[libc++] Implement P0980R1 (constexpr std::string)

[philnik]

• Changed split non-/constexpr tests into one and used TEST_IS_CONSTANT_EVALUATED
• replaced main() with main(int, char**)
• Updated feature-test macro

[philnik]

ping

[philnik]

Maybe @ldionne should take a look at this PR too, since it's his paper.

[philnik]

Rebased and added GCC workaround

[philnik]

Rebased

[philnik]

• Rebased
• check for nullptr

[philnik]

removed XFAIL for GCC

[philnik]

• Removed debugging stuff for constant evaluation
• fixed some tests

[philnik]

• Added check
• Using __wrap_iter now only during runtime

[ldionne]

In D110598#3135267, @philnik wrote:

Maybe @ldionne should take a look at this PR too, since it's his paper.

Will do, but I was waiting for the build to be green. Can you ping me (on Discord) when you want me to take a look?

[philnik]

• Rebased onto D114733
• Make test pass

[philnik]

• rebased
• debug iterators should work now

[philnik]

• Some formatting changes

[philnik]

ping @ldionne , @Quuxplusone

[• Quuxplusone]

Please update the PR summary (i.e. the proposed commit message and general info about the patch). Particularly, I'd like to see some paragraph-form text explaining the general approach. I'm sure it's been explained and discussed several times, e.g. on Discord, probably a couple times in this PR's comments... but IIUC the details have also changed over time. An up-to-date description belongs in the PR summary, and ultimately in the commit message. It would even be appropriate to add something to libcxx/docs/DesignDocs/. All of this will benefit the next person to maintain this and/or to implement constexpr vector or constexpr whatever-they-do-in-C++23.

It would help if this were split into relatively orthogonal chunks, most of which could be "NFC." For example:

• The test_iterators.h diff, which is just adding constexpr to one type, can be landed today AFAIC.
• The test_macros.h diff is probably unnecessary?
• A huge number of diffs in <string> are just adding the word _LIBCPP_CONSTEXPR_AFTER_CXX17; since this is all template code where the compiler shouldn't complain about incorrect constexpr keywords, can we land those as a separate patch, before the behavioral changes?
• Vice versa, can we land the behavioral changes first and then the _LIBCPP_CONSTEXPR_AFTER_CXX17 markings second?
• Several new member functions: __begin_lifetime, __finish_replace, __free_memory_constexpr, __insert_not_in_range. Can we introduce those first (changing the "shape" of the control flow), and then in a second step change their behavior to include the constexpr-friendly codepaths?
• The constexpr-ified tests do need to be landed in the same commit as the behavior they test... but for review purposes they are orthogonal/distracting.
• Even if we land this PR as a monolith, it would still be vastly easier to review in small pieces like that. If you're trying to review the control flow, all the _LIBCPP_CONSTEXPR_AFTER_CXX17 markings are distracting; and vice versa.

I don't find Phab's interface very convenient for splitting up patches in these ways. You can make a bunch of Phab PRs and link them together with dependencies, but really it might be easier to just edit the issue summary to include [the design info, plus] a link to a branch on your own GitHub (I mean a URL like https://github.com/Quuxplusone/llvm-project/compare/main...trivially-relocatable ) and say "Hey reviewers, if you prefer to see this PR broken down into its orthogonal component parts, take a look at this series of commits in GitHub."

libcxx/include/string

1819–1821

_VSTD::construct_at(_VSTD::addressof(__p[__i]), value_type())); for ADL.
On line 1785, please linebreak before void.

4223–4231

The PR summary (and possibly a design doc) should explain why the class invariants need to change in constexpr-land.
But aside from that, I find this diff hard to reason about. After some fiddling with it, I think the cleanest version introduces the concept of "SSO capacity," like this:

size_type __sso_capacity = __libcpp_is_constant_evaluated() ? 0 : (__min_cap - 1);
if (size() > capacity())
    return false;
if (capacity() < __sso_capacity)
    return false;
if (data() == nullptr)
    return false;
if (data()[size()] != value_type())
    return false;
return true;

IIUC, you use this calculation several places in this PR. Consider introducing a function __sso_capacity() for this purpose... except that at first glance I'm worried about any function whose return value changes depending on constexprness. I'm not sure that such a function is safe to use. So maybe pulling it out into a function would be very bad, I'm not sure.

[philnik]

• Rebased onto D115599

[philnik]

Rebased

[philnik]

Updates; There are still a few test changes but most are already done.

[philnik]

Poke CI

[philnik]

• generate files

[philnik]

• Fix CI

[philnik]

• Initialize union before allocating

[philnik]

• Fix alternate string layout

[philnik]

• Try to fix 32bit builds

[Mordante]

Thanks for working on this huge task!
In general I'm happy, but I'm not familiar enough with the internals of our string implementation to validate you didn't miss any important parts. Therefore I leave approval to somebody more familiar with string.

(This patch basically touches every function in string and thus "breaks" other patches for string. Would it be an idea to make some cleanup patches after this patch lands? Then we can use _LIBCPP_HIDE_FROM_ABI and std::.)

libcxx/include/string

77

Please update the synopsis.

