Add error on string addition

src/V3Width.cpp

@@ -7307,6 +7307,18 @@ class WidthVisitor final : public VNVisitor {
                 iterateCheckReal(nodep, "LHS", nodep->lhsp(), FINAL);
                 iterateCheckReal(nodep, "RHS", nodep->rhsp(), FINAL);
                 return;
+            } else if (nodep->lhsp()->isString() || nodep->rhsp()->isString()) {
+                nodep->v3error(
+                    "Operator "
+                    << nodep->prettyTypeName()
+                    << " is not legal on string data types (IEEE 1800-2023 6.16)\n"
+                    << (VN_IS(nodep, Add)
+                            ? (nodep->warnMore()
+                               + "... Suggest to concatenate strings use '{LHS, RHS, ...}'")
+                            : ""));
+                nodep->replaceWith(new AstConst{nodep->fileline(), AstConst::String{}, ""});
+                VL_DO_DANGLING(pushDeletep(nodep), nodep);
+                return;
             } else {
                 const int width = std::max(nodep->lhsp()->width(), nodep->rhsp()->width());
                 const int mwidth = std::max(nodep->lhsp()->widthMin(), nodep->rhsp()->widthMin());

test_regress/t/t_string_add_bad.out

@@ -0,0 +1,12 @@
+%Error: t/t_string_add_bad.v:13:9: Operator ADD is not legal on string data types (IEEE 1800-2023 6.16)
+                                 : ... note: In instance 't'
+                                 : ... Suggest to concatenate strings use '{LHS, RHS, ...}'
+   13 |       s += $sformatf(" a%0d", a);   
+      |         ^~
+        ... See the manual at https://verilator.org/verilator_doc.html?v=latest for more assistance.
+%Error: t/t_string_add_bad.v:14:13: Operator ADD is not legal on string data types (IEEE 1800-2023 6.16)
+                                  : ... note: In instance 't'
+                                  : ... Suggest to concatenate strings use '{LHS, RHS, ...}'
+   14 |       s = s + s;   
+      |             ^
+%Error: Exiting due to

test_regress/t/t_string_add_bad.py

@@ -0,0 +1,16 @@
+#!/usr/bin/env python3
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2025 by Wilson Snyder. 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-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_string_add_bad.v

@@ -0,0 +1,20 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain, for
+// any use, without warranty, 2025 by Wilson Snyder.
+// SPDX-License-Identifier: CC0-1.0
+
+module t;
+
+  string s;
+
+  initial begin
+    for (int a = 0; a < 3; ++a) begin : a_loop
+      s += $sformatf(" a%0d", a);  // <--- Error: += is not legal on strings
+      s = s + s;  // <--- Error: += is not legal on strings
+    end
+
+    $stop;
+  end
+
+endmodule