[PATCH] libstdc++: Make __max_size_type and __max_diff_type structural

[Patrick Palka via Gcc-patches]

This patch makes these integer-class type structural types by changing
their private data members into public ones, which allows them to be
used as NTTP types.  I'm not sure if this is required by the standard
but it seems handy.

Tested on x86_64-pc-linux-gnu, does this look OK for trunk?

libstdc++-v3/ChangeLog:

        * include/bits/max_size_type.h (__max_size_type::_M_val): Make
        public instead of private.
        (__max_size_type::_M_msb): Likewise.
        (__max_diff_type::_M_rep): Likewise.
        * testsuite/std/ranges/iota/max_size_type.cc: Verify
        __max_diff_type and __max_size_type are structural.
---
 libstdc++-v3/include/bits/max_size_type.h               | 4 ++--
 libstdc++-v3/testsuite/std/ranges/iota/max_size_type.cc | 7 +++++++
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/bits/max_size_type.h 
b/libstdc++-v3/include/bits/max_size_type.h
index 4796135d073..d6705bbe4c8 100644
--- a/libstdc++-v3/include/bits/max_size_type.h
+++ b/libstdc++-v3/include/bits/max_size_type.h
@@ -423,10 +423,11 @@ namespace ranges
       using __rep = unsigned long long;
 #endif
       static constexpr size_t _S_rep_bits = sizeof(__rep) * __CHAR_BIT__;
-    private:
+
       __rep _M_val = 0;
       unsigned _M_msb:1 = 0;
 
+    private:
       constexpr explicit
       __max_size_type(__rep __val, int __msb) noexcept
        : _M_val(__val), _M_msb(__msb)
@@ -750,7 +751,6 @@ namespace ranges
       { return !(__l < __r); }
 #endif
 
-    private:
       __max_size_type _M_rep = 0;
 
       friend class __max_size_type;
diff --git a/libstdc++-v3/testsuite/std/ranges/iota/max_size_type.cc 
b/libstdc++-v3/testsuite/std/ranges/iota/max_size_type.cc
index 985acd5a803..9afd05d5acf 100644
--- a/libstdc++-v3/testsuite/std/ranges/iota/max_size_type.cc
+++ b/libstdc++-v3/testsuite/std/ranges/iota/max_size_type.cc
@@ -400,6 +400,13 @@ static_assert(max_diff_t(max_size_t(1)
                         << (numeric_limits<max_size_t>::digits-1))
              == numeric_limits<max_diff_t>::min());
 
+// Verify that the types are structural types and can therefore be used
+// as NTTP types.
+template<max_size_t V> struct Su { static_assert(V*V == V+132); };
+template<max_diff_t V> struct Ss { static_assert(V*V == V+132); };
+template struct Su<12>;
+template struct Ss<12>;
+
 int
 main()
 {
-- 
2.40.0.374.g7580f92ffa