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<format>: Conditionally emit special diagnostic message for lack of const #4461

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merged 2 commits into from
Mar 16, 2024
Merged

<format>: Conditionally emit special diagnostic message for lack of const #4461

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ae8a5c5
Conditionally emit special diagnostic message for lack of `const`
frederick-vs-ja Mar 9, 2024
523bf0e
Implement `make_wformat_args` with Effects Equivalent To.
StephanTLavavej Mar 13, 2024
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46 changes: 32 additions & 14 deletions stl/inc/format
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Original file line number Diff line number Diff line change
Expand Up @@ -654,6 +654,14 @@ concept _Formattable_with = semiregular<_Formatter>
{ __cf.format(__t, __fc) } -> same_as<typename _Context::iterator>;
};

template <class _Ty, class _Context, class _Formatter = _Context::template formatter_type<remove_const_t<_Ty>>>
concept _Formattable_with_non_const = semiregular<_Formatter>
&& requires(_Formatter& __f, _Ty&& __t, _Context __fc,
basic_format_parse_context<typename _Context::char_type> __pc) {
{ __f.parse(__pc) } -> same_as<typename decltype(__pc)::iterator>;
{ __f.format(__t, __fc) } -> same_as<typename _Context::iterator>;
};

template <class _Ty, class _CharT>
inline constexpr bool _Is_basic_string_like_for = false;

Expand Down Expand Up @@ -707,15 +715,23 @@ public:
using _Td = remove_const_t<_Ty>;
// doesn't drop const-qualifier per an unnumbered LWG issue
using _Tq = conditional_t<_Formattable_with<const _Ty, _Context>, const _Ty, _Ty>;
if constexpr (_Formattable_with_non_const<_Tq, _Context>) {
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This ctor is exposition-only per [format.arg]/10 but is public in MSVC STL...
Perhaps we should consider making it private (in another PR, together with non-Standard public ctors of basic_format_context).

static_assert(_Formattable_with<_Tq, _Context>,
"The format() member function can't be called on const formatter<T>. "
"To make the formatter usable, add const to format(). "
"See N4971 [format.arg]/12, [format.formattable], and [formatter.requirements].");
} else {
static_assert(_Formattable_with<_Tq, _Context>,
"Cannot format an argument. "
"To make this type formattable, provide a formatter<T> specialization. "
"See N4971 [format.arg]/12, [format.formattable], and [formatter.requirements].");
}

typename _Context::template formatter_type<_Td> _Formatter;
_Parse_ctx.advance_to(_Formatter.parse(_Parse_ctx));
_Format_ctx.advance_to(
_Formatter.format(*const_cast<_Tq*>(static_cast<const _Td*>(_Ptr)), _Format_ctx));
}) {
// ditto doesn't drop const-qualifier
using _Tq = conditional_t<_Formattable_with<const _Ty, _Context>, const _Ty, _Ty>;
static_assert(_Formattable_with<_Tq, _Context>);
}
}) {}

void format(basic_format_parse_context<_CharType>& _Parse_ctx, _Context& _Format_ctx) const {
_Format(_Parse_ctx, _Format_ctx, _Ptr);
Expand Down Expand Up @@ -3711,20 +3727,22 @@ _EXPORT_STD using wformat_args = basic_format_args<wformat_context>;

_EXPORT_STD template <class _Context = format_context, class... _Args>
_NODISCARD auto make_format_args(_Args&... _Vals) {
// TRANSITION, should cite the new working draft
static_assert((_Formattable_with<remove_const_t<_Args>, _Context> && ...),
"Cannot format an argument. To make type T formattable, provide a formatter<T> specialization. "
"See N4964 [format.arg.store]/2 (along with modification in P2905R2) and [formatter.requirements].");
if constexpr ((_Formattable_with_non_const<remove_const_t<_Args>, _Context> && ...)) {
static_assert((_Formattable_with<remove_const_t<_Args>, _Context> && ...),
"The format() member function can't be called on const formatter<T>. "
"To make the formatter usable, add const to format(). "
"See N4971 [format.arg.store]/2 and [formatter.requirements].");
} else {
static_assert((_Formattable_with<remove_const_t<_Args>, _Context> && ...),
"Cannot format an argument. To make T formattable, provide a formatter<T> specialization. "
"See N4971 [format.arg.store]/2 and [formatter.requirements].");
}
return _Format_arg_store<_Context, _Args...>{_Vals...};
}

_EXPORT_STD template <class... _Args>
_NODISCARD auto make_wformat_args(_Args&... _Vals) {
// TRANSITION, should cite the new working draft
static_assert((_Formattable_with<remove_const_t<_Args>, wformat_context> && ...),
"Cannot format an argument. To make type T formattable, provide a formatter<T> specialization. "
"See N4964 [format.arg.store]/2 (along with modification in P2905R2) and [formatter.requirements].");
return _Format_arg_store<wformat_context, _Args...>{_Vals...};
return _STD make_format_args<wformat_context>(_Vals...);
}

_FMT_P2286_BEGIN
Expand Down