Fix bad-syntax crashes, bug1582.

Changes

@@ -27,7 +27,7 @@ The contributors that suggested a given feature are shown in []. Thanks!
 ****  Fix multithreaded yield behavior when no work.  [Patrick Stewart]
 
 ****  Fix bad-syntax crashes, bug1548, bug1550-1553, bug1557-1560, bug1563,
-      bug1573-1577. [Eric Rippey]
+      bug1573-1577, bug1582. [Eric Rippey]
 
 ****  Fix false CMPCONST/UNSIGNED warnings on "inside", bug1581. [Mitch Hayenga]
 

src/V3Const.cpp

@@ -95,6 +95,7 @@ private:
     // ** must track down everywhere V3Const is called and make sure no overlaps.
     // AstVar::user4p           -> Used by ConstVarMarkVisitor/ConstVarFindVisitor
     // AstJumpLabel::user4      -> bool.  Set when AstJumpGo uses this label
+    // AstEnum::user4           -> bool.  Recursing.
 
     // STATE
     bool        m_params;       // If true, propagate parameterized and true numbers only
@@ -1630,7 +1631,13 @@ private:
         bool did = false;
         if (nodep->itemp()->valuep()) {
             //if (debug()) nodep->itemp()->valuep()->dumpTree(cout, "  visitvaref: ");
-            iterateAndNextNull(nodep->itemp()->valuep());
+            if (nodep->itemp()->user4()) {
+                nodep->v3error("Recursive enum value: "<<nodep->itemp()->prettyNameQ());
+            } else {
+                nodep->itemp()->user4(true);
+                iterateAndNextNull(nodep->itemp()->valuep());
+                nodep->itemp()->user4(false);
+            }
             if (AstConst* valuep = VN_CAST(nodep->itemp()->valuep(), Const)) {
                 const V3Number& num = valuep->num();
                 replaceNum(nodep, num); VL_DANGLING(nodep);

test_regress/t/t_enum_recurse_bad.out

@@ -0,0 +1,10 @@
+%Error: t/t_enum_recurse_bad.v:6: Recursive enum value: 'u'
+enum {u=u} e_t;
+        ^
+%Error: t/t_enum_recurse_bad.v:6: Expecting expression to be constant, but variable isn't const: 'u'
+enum {u=u} e_t;
+        ^
+%Error: t/t_enum_recurse_bad.v:6: Enum value isn't a constant
+enum {u=u} e_t;
+        ^
+%Error: Exiting due to

test_regress/t/t_enum_recurse_bad.pl

@@ -0,0 +1,18 @@
+#!/usr/bin/perl
+if (!$::Driver) { use FindBin; exec("$FindBin::Bin/bootstrap.pl", @ARGV, $0); die; }
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2004 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.
+
+scenarios(simulator => 1);
+
+compile(
+    fails => 1,
+    expect_filename => $Self->{golden_filename},
+    );
+
+ok(1);
+1;

test_regress/t/t_enum_recurse_bad.v

@@ -0,0 +1,6 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed into the Public Domain, for any use,
+// without warranty, 2019 by Wilson Snyder.
+
+enum {u=u} e_t;