1003–1004

The test test/std/strings/basic.string/string.capacity/resize_and_overwrite.pass.cpp can be updated by removing
#if defined(__cpp_lib_constexpr_string) && __cpp_lib_constexpr_string >= 201907L

1466–1467

Nit: I would keep the order LIBCPP_INLINE_VISIBILITY constexpr for consistency with the other changes.

1521

Should these new functions be private members?

libcxx/test/std/strings/basic.string/string.access/at.pass.cpp

12

Please update the synopsis in all tests.

libcxx/test/std/strings/basic.string/string.capacity/max_size.pass.cpp

82 ↗

(On Diff #416937)

Why don't you test this in constexpr? The signature has changed in the paper.

libcxx/test/std/strings/basic.string/string.modifiers/string_insert/size_pointer_size.pass.cpp

53 ↗

(On Diff #416937)

Why is this part removed?

libcxx/test/std/strings/basic.string/string.starts_with/starts_with.char.pass.cpp

19 ↗

(On Diff #416937)

These functions are only available in C++20. (The same for ends_with and contains.)

[philnik]

• Update synopsis
• Address comments

[philnik]

libcxx/include/string

1466–1467

I'd keep it as-is for now. I don't want to make this patch unnecessarily larger.

libcxx/test/std/strings/basic.string/string.capacity/max_size.pass.cpp

82 ↗

(On Diff #416937)

This test makes extremely large allocations, like numeric_limits<size_t>::max() large. Because of this the constant evaluations fails.

[philnik]

libcxx/test/std/strings/basic.string/string.modifiers/string_insert/size_pointer_size.pass.cpp

53 ↗

(On Diff #416937)

This is duplicated for std::string and std::basic_string<char, std::char_traits<char>, min_allocator<char>>. I just made this a template and removed the duplicated code.

[Mordante]

SGTM modulo some small issues.

libcxx/test/std/strings/basic.string/string.capacity/max_size.pass.cpp

82 ↗

(On Diff #416937)

Then please a test to validate the function can be called compile-time and add comment why we done the minimal testing for compile-time testing.

libcxx/test/std/strings/basic.string/string.contains/contains.char.pass.cpp

19 ↗

(On Diff #418040)

[philnik]

• Address comments

[ldionne]

Thanks for the patch! Mostly small comments about forgetting stuff in the tests, however I think we'll need to figure out a better story for the string's invariant inside constexpr. I think we agreed on a direction to investigate during our live review, so let's try that and re-evaluate after. Thanks!

cross-project-tests/debuginfo-tests/llvm-prettyprinters/gdb/mlir-support.cpp

12 ↗

(On Diff #391005)

I don't think you intended to upload that.

libcxx/include/string

918–919

Please consider not reformatting as part of this patch since it's good to keep it as mechanical as possible.

1952–1953

What if instead of doing this, we made sure to have the invariants of the string hold before calling __init, and then we'd "simply" have to check whether we already have a long string allocated at the beginning of __init?

2106

This line does not look correct to me. Indeed, if __str is a short string, after the change we are accessing __str's long representation instead of its raw representation, which means that we might not be copying some part of the small buffer, if the small buffer is larger than the long representation.

This could happen if we had a very large character type where 2 * sizeof(CharType) is larger than sizeof(LongRepresentation). In that case, our short string buffer will contain two CharTypes at least, yet our long representation will be smaller than that. Please add a test to catch this, or explain why that doesn't work (you mentioned a bug related to large character types during live review).

2377–2383

Why don't we call __default_init() on a moved-from string instead?

Right now, after calling __zero() during constexpr evaluation, the string is invalid to access, thus we don't have the invariant __is_long(). Instead, I think we should make sure that the string satisfies __is_long() at all times during constexpr evaluation, which probably means replacing a few calls to __zero() by calls to __default_init().

This would allow removing the diff above.

2588

Same here -- if the invariant __is_long() held during constexpr evaluation, this nullptr check could go away.

2606

For instance, I think this is one place where we should call __default_init() instead.

2916–2922

I'd like to understand why this diff is needed. What happens if you remove it?

3158–3161

Same question here -- why is this required?

4225–4226

If we never put the string in an invalid state inside constexpr, is it possible to remove this diff entirely?

4462–4463

Could we investigate adding a private constructor that allows passing a capacity to pre-allocate, so this could be implemented like this:

using _String = basic_string<_CharT, _Traits, _Allocator>;
typename _String::size_type __lhs_sz = __lhs.size();
typename _String::size_type __rhs_sz = __rhs.size();
_String __r(_String::__alloc_traits::select_on_container_copy_construction(__lhs.get_allocator()), __capacity_tag{}, __lhs_sz + __rhs_sz);
__r.append(__lhs.data(), __lhs_sz);
__r.append(__rhs.data(), __rhs_sz);
return __r;

Can we please do this as a separate review? That should be easy to split out.

libcxx/test/std/strings/basic.string/string.cons/default_noexcept.pass.cpp

26–37 ↗

(On Diff #419393)

Unless I am mistaken, we don't seem to have any test for basic_string::basic_string() that runs at runtime. This is kind of crazy.

