Change metacomment extra underscore error to warning (#6968)

Used to throw a hard v3error on stray underscores in metacomments.
Issue a BADVLTPRAGMA that can be turned off instead.

src/V3PreProc.cpp

@@ -481,17 +481,21 @@ void V3PreProcImp::comment(const string& text) {
     if ((cp[0] == 'v' || cp[0] == 'V') && VString::startsWith(cp + 1, "erilator")) {
         cp += std::strlen("verilator");
         if (*cp == '_') {
-            fileline()->v3error("Extra underscore in meta-comment;"
+            V3Control::applyIgnores(fileline());
-                                " use /*verilator {...}*/ not /*verilator_{...}*/");
+            fileline()->v3warn(BADVLTPRAGMA, "Extra underscore in meta-comment, ignoring comment;"
+                                             " use /*verilator {...}*/ not /*verilator_{...}*/");
+            return;
         }
         vlcomment = true;
     } else if (VString::startsWith(cp, "synopsys")) {
         cp += std::strlen("synopsys");
-        synth = true;
         if (*cp == '_') {
-            fileline()->v3error("Extra underscore in meta-comment;"
+            V3Control::applyIgnores(fileline());
-                                " use /*synopsys {...}*/ not /*synopsys_{...}*/");
+            fileline()->v3warn(BADVLTPRAGMA, "Extra underscore in meta-comment, ignoring comment;"
+                                             " use /*synopsys {...}*/ not /*synopsys_{...}*/");
+            return;
         }
+        synth = true;
     } else if (VString::startsWith(cp, "cadence")) {
         cp += std::strlen("cadence");
         synth = true;
@@ -505,8 +509,6 @@ void V3PreProcImp::comment(const string& text) {
         return;
     }
 
-    if (!vlcomment && !synth) return;  // Short-circuit
-
     while (std::isspace(*cp)) ++cp;
     string cmd = commentCleanup(string{cp});
     // cmd now is comment without extra spaces and "verilator" prefix

test_regress/t/t_pp_underline.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('vlt')
+
+test.top_filename = "t/t_pp_underline_bad.v"
+
+test.lint(verilator_flags2=["-Wno-BADVLTPRAGMA"])
+
+test.passes()

test_regress/t/t_pp_underline_bad.out

@@ -1,12 +1,8 @@
-%Error: t/t_pp_underline_bad.v:8:4: Extra underscore in meta-comment; use /*verilator {...}*/ not /*verilator_{...}*/
+%Error-BADVLTPRAGMA: t/t_pp_underline_bad.v:8:4: Extra underscore in meta-comment, ignoring comment; use /*verilator {...}*/ not /*verilator_{...}*/
     8 |    // verilator_no_inline_module
       |    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-        ... See the manual at https://verilator.org/verilator_doc.html?v=latest for more assistance.
+                     ... For error description see https://verilator.org/warn/BADVLTPRAGMA?v=latest
-%Error: t/t_pp_underline_bad.v:10:19: Extra underscore in meta-comment; use /*synopsys {...}*/ not /*synopsys_{...}*/
+%Error-BADVLTPRAGMA: t/t_pp_underline_bad.v:10:19: Extra underscore in meta-comment, ignoring comment; use /*synopsys {...}*/ not /*synopsys_{...}*/
    10 |       case (1'b1) // synopsys_full_case
       |                   ^~~~~~~~~~~~~~~~~~~~~
-%Error-BADVLTPRAGMA: t/t_pp_underline_bad.v:8:4: Unknown verilator comment: '/*verilator _no_inline_module*/'
-    8 |    /*verilator _no_inline_module*/
-      |    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-                     ... For error description see https://verilator.org/warn/BADVLTPRAGMA?v=latest
 %Error: Exiting due to

test_regress/t/t_pp_underline_bad.v

@@ -9,6 +9,7 @@ module t;
    initial begin
       case (1'b1) // synopsys_full_case
         1'b0: $stop;
+        1'b1: $finish;
       endcase
       $stop; // Should have failed
    end

test_regress/t/t_pp_underline_bad_vlt.out

@@ -0,0 +1,5 @@
+%Error-BADVLTPRAGMA: t/t_pp_underline_bad.v:8:4: Extra underscore in meta-comment, ignoring comment; use /*verilator {...}*/ not /*verilator_{...}*/
+    8 |    // verilator_no_inline_module
+      |    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+                     ... For error description see https://verilator.org/warn/BADVLTPRAGMA?v=latest
+%Error: Exiting due to

test_regress/t/t_pp_underline_bad_vlt.py

@@ -0,0 +1,20 @@
+#!/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.top_filename = "t/t_pp_underline_bad.v"
+
+test.lint(verilator_flags2=["t/t_pp_underline_bad_vlt.vlt"],
+          fails=True,
+          expect_filename=test.golden_filename)
+
+test.passes()

test_regress/t/t_pp_underline_bad_vlt.vlt

@@ -0,0 +1,9 @@
+// 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
+
+`verilator_config
+
+lint_off -rule BADVLTPRAGMA -file "t/t_pp_underline_bad.v" -lines 10