[gcc r12-10249] libstdc++: Make std::istream_iterator copy ctor constexpr (LWG 3600)

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:af1efe9ffeb4d21e2895081be1b88031166cdb93

commit r12-10249-gaf1efe9ffeb4d21e2895081be1b88031166cdb93
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Mar 22 11:10:38 2023 +0000

    libstdc++: Make std::istream_iterator copy ctor constexpr (LWG 3600)
    
    As explained in LWG 3600, we never implemented a C++0x change that made
    the copy constructor of std::istream_iterator defined as defaulted. That
    would be an ABI break, so the resolution of LWG 3600 is to not require
    it to be trivial, but just constexpr and conditionally noexcept. This
    applies that resolution.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/stream_iterator.h (istream_iterator): Add
            constexpr to copy constructor, as per LWG 3600.
            * testsuite/24_iterators/istream_iterator/cons/constexpr.cc:
            Check copy construction.
    
    (cherry picked from commit ad0b9cf1a076fb9a802d9ba7fa2223aa3166dca2)

Diff:
---
 libstdc++-v3/include/bits/stream_iterator.h                          | 1 +
 .../testsuite/24_iterators/istream_iterator/cons/constexpr.cc        | 5 +++++
 2 files changed, 6 insertions(+)

diff --git a/libstdc++-v3/include/bits/stream_iterator.h b/libstdc++-v3/include/bits/stream_iterator.h
index 86c5845b835..019eaf840e7 100644
--- a/libstdc++-v3/include/bits/stream_iterator.h
+++ b/libstdc++-v3/include/bits/stream_iterator.h
@@ -77,6 +77,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       : _M_stream(std::__addressof(__s)), _M_ok(true)
       { _M_read(); }
 
+      _GLIBCXX_CONSTEXPR
       istream_iterator(const istream_iterator& __obj)
       _GLIBCXX_NOEXCEPT_IF(is_nothrow_copy_constructible<_Tp>::value)
       : _M_stream(__obj._M_stream), _M_value(__obj._M_value),
diff --git a/libstdc++-v3/testsuite/24_iterators/istream_iterator/cons/constexpr.cc b/libstdc++-v3/testsuite/24_iterators/istream_iterator/cons/constexpr.cc
index 6b973c611a7..9570e7b9280 100644
--- a/libstdc++-v3/testsuite/24_iterators/istream_iterator/cons/constexpr.cc
+++ b/libstdc++-v3/testsuite/24_iterators/istream_iterator/cons/constexpr.cc
@@ -24,5 +24,10 @@ int main()
 {
   __gnu_test::constexpr_default_constructible test;
   test.operator()<std::istream_iterator<char>>();
+
+  // LWG 3600. Making istream_iterator copy constructor trivial is an ABI break
+  __gnu_test::constexpr_single_value_constructible test2;
+  test2.operator()<std::istream_iterator<char>, std::istream_iterator<char>>();
+
   return 0;
 }