'/'.IO.mkdir must not segfault

RT#126976

geekosaur++ for explaining Windows side of things for this test
(http://irclog.perlgeek.de/perl6-dev/2016-07-05#i_12784679)

S32-io/mkdir_rmdir.t

@@ -1,7 +1,7 @@
 use v6;
 use Test;
 
-plan 7;
+plan 9;
 
 # Tests for IO::Path.mkdir and IO::Path.rmdir
 #
@@ -62,6 +62,35 @@ plan 7;
     isa_fatal_ok $err, X::IO::Mkdir;
 }
 
+# RT #126976
+subtest {
+    # This test is a bit tricky:
+    #   it generally should throw since we can't create '/' directory
+    #   on Windows, however, such .mkdir returns True due to a
+    #   backward compatibility wart. BUT, it fails if the test is run
+    #   from a root directory, such as C:\ [discussion:
+    #    http://irclog.perlgeek.de/perl6-dev/2016-07-05#i_12784679]
+    #
+    #   So what we're doing here is skipping the exception testing if
+    #     we are on Windows and got True. We also attempt to .mkdir
+    #     a few times to ensure segfaults aren't lurking in there.
+
+    my $result;
+    try {
+        $result = "/".IO.mkdir;
+        CATCH { default { $result = $_; } };
+    } for ^5;
+
+    if $*DISTRO ~~ /'mswin32'/ and $result ~~ Bool and $result {
+        skip '"/".IO.mkdir succeeds on Windows when not run in root dir', 2;
+    }
+    else {
+        isa-ok $result, X::IO::Mkdir, 'we received an exception';
+        like $result.message, /'Failed to create directory'/,
+            'exception has right message';
+    }
+}, '"/".IO.mkdir must not segfault';
+
 sub testdir {
     my $testdir = "testdir-" ~ 1000000.rand.floor;
     die if $testdir.path.e;