From f3cae495d325c654dd01883d83b0c5b8abb64a70 Mon Sep 17 00:00:00 2001
From: Peter Dimov <pdimov@gmail.com>
Date: Fri, 2 Jan 2026 13:40:47 +0200
Subject: [PATCH] Document is_placeholder

---
 doc/changes.qbk        |  6 ++++
 doc/core.qbk           |  1 +
 doc/is_placeholder.qbk | 69 ++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 76 insertions(+)
 create mode 100644 doc/is_placeholder.qbk

diff --git a/doc/changes.qbk b/doc/changes.qbk
index d2acc4b..53a3f60 100644
--- a/doc/changes.qbk
+++ b/doc/changes.qbk
@@ -7,6 +7,12 @@
 
 [section Revision History]
 
+[section Changes in 1.91.0]
+
+* The header [link core.is_placeholder `boost/is_placeholder.hpp`] has been moved from Bind to Core. ([github_issue 90])
+
+[endsect]
+
 [section Changes in 1.90.0]
 
 * The implementation of `BOOST_TEST_THROWS` and `BOOST_TEST_NO_THROW` macros defined in
diff --git a/doc/core.qbk b/doc/core.qbk
index 2ec1b3f..666736d 100644
--- a/doc/core.qbk
+++ b/doc/core.qbk
@@ -61,6 +61,7 @@ criteria for inclusion is that the utility component be:
 [include functor.qbk]
 [include identity.qbk]
 [include ignore_unused.qbk]
+[include is_placeholder.qbk]
 [include is_same.qbk]
 [include launder.qbk]
 [include lightweight_test.qbk]
diff --git a/doc/is_placeholder.qbk b/doc/is_placeholder.qbk
new file mode 100644
index 0000000..7e991a6
--- /dev/null
+++ b/doc/is_placeholder.qbk
@@ -0,0 +1,69 @@
+[/
+  Copyright 2014, 2025 Peter Dimov
+
+  Distributed under the Boost Software License, Version 1.0.
+
+  See accompanying file LICENSE_1_0.txt
+  or copy at http://boost.org/LICENSE_1_0.txt
+]
+
+[section:is_placeholder is_placeholder]
+
+[simplesect Authors]
+
+* Peter Dimov
+
+[endsimplesect]
+
+[section Header <boost/is_placeholder.hpp>]
+
+The header `<boost/is_placeholder.hpp>` defines the class template
+`boost::is_placeholder<T>`. It defines a nested integral constant
+`value` which is `0` when `T` is not a `boost::bind` placeholder
+type, and a positive value corresponding to the placeholder index
+otherwise.
+
+That is, `is_placeholder<_1>::value` is `1`,
+`is_placeholder<_2>::value` is `2`, and so on.
+
+`is_placeholder` can be specialized for user types. If it is,
+`boost::bind` will recognize these types as placeholders.
+
+[section Synopsis]
+
+``
+namespace boost
+{
+    template<class T> struct is_placeholder;
+}
+``
+
+[endsect]
+
+[section Example]
+
+``
+#include <boost/is_placeholder.hpp>
+#include <boost/bind.hpp>
+
+struct arg1_type {};
+constexpr arg1_type arg1{};
+
+template<> struct boost::is_placeholder<arg1_type>
+{
+    static constexpr int value = 1;
+};
+
+int f( int x ) { return x + 1; }
+
+int main()
+{
+    return boost::bind( f, arg1 )( -1 );
+}
+``
+
+[endsect]
+
+[endsect]
+
+[endsect]