With --Wpedantic, report forward typedefs that are unused.

src/V3LinkDot.cpp

@@ -1089,6 +1089,12 @@ class LinkDotFindVisitor : public AstNVisitor {
         iterateChildren(nodep);
         m_statep->insertSym(m_curSymp, nodep->name(), nodep, m_packagep);
     }
+    virtual void visit(AstTypedefFwd* nodep) VL_OVERRIDE {
+        UASSERT_OBJ(m_curSymp, nodep, "Typedef not under module/package/$unit");
+        iterateChildren(nodep);
+        // No need to insert, only the real typedef matters, but need to track for errors
+        nodep->user1p(m_curSymp);
+    }
     virtual void visit(AstParamTypeDType* nodep) VL_OVERRIDE {
         UASSERT_OBJ(m_curSymp, nodep, "Parameter type not under module/package/$unit");
         iterateChildren(nodep);
@@ -1339,6 +1345,19 @@ private:
         // We're done with implicit gates
         VL_DO_DANGLING(nodep->unlinkFrBack()->deleteTree(), nodep);
     }
+    virtual void visit(AstTypedefFwd* nodep) VL_OVERRIDE {
+        VSymEnt* foundp = m_statep->getNodeSym(nodep)->findIdFallback(nodep->name());
+        if (!foundp && v3Global.opt.pedantic()) {
+            // We only check it was ever really defined in pedantic mode, as it
+            // might have been in a header file referring to a module we never
+            // needed so there are false positives
+            nodep->v3error(
+                "Forward typedef unused or does not resolve to a data type (IEEE 1800-2017 6.18): "
+                << nodep->prettyNameQ());
+        }
+        // We only needed the forward declaration in order to parse correctly.
+        VL_DO_DANGLING(nodep->unlinkFrBack()->deleteTree(), nodep);
+    }
     virtual void visit(AstNode* nodep) VL_OVERRIDE {
         // Default: Just iterate
         iterateChildren(nodep);

src/V3LinkParse.cpp

@@ -382,13 +382,6 @@ private:
         VL_DO_DANGLING(nodep->deleteTree(), nodep);
     }
 
-    virtual void visit(AstTypedefFwd* nodep) VL_OVERRIDE {
-        // We only needed the forward declaration in order to parse correctly.
-        // We won't even check it was ever really defined, as it might have been in a header
-        // file referring to a module we never needed
-        VL_DO_DANGLING(nodep->unlinkFrBack()->deleteTree(), nodep);
-    }
-
     virtual void visit(AstForeach* nodep) VL_OVERRIDE {
         // FOREACH(array,loopvars,body)
         // -> BEGIN(declare vars, loopa=lowest; WHILE(loopa<=highest, ... body))

test_regress/t/t_typedef_unused_bad.out

@@ -0,0 +1,4 @@
+%Error: t/t_typedef_unused_bad.v:9:9: Forward typedef unused or does not resolve to a data type (IEEE 1800-2017 6.18): 'fwd_undecl_t'
+typedef fwd_undecl_t;
+        ^~~~~~~~~~~~
+%Error: Exiting due to

test_regress/t/t_typedef_unused_bad.pl

@@ -0,0 +1,20 @@
+#!/usr/bin/perl
+if (!$::Driver) { use FindBin; exec("$FindBin::Bin/bootstrap.pl", @ARGV, $0); die; }
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2003 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
+
+scenarios(linter => 1);
+
+lint(
+    verilator_flags2 => ["--Wpedantic"],
+    fails => 1,
+    expect_filename => $Self->{golden_filename},
+    );
+
+ok(1);
+1;

test_regress/t/t_typedef_unused_bad.v

@@ -0,0 +1,9 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain, for
+// any use, without warranty, 2020 by Wilson Snyder.
+// SPDX-License-Identifier: CC0-1.0
+
+typedef logic ok_t;
+
+typedef fwd_undecl_t;