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Implement LWG-4037 Static data members of ctype_base are not yet required to be usable in constant expressions #6079

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Merged
Stephan T. Lavavej (StephanTLavavej) merged 5 commits into from
Feb 12, 2026
Merged

Implement LWG-4037 Static data members of ctype_base are not yet required to be usable in constant expressions #6079

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3687ea0
Implement LWG-4037
frederick-vs-ja Jan 27, 2026
e76757b
Merge branch 'main' into lwg-4037
StephanTLavavej Feb 11, 2026
0fb1785
Add `__declspec(empty_bases)` to `ctype_base`.
StephanTLavavej Feb 11, 2026
da0918a
`<xiosbase>`: Remove enums.
StephanTLavavej Feb 11, 2026
b4e3ecb
Hiss.
StephanTLavavej Feb 11, 2026
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21 changes: 6 additions & 15 deletions stl/inc/xiosbase
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Original file line number Diff line number Diff line change
Expand Up @@ -20,12 +20,11 @@ _STL_DISABLE_CLANG_WARNINGS

_STD_BEGIN
template <class _Dummy>
class _Iosb { // define templatized bitmask/enumerated types, instantiate on demand
class _Iosb { // define templatized bitmask types, instantiate on demand
public:
enum _Fmtflags { // constants for formatting options
_Fmtmask = 0xffff,
_Fmtzero = 0
};
// constants for formatting options
static constexpr int _Fmtmask = 0xffff;
static constexpr int _Fmtzero = 0;

static constexpr int skipws = 0x0001;
static constexpr int unitbuf = 0x0002;
Expand All @@ -47,9 +46,8 @@ public:
static constexpr int basefield = dec | oct | hex;
static constexpr int floatfield = scientific | fixed;

enum _Iostate { // constants for stream states
_Statmask = 0x17
};
// constants for stream states
static constexpr int _Statmask = 0x17;

static constexpr int goodbit = 0x0;
static constexpr int eofbit = 0x1;
Expand All @@ -73,13 +71,6 @@ public:
static constexpr int end = 2;

static constexpr int _Default_open_prot = _SH_DENYNO; // constant for default file opening protection

// TRANSITION, ABI: These enums don't appear in the STL DLL's export surface, but MaxMPDCompat
// in the MSVC-internal build detects that they appear in the CompatibilityData baseline.
enum _Dummy_enum { _Dummy_enum_val = 1 };
enum _Openmode { _Openmask = 0xff };
enum _Seekdir { _Seekbeg, _Seekcur, _Seekend };
enum { _Openprot = _SH_DENYNO };
};

_EXPORT_STD extern "C++" class _CRTIMP2_PURE_IMPORT ios_base : public _Iosb<int> { // base class for ios
Expand Down
46 changes: 29 additions & 17 deletions stl/inc/xlocale
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Original file line number Diff line number Diff line change
Expand Up @@ -21,6 +21,10 @@ _STL_DISABLE_CLANG_WARNINGS
#pragma push_macro("new")
#undef new

// TRANSITION, non-_Ugly attribute tokens
#pragma push_macro("empty_bases")
#undef empty_bases

_STD_BEGIN
template <class _Dummy>
class _Locbase {}; // TRANSITION, ABI, affects sizeof(locale)
Expand Down Expand Up @@ -2373,25 +2377,29 @@ _STL_RESTORE_DEPRECATED_WARNING
#define _UP _UPPER // 'A'-'Z'
#define _XD _HEX // '0'-'9', 'A'-'F', 'a'-'f'

_EXPORT_STD extern "C++" struct _CRTIMP2_PURE_IMPORT ctype_base // base for ctype
: locale::facet // TRANSITION, ABI, shouldn't be derived from locale::facet
{
enum { // constants for character classifications
alnum = _DI | _LO | _UP | _XA,
alpha = _LO | _UP | _XA,
cntrl = _BB,
digit = _DI,
graph = _DI | _LO | _PU | _UP | _XA,
lower = _LO,
print = _DI | _LO | _PU | _SP | _UP | _XA | _XD,
punct = _PU,
space = _CN | _SP,
upper = _UP,
xdigit = _XD,
blank = _CN | _SP
};
struct _Ctype_constants_base {
using mask = short; // to match <ctype.h>

// constants for character classifications

static constexpr mask alnum = _DI | _LO | _UP | _XA;
static constexpr mask alpha = _LO | _UP | _XA;
static constexpr mask cntrl = _BB;
static constexpr mask digit = _DI;
static constexpr mask graph = _DI | _LO | _PU | _UP | _XA;
static constexpr mask lower = _LO;
static constexpr mask print = _DI | _LO | _PU | _SP | _UP | _XA | _XD;
static constexpr mask punct = _PU;
static constexpr mask space = _CN | _SP;
static constexpr mask upper = _UP;
static constexpr mask xdigit = _XD;
static constexpr mask blank = _CN | _SP;
};