As a different review, can you please move this test to default.pass.cpp, make it run the default constructor at runtime, and prepare it for constexpr-friendliness as well? That way, this diff can look like the other ones where you basically uncomment static_assert(test());.

libcxx/test/std/strings/basic.string/string.cons/dtor_noexcept.pass.cpp

13 ↗

(On Diff #419393)

Same thing here -- we don't have a runtime test for the string destructor outside of the static variable below. Please add one -- this can be done in the same review as the default ctor.

You could use a special allocator and check that deallocate() has been called after the string is destroyed.

libcxx/test/std/strings/basic.string/string.iterators/iterators.pass.cpp

81 ↗

(On Diff #419393)

I think this needs to change to TEST_STD_VER > 17?

Please also grep for __cpp_lib_constexpr_string after applying your patch just to make sure there aren't any stray ones left where they shouldn't be.

libcxx/test/std/strings/basic.string/string.modifiers/string_insert/size_T_size_size.pass.cpp

1867–1898 ↗

(On Diff #419393)

I think you missed those.

I'm also not a big fan of the 2k lines of test here, but this has nothing to do with this review.

libcxx/test/std/strings/basic.string/string.modifiers/string_insert/size_string_size_size.pass.cpp

1827–1857 ↗

(On Diff #419393)

You forgot those too! Please grep for // static_assert in the test suite just to make sure you haven't missed other ones.

libcxx/test/std/strings/basic.string/string.modifiers/string_replace/iter_iter_iter_iter.pass.cpp

1060–1069 ↗

(On Diff #419393)

Here!

libcxx/test/std/strings/basic.string/string.modifiers/string_replace/iter_iter_pointer_size.pass.cpp

995–1006 ↗

(On Diff #419393)

Here!

libcxx/test/std/strings/basic.string/string.modifiers/string_replace/size_size_T_size_size.pass.cpp

6079 ↗

(On Diff #419393)

Here!

libcxx/test/std/strings/basic.string/string.modifiers/string_replace/size_size_pointer_size.pass.cpp

1340 ↗

(On Diff #419393)

Here!

libcxx/test/std/strings/basic.string/string.modifiers/string_replace/size_size_string_size_size.pass.cpp

6069 ↗

(On Diff #419393)

Here!

Wow, 6k lines of test. I'm sure they are all 100% relevant -_-.

libcxx/test/std/strings/basic.string/string.nonmembers/string.io/get_line.pass.cpp

14 ↗

(On Diff #419393)

I don't think you meant to make this one constexpr.

libcxx/test/std/strings/basic.string/string.nonmembers/string.io/get_line_delim.pass.cpp

14 ↗

(On Diff #419393)

Same, not constexpr.

libcxx/test/std/strings/basic.string/string.nonmembers/string.io/get_line_delim_rv.pass.cpp

14 ↗

(On Diff #419393)

Same!

libcxx/test/std/strings/basic.string/string.nonmembers/string.io/get_line_rv.pass.cpp

14 ↗

(On Diff #419393)

Same, and for all string.io probably.

libcxx/test/std/strings/basic.string/string.ops/string_compare/size_size_T_size_size.pass.cpp

5955 ↗

(On Diff #419393)

We are missing some constexpr-ification here for sure.

libcxx/test/std/strings/basic.string/string.ops/string_compare/size_size_pointer_size.pass.cpp

1335 ↗

(On Diff #419393)

Missed these ones!

libcxx/test/std/strings/basic.string/string.ops/string_compare/size_size_string_size_size.pass.cpp

6041 ↗

(On Diff #419393)

Missed!

libcxx/test/std/strings/basic.string/string.ops/string_find.last.not.of/string_view_size.pass.cpp

154–157 ↗

(On Diff #419393)

Can you try uncommenting this and seeing what happens?

libcxx/test/support/constexpr_char_traits.h

121 ↗

(On Diff #419393)

// fails in constexpr because we might be comparing unrelated pointers

[philnik]

• Rebased
• Address comments

libcxx/include/string

1952–1953

I think we agreed that this is OK and we make sure that __init is only ever called inside constructors.

2916–2922

We may compare unrelated pointers with __p + __pos <= __s && __s < __p + __sz.

3158–3161

Same here - we may compare unrelated pointers.

4462–4463

This is D123058 now.

libcxx/test/std/strings/basic.string/string.cons/dtor_noexcept.pass.cpp

13 ↗

(On Diff #419393)

This is now D123129.

libcxx/test/std/strings/basic.string/string.ops/string_find.last.not.of/string_view_size.pass.cpp

154–157 ↗

(On Diff #419393)

Seems to just work.

[philnik]

• Fix CI

[ldionne]

Thanks! Still a few comments and questions, but this is looking really good now.

libcxx/include/string

1591–1592

I don't understand why this diff is required when running in constexpr. What happens if you remove it?

