clk);
+    $write("*-* All Finished *-*\n");
+    $finish;
+  end
+endmodule