Fix mailbox#(packed_struct) type mismatch with parameterized class (#7494) (#7495)

docs/CONTRIBUTORS

@@ -200,6 +200,7 @@ Nathan Graybeal
 Nathan Kohagen
 Nathan Myers
 Nick Brereton
+Nikolai Kumar
 Nikolay Puzanov
 Nolan Poe
 Oleh Maksymenko

src/V3AstNodeDType.h

@@ -275,9 +275,7 @@ public:
     int widthAlignBytes() const override;
     // (Slow) recurses - Width in bytes rounding up 1,2,4,8,12,...
     int widthTotalBytes() const override;
-    bool similarDTypeNode(const AstNodeDType* samep) const override {
-        return this == samep;  // We don't compare members, require exact equivalence
-    }
+    bool similarDTypeNode(const AstNodeDType* samep) const override;
     string name() const override VL_MT_STABLE { return m_name; }
     void name(const string& flag) override { m_name = flag; }
     bool packed() const VL_MT_SAFE { return m_packed; }

src/V3AstNodes.cpp

@@ -2003,6 +2003,21 @@ bool AstClassRefDType::similarDTypeNode(const AstNodeDType* samep) const {
     }
     return !lp && !rp;
 }
+bool AstNodeUOrStructDType::similarDTypeNode(const AstNodeDType* samep) const {
+    const AstNodeUOrStructDType* const sp = VN_DBG_AS(samep, NodeUOrStructDType);
+    if (m_packed != sp->m_packed) return false;
+    if (fileline()->tokenNum() != sp->fileline()->tokenNum()) return false;
+    const AstMemberDType* lp = membersp();
+    const AstMemberDType* rp = sp->membersp();
+    while (lp && rp) {
+        if (lp->name() != rp->name()) return false;
+        if (lp->width() != rp->width()) return false;
+        if (!lp->subDTypep()->similarDType(rp->subDTypep())) return false;
+        lp = VN_CAST(lp->nextp(), MemberDType);
+        rp = VN_CAST(rp->nextp(), MemberDType);
+    }
+    return !lp && !rp;
+}
 void AstNodeCoverOrAssert::dump(std::ostream& str) const {
     this->AstNodeStmt::dump(str);
     str << " ["s + this->userType().ascii() + "]";

test_regress/t/t_mailbox_struct_param.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_mailbox_struct_param.v

@@ -0,0 +1,51 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain.
+// SPDX-FileCopyrightText: 2026 Nikolai Kumar
+// SPDX-License-Identifier: CC0-1.0
+
+package pkg;
+  class C #(parameter P = 0);
+  typedef struct packed {
+    bit [7:0] x;
+    } my_t;
+
+    mailbox #(my_t) mb = new();
+
+    task run(output bit [7:0] got);
+      my_t v;
+      mb.get(v);
+      got = v.x;
+    endtask
+  endclass
+endpackage
+
+module top;
+  import pkg::*;
+
+  initial begin
+    C #(0) c0;
+    C #(1) c1;
+    C#(0)::my_t s0;
+    C#(1)::my_t s1;
+    bit [7:0] got0;
+    bit [7:0] got1;
+
+    c0 = new();
+    c1 = new();
+
+    s0.x = 8'hA5;
+    s1.x = 8'h5A;
+    c0.mb.put(s0);
+    c1.mb.put(s1);
+
+    c0.run(got0);
+    c1.run(got1);
+
+    if(got0 !== 8'hA5) $stop;
+    if(got0 !== 8'hA5) $stop;
+
+    $write("*-* All Finished *-*\n");
+    $finish;
+  end
+endmodule