1624

Is there any reason why we don't simply always do __r_.first() = __rep()? That would work both in constexpr and non-constexpr, and it seems to be pretty much equivalent: https://godbolt.org/z/T85f13ozT

And when that's done, perhaps we can even remove __zero() altogether?

1794–1795

This needs a comment!

3158–3160

Note to future me: I looked at this and it seems OK, no need to spend another 15 minutes validating.

3535

Can we repace this by a call to __fits_in_sso(__target_capacity)? (or maybe __target_capacity +/- 1, please double-check)

libcxx/test/support/constexpr_char_traits.h

130 ↗

(On Diff #421756)

@daltenty The CI is currently failing in pretty bad ways on AIX. The compiler is being reported as IBM Open XL C/C++ for AIX 17.1.0 (5725-C72, 5765-J18), clang version 13.0.0, which we claim to support on our page.

I believe there might be some patches to the constant evaluator on top of Clang 13 that have not been cherry-picked to your internal branch. I suspect there are probably no reasonable workarounds to make std::string work inside constexpr on that compiler without fixing the compiler itself. Hence, I am unsure what's the best course of action here. I think what we should do is either:

1. Mark those tests as XFAIL temporarily on AIX (LIBCXX-AIX-FIXME). This has the downside that you'll lose coverage for std::string at runtime too. Otherwise,
2. Introduce a macro and #ifdef out only the constexpr part of the tests on AIX until the compiler is fixed or updated. I'm fine with doing that, but only if we know that it's a temporary state (< 6 months roughly).

Do you have thoughts on that?

Note to @philnik: Since I'm about to go OOO, if there is no resolution for this by EOW, feel free to land this with option (1) above (which is simplest) and we can always do (2) afterwards if that's the preferred solution.

[philnik]

• Rebased onto D123580
• Address comments

libcxx/include/string

1591–1592

Fixed the bug!

[philnik]

Rebased

[philnik]

Rebased

[ldionne]

libcxx/include/string

1641–1643

Can you explain this?

[philnik]

libcxx/include/string

1641–1643

During runtime we never actually allocate __min_cap bytes, so it's OK that we return an even number in this case. It only exists for checking if the string fits in SSO or is in SSO mode AFAICT. (This should probably be changed at some point) During constant evaluation we have to return an odd number to make the allocation size even, otherwise we save garbage in the capacity member. That's why the other way to fix the bug was saving the actual capacity.

[ldionne]

Please rebase onto main to get a clean CI run, and ship it. Thanks!

libcxx/include/string

1641–1643

Ok. As discussed offline, let's land this as-is, but let's also make a follow-up patch that tries to fix places that assume __min_cap - 1 is returned when the string fits in the SSO, so we can remove this if and always return static_cast<size_type>(__min_cap);.

[philnik]

• Rebased
• XFAIL AIX

[philnik]

• Add a few more XFAILs

libcxx/docs/Status/Cxx20Papers.csv

 "`P0325 <https://wg21.link/P0325>`__","LWG","to_array from LFTS with updates","Cologne","|Complete|","10.0"
 "`P0408 <https://wg21.link/P0408>`__","LWG","Efficient Access to basic_stringbuf's Buffer","Cologne","",""
 "`P0466 <https://wg21.link/P0466>`__","LWG","Layout-compatibility and Pointer-interconvertibility Traits","Cologne","",""
 "`P0553 <https://wg21.link/P0553>`__","LWG","Bit operations","Cologne","|Complete|","9.0"
 "`P0631 <https://wg21.link/P0631>`__","LWG","Math Constants","Cologne","|Complete|","11.0"
 "`P0645 <https://wg21.link/P0645>`__","LWG","Text Formatting","Cologne","|In Progress|",""
 "`P0660 <https://wg21.link/P0660>`__","LWG","Stop Token and Joining Thread, Rev 10","Cologne","",""
 "`P0784 <https://wg21.link/P0784>`__","CWG","More constexpr containers","Cologne","|Complete|","12.0"
-"`P0980 <https://wg21.link/P0980>`__","LWG","Making std::string constexpr","Cologne","",""
+"`P0980 <https://wg21.link/P0980>`__","LWG","Making std::string constexpr","Cologne","|In Progress|",""
 "`P1004 <https://wg21.link/P1004>`__","LWG","Making std::vector constexpr","Cologne","",""
 "`P1035 <https://wg21.link/P1035>`__","LWG","Input Range Adaptors","Cologne","",""
 "`P1065 <https://wg21.link/P1065>`__","LWG","Constexpr INVOKE","Cologne","|Complete|","12.0"
 "`P1135 <https://wg21.link/P1135>`__","LWG","The C++20 Synchronization Library","Cologne","|Complete|","11.0"
 "`P1207 <https://wg21.link/P1207>`__","LWG","Movability of Single-pass Iterators","Cologne","",""
 "`P1208 <https://wg21.link/P1208>`__","LWG","Adopt source_location for C++20","Cologne","",""
 "`P1355 <https://wg21.link/P1355>`__","LWG","Exposing a narrow contract for ceil2","Cologne","|Complete|","9.0"
 "`P1361 <https://wg21.link/P1361>`__","LWG","Integration of chrono with text formatting","Cologne","",""

libcxx/include/__memory/compressed_pair.h

 #ifndef _LIBCPP_CXX03_LANG
   template <class... _Args, size_t... _Indexes>
   _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
   __compressed_pair_elem(piecewise_construct_t, tuple<_Args...> __args,
                          __tuple_indices<_Indexes...>)
       : __value_(_VSTD::forward<_Args>(_VSTD::get<_Indexes>(__args))...) {}
 #endif
 
