Fix display of streaming concat arguments

docs/CONTRIBUTORS

@@ -110,6 +110,7 @@ Ilya Barkov
 Iru Cai
 Ivan Vnučec
 Iztok Jeras
+Jaeuk Lee (이재욱)
 Jake Merdich
 Jakub Michalski
 Jakub Wasilewski

src/V3Width.cpp

@@ -1002,6 +1002,14 @@ class WidthVisitor final : public VNVisitor {
             }
         }
         if (m_vup->final()) {
+            const AstNode* backp = nodep->backp();
+            if (VN_IS(backp, SFormatArg)) backp = backp->backp();
+            if (VN_IS(backp, SFormatF)) {
+                nodep->v3error(
+                    "Streaming concatenation cannot be used in an implicitly cast context "
+                    "(IEEE 1800-2023 11.4.17)\n"
+                    << nodep->warnMore() << "... Suggest use a cast");
+            }
             if (!nodep->dtypep()->widthSized()) {
                 // See also error in V3Number
                 nodeForUnsizedWarning(nodep)->v3warn(

test_regress/t/t_display_stream_bad.out

@@ -0,0 +1,17 @@
+%Error: t/t_display_stream_bad.v:11:15: Streaming concatenation cannot be used in an implicitly cast context (IEEE 1800-2023 11.4.17)
+                                      : ... note: In instance 't'
+                                      : ... Suggest use a cast
+   11 |     $display({<<{value}});
+      |               ^~
+        ... See the manual at https://verilator.org/verilator_doc.html?v=latest for more assistance.
+%Error: t/t_display_stream_bad.v:12:22: Streaming concatenation cannot be used in an implicitly cast context (IEEE 1800-2023 11.4.17)
+                                      : ... note: In instance 't'
+                                      : ... Suggest use a cast
+   12 |     $display("%0d", {<<{value}});
+      |                      ^~
+%Error: t/t_display_stream_bad.v:13:29: Streaming concatenation cannot be used in an implicitly cast context (IEEE 1800-2023 11.4.17)
+                                      : ... note: In instance 't'
+                                      : ... Suggest use a cast
+   13 |     void'($sformatf("%0d", {<<{value}}));
+      |                             ^~
+%Error: Exiting due to

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()

test_regress/t/t_display_stream_bad.v

@@ -0,0 +1,15 @@
+// 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}});
+    $display("%0d", {<<{value}});
+    void'($sformatf("%0d", {<<{value}}));
+  end
+endmodule