WIP: preserve Perl scalar dual-value semantics

docs/about/changelog.md

@@ -6,6 +6,9 @@ priorities and future plans.
 
 ## Work in progress
 
+- Preserve Data::Dumper's pure-Perl numeric-string behavior for
+  Test::Differences, including copied `qw` values and numeric zero fixtures.
+
 - Correct named-unary operand precedence, so `! scalar @array % 2` evaluates
   the modulo operation before its logical negation.
 

src/main/perl/lib/Data/Dumper.pm

@@ -580,7 +580,12 @@ sub _dump {
       $out .= sprintf "v%vd", $val;
     }
     # \d here would treat "1\x{660}" as a safe decimal number
-    elsif (defined &Data::Dumper::_perlonjava_numified_safe_decimal
+    # Test::Differences explicitly selects Data::Dumper's pure-Perl renderer.
+    # That renderer has always used the safe-decimal fallback below, which
+    # renders numeric-looking strings without quotes.  Keep that behavior in
+    # Useperl mode; otherwise use PerlOnJava's scalar-aware hook so an
+    # untouched string remains distinguishable from a numeric scalar.
+    elsif ((!$s->{useperl} && defined &Data::Dumper::_perlonjava_numified_safe_decimal)
        ? Data::Dumper::_perlonjava_numified_safe_decimal($val)
        : $val =~ /^(?:0|-?[1-9][0-9]{0,8})\z/) {
       $out .= $val;

src/test/resources/unit/scalar_dual_value_cpan_regressions.t

@@ -0,0 +1,79 @@
+use strict;
+use warnings;
+use Test::More;
+use Data::Dumper;
+use JSON::PP qw(decode_json);
+use B ();
+
+sub eq_or_diff_via_data_dumper {
+    my ($got, $expected, $name) = @_;
+
+    # Test::Differences enables this pure-Perl Data::Dumper configuration
+    # before comparing its serialized values.  Keep the project unit test
+    # self-contained: Test::Differences is a CPAN test dependency and is not
+    # installed by the clean CI image.
+    local $Data::Dumper::Deparse   = 1;
+    local $Data::Dumper::Indent    = 1;
+    local $Data::Dumper::Purity    = 0;
+    local $Data::Dumper::Terse     = 1;
+    local $Data::Dumper::Deepcopy  = 1;
+    local $Data::Dumper::Quotekeys = 0;
+    local $Data::Dumper::Useperl   = 1;
+    local $Data::Dumper::Sortkeys  = 1;
+
+    is(Dumper($got), Dumper($expected), $name);
+}
+
+# Equivalent to List::PowerSet 0.01.  Its recursive list copies must retain
+# the scalar behavior Test::Differences observes with stock Perl.
+sub powerset {
+    return [[]] if @_ == 0;
+    my $first = shift;
+    my $pow = powerset(@_);
+    return [ map { [$first, @$_], [@$_] } @$pow ];
+}
+
+eq_or_diff_via_data_dumper(
+    powerset(qw(1 2 3)),
+    [[1, 2, 3], [2, 3], [1, 3], [3], [1, 2], [2], [1], []],
+    'recursive copies of numeric-looking qw values match numeric literals',
+);
+
+# HTTP::BrowserDetect compares a computed numeric zero with a JSON string
+# fixture through this same Data::Dumper-backed comparison path.
+sub browser_major { return 0 }
+my $browser_fixture = decode_json('{"browser_major":"0"}');
+eq_or_diff_via_data_dumper(browser_major(), $browser_fixture->{browser_major},
+    'method-returned numeric zero and JSON string zero compare through Test::Differences');
+
+my $document = decode_json('{"number":1,"string":"1"}');
+my $number_flags = B::svref_2object(\$document->{number})->FLAGS;
+my $string_flags = B::svref_2object(\$document->{string})->FLAGS;
+ok($number_flags & (B::SVp_IOK() | B::SVp_NOK()),
+    'JSON numeric token has numeric scalar flags');
+ok(!($number_flags & B::SVp_POK()),
+    'JSON numeric token does not have string-only flags');
+ok($string_flags & B::SVp_POK(),
+    'JSON string token has string flags');
+ok(!($string_flags & (B::SVp_IOK() | B::SVp_NOK())),
+    'JSON string token does not gain numeric flags');
+
+sub abs2rel {
+    return if !@_;
+    my @result = $_[0];
+    for my $i (1 .. $#_) {
+        push @result, $_[$i] - $_[$i - 1];
+    }
+    return @result;
+}
+
+eq_or_diff_via_data_dumper(
+    [abs2rel(qw(1 2 3))],
+    [qw(1 1 1)],
+    'arithmetic on string inputs preserves the comparison-visible string channel',
+);
+
+local $Data::Dumper::Terse = 1;
+is(Dumper('nonnumeric'), "'nonnumeric'\n", 'ordinary nonnumeric strings remain strings');
+
+done_testing;