-
-  _LIBCPP_INLINE_VISIBILITY reference __get() _NOEXCEPT { return __value_; }
-  _LIBCPP_INLINE_VISIBILITY
-  const_reference __get() const _NOEXCEPT { return __value_; }
+  _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR reference __get() _NOEXCEPT { return __value_; }
+  _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR const_reference __get() const _NOEXCEPT { return __value_; }
 
 private:
   _Tp __value_;
 };
 
 template <class _Tp, int _Idx>
 struct __compressed_pair_elem<_Tp, _Idx, true> : private _Tp {
   typedef _Tp _ParamT;
   typedef _Tp& reference;

 #ifndef _LIBCPP_CXX03_LANG
   template <class... _Args, size_t... _Indexes>
   _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
   __compressed_pair_elem(piecewise_construct_t, tuple<_Args...> __args,
                          __tuple_indices<_Indexes...>)
       : __value_type(_VSTD::forward<_Args>(_VSTD::get<_Indexes>(__args))...) {}
 #endif
 
-  _LIBCPP_INLINE_VISIBILITY reference __get() _NOEXCEPT { return *this; }
-  _LIBCPP_INLINE_VISIBILITY
-  const_reference __get() const _NOEXCEPT { return *this; }
+  _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR reference __get() _NOEXCEPT { return *this; }
+  _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR const_reference __get() const _NOEXCEPT { return *this; }
 };
 
 template <class _T1, class _T2>
 class __compressed_pair : private __compressed_pair_elem<_T1, 0>,
                           private __compressed_pair_elem<_T2, 1> {
 public:
   // NOTE: This static assert should never fire because __compressed_pair
   // is *almost never* used in a scenario where it's possible for T1 == T2.
#ifndef _LIBCPP_CXX03_LANG
   __compressed_pair(piecewise_construct_t __pc, tuple<_Args1...> __first_args,
                     tuple<_Args2...> __second_args)
       : _Base1(__pc, _VSTD::move(__first_args),
                typename __make_tuple_indices<sizeof...(_Args1)>::type()),
         _Base2(__pc, _VSTD::move(__second_args),
                typename __make_tuple_indices<sizeof...(_Args2)>::type()) {}
 #endif
 
-  _LIBCPP_INLINE_VISIBILITY
-  typename _Base1::reference first() _NOEXCEPT {
+  _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR typename _Base1::reference first() _NOEXCEPT {
     return static_cast<_Base1&>(*this).__get();
   }
 
-  _LIBCPP_INLINE_VISIBILITY
-  typename _Base1::const_reference first() const _NOEXCEPT {
+  _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR typename _Base1::const_reference first() const _NOEXCEPT {
     return static_cast<_Base1 const&>(*this).__get();
   }
 
-  _LIBCPP_INLINE_VISIBILITY
-  typename _Base2::reference second() _NOEXCEPT {
+  _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR typename _Base2::reference second() _NOEXCEPT {
     return static_cast<_Base2&>(*this).__get();
   }
 
-  _LIBCPP_INLINE_VISIBILITY
-  typename _Base2::const_reference second() const _NOEXCEPT {
+  _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR typename _Base2::const_reference second() const _NOEXCEPT {
     return static_cast<_Base2 const&>(*this).__get();
   }
 
   _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
   static _Base1* __get_first_base(__compressed_pair* __pair) _NOEXCEPT {
     return static_cast<_Base1*>(__pair);
   }
   _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR

libcxx/include/string

 
 template <> struct char_traits<char>;
 template <> struct char_traits<wchar_t>;
 template <> struct char_traits<char8_t>;  // C++20
 template <> struct char_traits<char16_t>;
 template <> struct char_traits<char32_t>;
 
 template<class charT, class traits = char_traits<charT>, class Allocator = allocator<charT> >
 class basic_string
 {
 public:
 // types:
     typedef traits traits_type;
     typedef typename traits_type::char_type value_type;
     typedef Allocator allocator_type;
     typedef typename allocator_type::size_type size_type;
     typedef typename allocator_type::difference_type difference_type;

 }  // std
 
 */
 
 #include <__config>
 #include <__debug>
 #include <__functional_base>
 #include <__iterator/wrap_iter.h>
+#include <__memory/construct_at.h>
 #include <algorithm>
 #include <compare>
 #include <cstdio>  // EOF
 #include <cstdlib>
 #include <cstring>
 #include <cwchar>
 #include <initializer_list>
 #include <iosfwd>

 template <class _StateT>
 inline _LIBCPP_INLINE_VISIBILITY
 bool operator!=(const fpos<_StateT>& __x, const fpos<_StateT>& __y)
     {return streamoff(__x) != streamoff(__y);}
 
