From a2b125a33eec3c9c55f66b331a65c518f3c388f9 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=EC=9D=B4=EC=9E=AC=EC=9A=B1?=
 <126692701+skku970412@users.noreply.github.com>
Date: Tue, 7 Jul 2026 08:37:45 +0900
Subject: [PATCH] Fix display of streaming concat arguments

---
 src/V3Width.cpp                         |  4 ++++
 test_regress/t/t_display_stream_bad.out |  6 ++++++
 test_regress/t/t_display_stream_bad.py  | 16 ++++++++++++++++
 test_regress/t/t_display_stream_bad.v   | 13 +++++++++++++
 4 files changed, 39 insertions(+)
 create mode 100644 test_regress/t/t_display_stream_bad.out
 create mode 100755 test_regress/t/t_display_stream_bad.py
 create mode 100644 test_regress/t/t_display_stream_bad.v

diff --git a/src/V3Width.cpp b/src/V3Width.cpp
index 802fe155d..9d3c02a4b 100644
--- a/src/V3Width.cpp
+++ b/src/V3Width.cpp
@@ -8567,6 +8567,10 @@ class WidthVisitor final : public VNVisitor {
                 ch = std::tolower(ch);
                 switch (ch) {
                 case '?':  // Unspecified by user, guess
+                    if (VN_IS(subargp, StreamL) || VN_IS(subargp, StreamR)) {
+                        subargp->v3error("Streaming concatenation cannot be used as an implicit "
+                                         "$display-like argument.");
+                    }
                     if (dtypep->isDouble()) {
                         ch = 'g';
                     } else if (dtypep->isString()) {
diff --git a/test_regress/t/t_display_stream_bad.out b/test_regress/t/t_display_stream_bad.out
new file mode 100644
index 000000000..0b92967eb
--- /dev/null
+++ b/test_regress/t/t_display_stream_bad.out
@@ -0,0 +1,6 @@
+%Error: t/t_display_stream_bad.v:11:15: Streaming concatenation cannot be used as an implicit $display-like argument.
+                                      : ... note: In instance 't'
+   11 |     $display({<<{value}});
+      |               ^~
+        ... See the manual at https://verilator.org/verilator_doc.html?v=latest for more assistance.
+%Error: Exiting due to
diff --git a/test_regress/t/t_display_stream_bad.py b/test_regress/t/t_display_stream_bad.py
new file mode 100755
index 000000000..38cf36b43
--- /dev/null
+++ b/test_regress/t/t_display_stream_bad.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('linter')
+
+test.lint(fails=True, expect_filename=test.golden_filename)
+
+test.passes()
diff --git a/test_regress/t/t_display_stream_bad.v b/test_regress/t/t_display_stream_bad.v
new file mode 100644
index 000000000..a9b7e5a04
--- /dev/null
+++ b/test_regress/t/t_display_stream_bad.v
@@ -0,0 +1,13 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain.
+// SPDX-FileCopyrightText: 2026 Wilson Snyder
+// SPDX-License-Identifier: CC0-1.0
+
+module t;
+  int value = 1;
+
+  initial begin
+    $display({<<{value}});
+  end
+endmodule