[gcc r16-1395] libstdc++: Make __max_size_type and __max_diff_type structural

[Patrick Palka via Gcc-cvs]

https://gcc.gnu.org/g:1f402fe23b0d4cf024688a729f4c86c37144d54a

commit r16-1395-g1f402fe23b0d4cf024688a729f4c86c37144d54a
Author: Patrick Palka <ppa...@redhat.com>
Date:   Tue Jun 10 10:15:25 2025 -0400

    libstdc++: Make __max_size_type and __max_diff_type structural
    
    This patch makes these integer-class types structural types by
    public-izing their data members so that they could be used as NTTP
    types.  I don't think this is required by the standard, but it seems
    like a useful extension.
    
    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.
    
    Reviewed-by: Tomasz Kamiński <tkami...@redhat.com>
    Reviewed-by: Jonathan Wakely <jwak...@redhat.com>

Diff:
---
 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 5bec0b5a519a..e602b1b4bee5 100644
--- a/libstdc++-v3/include/bits/max_size_type.h
+++ b/libstdc++-v3/include/bits/max_size_type.h
@@ -425,10 +425,11 @@ 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)
@@ -752,7 +753,6 @@ 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 3e6f954ceb0c..4739d9e2f790 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()
 {