 // basic_string
 
 template<class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>
 operator+(const basic_string<_CharT, _Traits, _Allocator>& __x,
           const basic_string<_CharT, _Traits, _Allocator>& __y);
 
 template<class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>
 operator+(const _CharT* __x, const basic_string<_CharT,_Traits,_Allocator>& __y);
 
 template<class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>
 operator+(_CharT __x, const basic_string<_CharT,_Traits,_Allocator>& __y);
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>
 operator+(const basic_string<_CharT, _Traits, _Allocator>& __x, const _CharT* __y);
 
 template<class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>
 operator+(const basic_string<_CharT, _Traits, _Allocator>& __x, _CharT __y);
 
 _LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS string operator+<char, char_traits<char>, allocator<char> >(char const*, string const&))
 
 template <bool>
 class _LIBCPP_TEMPLATE_VIS __basic_string_common
 {
 protected:
     _LIBCPP_NORETURN void __throw_length_error() const;
-    _LIBCPP_NORETURN void __throw_out_of_range() const;
+    _LIBCPP_NORETURN  void __throw_out_of_range() const;
 };
 
 template <bool __b>
 void
 __basic_string_common<__b>::__throw_length_error() const
 {
     _VSTD::__throw_length_error("basic_string");
 }
#endif // _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
     };
 
     __compressed_pair<__rep, allocator_type> __r_;
 
 public:
     _LIBCPP_TEMPLATE_DATA_VIS
     static const size_type npos = -1;
 
-    _LIBCPP_INLINE_VISIBILITY basic_string()
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    basic_string()
         _NOEXCEPT_(is_nothrow_default_constructible<allocator_type>::value);
 
-    _LIBCPP_INLINE_VISIBILITY explicit basic_string(const allocator_type& __a)
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    explicit basic_string(const allocator_type& __a)
 #if _LIBCPP_STD_VER <= 14
         _NOEXCEPT_(is_nothrow_copy_constructible<allocator_type>::value);
 #else
         _NOEXCEPT;
 #endif
 
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string(const basic_string& __str);
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string(const basic_string& __str, const allocator_type& __a);
 
 #ifndef _LIBCPP_CXX03_LANG
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string(basic_string&& __str)
 #if _LIBCPP_STD_VER <= 14
         _NOEXCEPT_(is_nothrow_move_constructible<allocator_type>::value);
 #else
         _NOEXCEPT;
 #endif
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string(basic_string&& __str, const allocator_type& __a);
 #endif // _LIBCPP_CXX03_LANG
 
     template <class = __enable_if_t<__is_allocator<_Allocator>::value, nullptr_t> >
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string(const _CharT* __s) : __r_(__default_init_tag(), __default_init_tag()) {
       _LIBCPP_ASSERT(__s != nullptr, "basic_string(const char*) detected nullptr");
       __init(__s, traits_type::length(__s));
 #   if _LIBCPP_DEBUG_LEVEL == 2
       __get_db()->__insert_c(this);
 #   endif
     }
 
     template <class = __enable_if_t<__is_allocator<_Allocator>::value, nullptr_t> >
-        _LIBCPP_INLINE_VISIBILITY
+        _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
         basic_string(const _CharT* __s, const _Allocator& __a);
 
 #if _LIBCPP_STD_VER > 20
     basic_string(nullptr_t) = delete;
 #endif
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string(const _CharT* __s, size_type __n);
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string(const _CharT* __s, size_type __n, const _Allocator& __a);
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string(size_type __n, _CharT __c);
 
     template <class = __enable_if_t<__is_allocator<_Allocator>::value, nullptr_t> >
-        _LIBCPP_INLINE_VISIBILITY
+        _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
         basic_string(size_type __n, _CharT __c, const _Allocator& __a);
 
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string(const basic_string& __str, size_type __pos, size_type __n,
                  const _Allocator& __a = _Allocator());
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string(const basic_string& __str, size_type __pos,
                  const _Allocator& __a = _Allocator());
 
     template<class _Tp, class = __enable_if_t<__can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value && !__is_same_uncvref<_Tp, basic_string>::value> >
-        _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
+        _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS _LIBCPP_CONSTEXPR_AFTER_CXX17
         basic_string(const _Tp& __t, size_type __pos, size_type __n,
                      const allocator_type& __a = allocator_type());
 
     template<class _Tp, class = __enable_if_t<__can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value &&
                                           !__is_same_uncvref<_Tp, basic_string>::value> >
-        _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
+        _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS _LIBCPP_CONSTEXPR_AFTER_CXX17
         explicit basic_string(const _Tp& __t);
 
     template<class _Tp, class = __enable_if_t<__can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value && !__is_same_uncvref<_Tp, basic_string>::value> >
-        _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
+        _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS _LIBCPP_CONSTEXPR_AFTER_CXX17
         explicit basic_string(const _Tp& __t, const allocator_type& __a);
 