_EXPORT_STD extern "C++" struct _CRTIMP2_PURE_IMPORT __declspec(empty_bases) ctype_base // base for ctype
: locale::facet, // TRANSITION, ABI, shouldn't be derived from locale::facet
_Ctype_constants_base // TRANSITION, ABI, avoid affecting DLL interface
{
__CLR_OR_THIS_CALL ctype_base(size_t _Refs = 0) noexcept // strengthened
: locale::facet(_Refs) {}

Expand Down Expand Up @@ -3312,6 +3320,10 @@ template class _CRTIMP2_PURE_IMPORT codecvt<char, char, mbstate_t>;
#endif // !defined(_CRTBLD) || defined(__FORCE_INSTANCE)
#endif // defined(_DLL_CPPLIB)
_STD_END

// TRANSITION, non-_Ugly attribute tokens
#pragma pop_macro("empty_bases")

#pragma pop_macro("new")
_STL_RESTORE_CLANG_WARNINGS
#pragma warning(pop)
Expand Down
34 changes: 34 additions & 0 deletions tests/std/tests/Dev11_1074023_constexpr/test.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -801,6 +801,12 @@ void test_all_constants() {

test_constants<ios_base::seekdir, ios_base::beg, ios_base::cur, ios_base::end>();

// LWG-4037 "Static data members of ctype_base are not yet required to be usable in constant expressions"
// The following furtherly requires ctype_base::mask to be usable as a type of a constant template parameter.
test_constants<ctype_base::mask, ctype_base::alnum, ctype_base::alpha, ctype_base::cntrl, ctype_base::digit,
ctype_base::graph, ctype_base::lower, ctype_base::print, ctype_base::punct, ctype_base::space,
ctype_base::upper, ctype_base::xdigit, ctype_base::blank>();

test_constants<RC::syntax_option_type, RC::icase, RC::nosubs, RC::optimize, RC::collate, RC::ECMAScript, RC::basic,
RC::extended, RC::awk, RC::grep, RC::egrep, regex::icase, regex::nosubs, regex::optimize, regex::collate,
regex::ECMAScript, regex::basic, regex::extended, regex::awk, regex::grep, regex::egrep>();
Expand All @@ -814,6 +820,34 @@ void test_all_constants() {
RC::error_badrepeat, RC::error_complexity, RC::error_stack>();
}

// Ensure that these members of ctype_base are actually implemented as static const members of the correct type.

STATIC_ASSERT(is_same_v<decltype(ctype_base::alnum), const ctype_base::mask>);
STATIC_ASSERT(is_same_v<decltype(ctype_base::alpha), const ctype_base::mask>);
STATIC_ASSERT(is_same_v<decltype(ctype_base::cntrl), const ctype_base::mask>);
STATIC_ASSERT(is_same_v<decltype(ctype_base::digit), const ctype_base::mask>);
STATIC_ASSERT(is_same_v<decltype(ctype_base::graph), const ctype_base::mask>);
STATIC_ASSERT(is_same_v<decltype(ctype_base::lower), const ctype_base::mask>);
STATIC_ASSERT(is_same_v<decltype(ctype_base::print), const ctype_base::mask>);
STATIC_ASSERT(is_same_v<decltype(ctype_base::punct), const ctype_base::mask>);
STATIC_ASSERT(is_same_v<decltype(ctype_base::space), const ctype_base::mask>);
STATIC_ASSERT(is_same_v<decltype(ctype_base::upper), const ctype_base::mask>);
STATIC_ASSERT(is_same_v<decltype(ctype_base::xdigit), const ctype_base::mask>);
STATIC_ASSERT(is_same_v<decltype(ctype_base::blank), const ctype_base::mask>);

STATIC_ASSERT(is_same_v<decltype((ctype_base::alnum)), const ctype_base::mask&>);
STATIC_ASSERT(is_same_v<decltype((ctype_base::alpha)), const ctype_base::mask&>);
STATIC_ASSERT(is_same_v<decltype((ctype_base::cntrl)), const ctype_base::mask&>);
STATIC_ASSERT(is_same_v<decltype((ctype_base::digit)), const ctype_base::mask&>);
STATIC_ASSERT(is_same_v<decltype((ctype_base::graph)), const ctype_base::mask&>);
STATIC_ASSERT(is_same_v<decltype((ctype_base::lower)), const ctype_base::mask&>);
STATIC_ASSERT(is_same_v<decltype((ctype_base::print)), const ctype_base::mask&>);
STATIC_ASSERT(is_same_v<decltype((ctype_base::punct)), const ctype_base::mask&>);
STATIC_ASSERT(is_same_v<decltype((ctype_base::space)), const ctype_base::mask&>);
STATIC_ASSERT(is_same_v<decltype((ctype_base::upper)), const ctype_base::mask&>);
STATIC_ASSERT(is_same_v<decltype((ctype_base::xdigit)), const ctype_base::mask&>);
STATIC_ASSERT(is_same_v<decltype((ctype_base::blank)), const ctype_base::mask&>);

constexpr csub_match sm{};
STATIC_ASSERT(sm.first == nullptr);
STATIC_ASSERT(sm.second == nullptr);
Expand Down