Test equivalence of 'true' and 'false' to negated statements

The documentation for lib/builtin.pm asserts that 'true' is equivalent
to !!1 or !0, and that false is equivalent to !!0 or !1.  Demonstrate
that this is so.

Per review by Paul Evans in https://github.com/Perl/perl5/pull/19252,
we'll use cmp_ok() rather than is() and test for each of 'eq' and '=='.

1 files changed, 12 insertions, 0 deletions

diff --git a/lib/builtin.t b/lib/builtin.t
index 4f4e33a49e..b94cef4e40 100644
--- a/lib/builtin.t
+++ b/lib/builtin.t
@@ -112,4 +112,16 @@ package FetchStoreCounter {
     ok($recursecoderef->("rec"), 'true in self-recursive anon sub');
 }
 
+{
+    use builtin qw( true false );
+
+    my $val = true;
+    cmp_ok($val, $_, !!1, "true is equivalent to !!1 by $_") for qw( eq == );
+    cmp_ok($val, $_,  !0, "true is equivalent to  !0 by $_") for qw( eq == );
+
+    $val = false;
+    cmp_ok($val, $_, !!0, "false is equivalent to !!0 by $_") for qw( eq == );
+    cmp_ok($val, $_,  !1, "false is equivalent to  !1 by $_") for qw( eq == );
+}
+
 done_testing();