Improve error that assert-under-assert is unsupported

src/V3LinkResolve.cpp

@@ -89,7 +89,9 @@ private:
         }
     }
     void visit(AstNodeCoverOrAssert* nodep) override {
-        if (m_assertp) nodep->v3error("Assert not allowed under another assert");
+        if (m_assertp) {
+            nodep->v3warn(E_UNSUPPORTED, "Unsupported: Assert not allowed under another assert");
+        }
         m_assertp = nodep;
         iterateChildren(nodep);
         m_assertp = nullptr;

test_regress/t/t_cover_assert.out

@@ -0,0 +1,5 @@
+%Error-UNSUPPORTED: t/t_cover_assert.v:42:16: Unsupported: Assert not allowed under another assert
+   42 |            A2: assert (b);
+      |                ^~~~~~
+                    ... For error description see https://verilator.org/warn/UNSUPPORTED?v=latest
+%Error: Exiting due to

test_regress/t/t_cover_assert.pl

@@ -0,0 +1,20 @@
+#!/usr/bin/env perl
+if (!$::Driver) { use FindBin; exec("$FindBin::Bin/bootstrap.pl", @ARGV, $0); die; }
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2003-2009 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(vlt => 1);
+
+lint(
+    verilator_flags2 => ["-Wall -Wno-DECLFILENAME --coverage"],
+    fails => 1,
+    expect_filename => $Self->{golden_filename},
+    );
+
+ok(1);
+1;

test_regress/t/t_cover_assert.v

@@ -0,0 +1,46 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain, for
+// any use, without warranty, 2023 by Wilson Snyder.
+// SPDX-License-Identifier: CC0-1.0
+
+module t(/*AUTOARG*/
+   // Inputs
+   clk
+   );
+   input clk;
+
+   integer cyc = 0;
+   bit     a;
+   bit     b;
+
+   // Test loop
+   always @ (posedge clk) begin
+      cyc <= cyc + 1;
+      if (cyc == 0) begin
+         a <= '0;
+         b <= '0;
+      end
+      else if (cyc == 10) begin
+         a <= '1;
+         b <= '1;
+      end
+      else if (cyc == 11) begin
+         a <= '0;
+         b <= '1;
+      end
+      else if (cyc == 99) begin
+         $write("*-* All Finished *-*\n");
+         $finish;
+      end
+   end
+
+   always_ff @(posedge clk) begin
+      C1: cover property(a)
+        begin
+           // Assert under cover legal in some other simulators
+           A2: assert (b);
+        end
+   end
+
+endmodule

test_regress/t/t_dist_warn_coverage.pl

@@ -37,7 +37,6 @@ foreach my $s (
     '/*verilator sformat*/ can only be applied to last argument of ',
     'Argument needed for string.',
     'Array initialization has too few elements, need element ',
-    'Assert not allowed under another assert',
     'Assigned pin is neither input nor output',
     'Assignment pattern with no members',
     'Attempted parameter setting of non-parameter: Param ',