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P3931R0: consteval all the non-allocating operator"" things

This paper makes all non-allocating operator"" in the standard library marked consteval. This doesn't break any code.

Motivation

Functions which don't have any runtime dependency and just creates an object should be consteval, unless they have some implicit external dependency, then they should be only constexpr, so they are still usable in constant evaluation.

Implementation

Basically just a replacement of constexpr with consteval in my fork of libc++.

No breakage

A dear reader can think this change will break some code by making pointers to these functions consteval-only values, but the standard already disallow us from taking address of most functions in the standard library, hence this is not a breakage.

Debugability

Any problematic code or a bug inside these functions will result in a compilation error instead runtime bug which needs to be debugged. This is seen as strict improvement, and even so users tends to prefer to skip over std:: namespaced functions when looking for bugs.

Wording

Note: only operator which is not changed is std::basic_string operators ""s as we still don't have non-transient allocation out of constant evaluation to runtime.

27.3.7 Suffix for basic_string_view literals [string.view.literals]

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constexprconsteval string_view operator""sv(const char* str, size_t len) noexcept;
1
#
Returns: string_view{str, len}.
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constexprconsteval u8string_view operator""sv(const char8_t* str, size_t len) noexcept;
2
#
Returns: u8string_view{str, len}.
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constexprconsteval u16string_view operator""sv(const char16_t* str, size_t len) noexcept;
3
#
Returns: u16string_view{str, len}.
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constexprconsteval u32string_view operator""sv(const char32_t* str, size_t len) noexcept;
4
#
Returns: u32string_view{str, len}.
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constexprconsteval wstring_view operator""sv(const wchar_t* str, size_t len) noexcept;
5
#
Returns: wstring_view{str, len}.

29.4.11 Suffixes for complex number literals [complex.literals]

1
#
This subclause describes literal suffixes for constructing complex number literals.
The suffixes i, il, and if create complex numbers of the types complex<double>, complex<long double>, and complex<float> respectively, with their imaginary part denoted by the given literal number and the real part being zero.
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constexprconsteval complex<long double> operator""il(long double d); constexprconsteval complex<long double> operator""il(unsigned long long d);
2
#
Returns: complex<long double>{0.0L, static_cast<long double>(d)}.
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constexprconsteval complex<double> operator""i(long double d); constexprconsteval complex<double> operator""i(unsigned long long d);
3
#
Returns: complex<double>{0.0, static_cast<double>(d)}.
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constexprconsteval complex<float> operator""if(long double d); constexprconsteval complex<float> operator""if(unsigned long long d);
4
#
Returns: complex<float>{0.0f, static_cast<float>(d)}.

30.5.9 Suffixes for duration literals [time.duration.literals]

1
#
This subclause describes literal suffixes for constructing duration literals.
The suffixes h, min, s, ms, us, ns denote duration values of the corresponding types hours, minutes, seconds, milliseconds, microseconds, and nanoseconds respectively if they are applied to integer-literals.
2
#
If any of these suffixes are applied to a floating-point-literal the result is a chrono​::​duration literal with an unspecified floating-point representation.
3
#
If any of these suffixes are applied to an integer-literal and the resulting chrono​::​duration value cannot be represented in the result type because of overflow, the program is ill-formed.
4
#
[Example 1: 
The following code shows some duration literals.
using namespace std::chrono_literals; auto constexpr aday=24h; auto constexpr lesson=45min; auto constexpr halfanhour=0.5h; — end example]
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constexprconsteval chrono::hours operator""h(unsigned long long hours); constexprconsteval chrono::duration<unspecified, ratio<3600, 1>> operator""h(long double hours);
5
#
Returns: A duration literal representing hours hours.
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constexprconsteval chrono::minutes operator""min(unsigned long long minutes); constexprconsteval chrono::duration<unspecified, ratio<60, 1>> operator""min(long double minutes);
6
#
Returns: A duration literal representing minutes minutes.
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constexprconsteval chrono::seconds operator""s(unsigned long long sec); constexprconsteval chrono::duration<unspecified> operator""s(long double sec);
7
#
Returns: A duration literal representing sec seconds.
8
#
[Note 1: 
The same suffix s is used for basic_string but there is no conflict, since duration suffixes apply to numbers and string literal suffixes apply to character array literals.
— end note]
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constexprconsteval chrono::milliseconds operator""ms(unsigned long long msec); constexprconsteval chrono::duration<unspecified, milli> operator""ms(long double msec);
9
#
Returns: A duration literal representing msec milliseconds.
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constexprconsteval chrono::microseconds operator""us(unsigned long long usec); constexprconsteval chrono::duration<unspecified, micro> operator""us(long double usec);
10
#
Returns: A duration literal representing usec microseconds.
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constexprconsteval chrono::nanoseconds operator""ns(unsigned long long nsec); constexprconsteval chrono::duration<unspecified, nano> operator""ns(long double nsec);
11
#
Returns: A duration literal representing nsec nanoseconds.

30.8.3.3 Non-member functions [time.cal.day.nonmembers]

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constexprconsteval chrono::day operator""d(unsigned long long d) noexcept;
10
#
Returns: day{static_cast<unsigned>(d)}.

30.8.5.3 Non-member functions [time.cal.year.nonmembers]

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constexprconsteval chrono::year operator""y(unsigned long long y) noexcept;
10
#
Returns: year{static_cast<int>(y)}.

Feature test macro

17.3.2 Header <version> synopsis [version.syn]

#define __cpp_lib_consteval_literals 20????L