     template<class _InputIterator, class = __enable_if_t<__is_cpp17_input_iterator<_InputIterator>::value> >
-        _LIBCPP_INLINE_VISIBILITY
+        _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
         basic_string(_InputIterator __first, _InputIterator __last);
     template<class _InputIterator, class = __enable_if_t<__is_cpp17_input_iterator<_InputIterator>::value> >
-        _LIBCPP_INLINE_VISIBILITY
+        _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
         basic_string(_InputIterator __first, _InputIterator __last, const allocator_type& __a);
 #ifndef _LIBCPP_CXX03_LANG
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string(initializer_list<_CharT> __il);
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string(initializer_list<_CharT> __il, const _Allocator& __a);
 #endif // _LIBCPP_CXX03_LANG
 
-    inline ~basic_string();
+    _LIBCPP_CONSTEXPR_AFTER_CXX17 inline ~basic_string();
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     operator __self_view() const _NOEXCEPT { return __self_view(data(), size()); }
 
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string& operator=(const basic_string& __str);
 
     template <class _Tp, class = __enable_if_t<__can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value && !__is_same_uncvref<_Tp, basic_string>::value> >
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string& operator=(const _Tp& __t)
         {__self_view __sv = __t; return assign(__sv);}
 
 #ifndef _LIBCPP_CXX03_LANG
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string& operator=(basic_string&& __str)
         _NOEXCEPT_((__noexcept_move_assign_container<_Allocator, __alloc_traits>::value));
-     _LIBCPP_INLINE_VISIBILITY
+     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string& operator=(initializer_list<value_type> __il) {return assign(__il.begin(), __il.size());}
 #endif
-    _LIBCPP_INLINE_VISIBILITY basic_string& operator=(const value_type* __s) {return assign(__s);}
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    basic_string& operator=(const value_type* __s) {return assign(__s);}
 #if _LIBCPP_STD_VER > 20
     basic_string& operator=(nullptr_t) = delete;
 #endif
-    basic_string& operator=(value_type __c);
+    _LIBCPP_CONSTEXPR_AFTER_CXX17 basic_string& operator=(value_type __c);
 
 #if _LIBCPP_DEBUG_LEVEL == 2
     _LIBCPP_INLINE_VISIBILITY
     iterator begin() _NOEXCEPT
         {return iterator(this, __get_pointer());}
     _LIBCPP_INLINE_VISIBILITY
     const_iterator begin() const _NOEXCEPT
         {return const_iterator(this, __get_pointer());}
     _LIBCPP_INLINE_VISIBILITY
     iterator end() _NOEXCEPT
         {return iterator(this, __get_pointer() + size());}
     _LIBCPP_INLINE_VISIBILITY
     const_iterator end() const _NOEXCEPT
         {return const_iterator(this, __get_pointer() + size());}
 #else
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     iterator begin() _NOEXCEPT
         {return iterator(__get_pointer());}
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     const_iterator begin() const _NOEXCEPT
         {return const_iterator(__get_pointer());}
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     iterator end() _NOEXCEPT
         {return iterator(__get_pointer() + size());}
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     const_iterator end() const _NOEXCEPT
         {return const_iterator(__get_pointer() + size());}
 #endif // _LIBCPP_DEBUG_LEVEL == 2
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     reverse_iterator rbegin() _NOEXCEPT
         {return reverse_iterator(end());}
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     const_reverse_iterator rbegin() const _NOEXCEPT
         {return const_reverse_iterator(end());}
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     reverse_iterator rend() _NOEXCEPT
         {return reverse_iterator(begin());}
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     const_reverse_iterator rend() const _NOEXCEPT
         {return const_reverse_iterator(begin());}
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     const_iterator cbegin() const _NOEXCEPT
         {return begin();}
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     const_iterator cend() const _NOEXCEPT
         {return end();}
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     const_reverse_iterator crbegin() const _NOEXCEPT
         {return rbegin();}
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     const_reverse_iterator crend() const _NOEXCEPT
         {return rend();}
 
-    _LIBCPP_INLINE_VISIBILITY size_type size() const _NOEXCEPT
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    size_type size() const _NOEXCEPT
         {return __is_long() ? __get_long_size() : __get_short_size();}
-    _LIBCPP_INLINE_VISIBILITY size_type length() const _NOEXCEPT {return size();}
-    _LIBCPP_INLINE_VISIBILITY size_type max_size() const _NOEXCEPT;
-    _LIBCPP_INLINE_VISIBILITY size_type capacity() const _NOEXCEPT
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+     size_type length() const _NOEXCEPT {return size();}
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    size_type max_size() const _NOEXCEPT;
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    size_type capacity() const _NOEXCEPT
         {return (__is_long() ? __get_long_cap()
                              : static_cast<size_type>(__min_cap)) - 1;}
 
-    void resize(size_type __n, value_type __c);
-    _LIBCPP_INLINE_VISIBILITY void resize(size_type __n) {resize(__n, value_type());}
+    _LIBCPP_CONSTEXPR_AFTER_CXX17 void resize(size_type __n, value_type __c);
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    void resize(size_type __n) {resize(__n, value_type());}
 
