Fix tristate lowering for interface inout ports (#7134 repair) (#7708) (#7710)

Fixes #7708.

src/V3Tristate.cpp

@@ -739,7 +739,7 @@ class TristateVisitor final : public TristateBaseVisitor {
                 // Check if the var is owned by a different module (cross-module reference).
                 // For interface vars this is expected; for regular modules it's unsupported.
                 AstNodeModule* const ownerModp = findParentModule(invarp);
-                const bool isCrossModule = ownerModp && ownerModp != nodep && !invarp->isIO();
+                const bool isCrossModule = ownerModp && ownerModp != nodep;
                 const bool isIfaceTri = isCrossModule && VN_IS(ownerModp, Iface);
 
                 if (isCrossModule && !isIfaceTri) {
@@ -778,7 +778,7 @@ class TristateVisitor final : public TristateBaseVisitor {
                                               kv.second.inlinedDots,
                                               findModportForDotted(nodep, kv.first));
                     }
-                } else if (VN_IS(nodep, Iface)) {
+                } else if (VN_IS(nodep, Iface) && !invarp->isIO()) {
                     // Local driver in an interface module - use contribution mechanism
                     // so it can be combined with any external drivers later
                     insertTristatesSignal(nodep, invarp, refsp, true, "", "", nullptr);

test_regress/t/t_tri_iface_port.py

@@ -0,0 +1,16 @@
+#!/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('vlt')
+
+test.compile()
+
+test.passes()

test_regress/t/t_tri_iface_port.v

@@ -0,0 +1,65 @@
+// DESCRIPTION: Verilator: Interface inout port driven locally and externally
+//
+// 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
+
+interface zest_if_local (
+    inout [0:0] U27
+);
+  assign U27[0] = 1'b1;
+endinterface
+
+interface zest_if_ext (
+    inout [0:0] U28
+);
+endinterface
+
+module ext_drv (
+    zest_if_ext zif,
+    input drive_en
+);
+  assign zif.U28[0] = drive_en ? 1'b1 : 1'bz;
+endmodule
+
+interface zest_if_ext_mp (
+    inout [0:0] U29
+);
+  modport drv(inout U29);
+endinterface
+
+module ext_drv_mp (
+    zest_if_ext_mp.drv zif,
+    input drive_en
+);
+  assign zif.U29[0] = drive_en ? 1'b0 : 1'bz;
+endmodule
+
+module t (
+    inout [0:0] bus_local,
+    inout [0:0] bus_ext,
+    inout [0:0] bus_ext_mp,
+    input drive_en
+);
+  zest_if_local zif_local (
+      .U27(bus_local)
+  );
+
+  zest_if_ext zif_ext (
+      .U28(bus_ext)
+  );
+  ext_drv u_ext_drv (
+      .zif(zif_ext),
+      .drive_en(drive_en)
+  );
+
+  zest_if_ext_mp zif_ext_mp (
+      .U29(bus_ext_mp)
+  );
+  ext_drv_mp u_ext_drv_mp (
+      .zif(zif_ext_mp),
+      .drive_en(drive_en)
+  );
+endmodule