-    void reserve(size_type __requested_capacity);
+    _LIBCPP_CONSTEXPR_AFTER_CXX17 void reserve(size_type __requested_capacity);
     _LIBCPP_INLINE_VISIBILITY void __resize_default_init(size_type __n);
 
     _LIBCPP_DEPRECATED_IN_CXX20 _LIBCPP_INLINE_VISIBILITY
     void reserve() _NOEXCEPT {shrink_to_fit();}
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     void shrink_to_fit() _NOEXCEPT;
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     void clear() _NOEXCEPT;
-    _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     bool empty() const _NOEXCEPT {return size() == 0;}
 
-    _LIBCPP_INLINE_VISIBILITY const_reference operator[](size_type __pos) const _NOEXCEPT;
-    _LIBCPP_INLINE_VISIBILITY reference       operator[](size_type __pos)       _NOEXCEPT;
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    const_reference operator[](size_type __pos) const _NOEXCEPT;
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    reference       operator[](size_type __pos)       _NOEXCEPT;
 
-    const_reference at(size_type __n) const;
-    reference       at(size_type __n);
+    _LIBCPP_CONSTEXPR_AFTER_CXX17 const_reference at(size_type __n) const;
+    _LIBCPP_CONSTEXPR_AFTER_CXX17 reference       at(size_type __n);
 
-    _LIBCPP_INLINE_VISIBILITY basic_string& operator+=(const basic_string& __str) {return append(__str);}
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    basic_string& operator+=(const basic_string& __str) {return append(__str);}
 
     template <class _Tp>
-    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
+    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS _LIBCPP_CONSTEXPR_AFTER_CXX17
     __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value
             && !__is_same_uncvref<_Tp, basic_string >::value,
             basic_string&
         >
-                                            operator+=(const _Tp& __t)            {__self_view __sv = __t; return append(__sv);}
-    _LIBCPP_INLINE_VISIBILITY basic_string& operator+=(const value_type* __s)     {return append(__s);}
-    _LIBCPP_INLINE_VISIBILITY basic_string& operator+=(value_type __c)            {push_back(__c); return *this;}
+        operator+=(const _Tp& __t) {__self_view __sv = __t; return append(__sv);}
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    basic_string& operator+=(const value_type* __s) {return append(__s);}
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    basic_string& operator+=(value_type __c) {push_back(__c); return *this;}
 #ifndef _LIBCPP_CXX03_LANG
-    _LIBCPP_INLINE_VISIBILITY basic_string& operator+=(initializer_list<value_type> __il) {return append(__il);}
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    basic_string& operator+=(initializer_list<value_type> __il) {return append(__il);}
 #endif // _LIBCPP_CXX03_LANG
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string& append(const basic_string& __str);
 
     template <class _Tp>
-    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
+    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS _LIBCPP_CONSTEXPR_AFTER_CXX17
     __enable_if_t<
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value
             && !__is_same_uncvref<_Tp, basic_string>::value,
             basic_string&
         >
                   append(const _Tp& __t) { __self_view __sv = __t; return append(__sv.data(), __sv.size()); }
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string& append(const basic_string& __str, size_type __pos, size_type __n=npos);
 
     template <class _Tp>
-    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
+    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS _LIBCPP_CONSTEXPR_AFTER_CXX17
     __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value
             && !__is_same_uncvref<_Tp, basic_string>::value,
             basic_string&
         >
                   append(const _Tp& __t, size_type __pos, size_type __n=npos);
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string& append(const value_type* __s, size_type __n);
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string& append(const value_type* __s);
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string& append(size_type __n, value_type __c);
 
     _LIBCPP_INLINE_VISIBILITY
     void __append_default_init(size_type __n);
 
     template<class _InputIterator>
     _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
     __enable_if_t
         <
             __is_exactly_cpp17_input_iterator<_InputIterator>::value,
             basic_string&
         >
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     append(_InputIterator __first, _InputIterator __last) {
       const basic_string __temp(__first, __last, __alloc());
       append(__temp.data(), __temp.size());
       return *this;
     }
     template<class _ForwardIterator>
     _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
     __enable_if_t
         <
             __is_cpp17_forward_iterator<_ForwardIterator>::value,
             basic_string&
         >
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     append(_ForwardIterator __first, _ForwardIterator __last);
 
 #ifndef _LIBCPP_CXX03_LANG
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string& append(initializer_list<value_type> __il) {return append(__il.begin(), __il.size());}
 #endif // _LIBCPP_CXX03_LANG
 
-    void push_back(value_type __c);
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_CONSTEXPR_AFTER_CXX17 void push_back(value_type __c);
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     void pop_back();
-    _LIBCPP_INLINE_VISIBILITY reference       front() _NOEXCEPT;
-    _LIBCPP_INLINE_VISIBILITY const_reference front() const _NOEXCEPT;
-    _LIBCPP_INLINE_VISIBILITY reference       back() _NOEXCEPT;
-    _LIBCPP_INLINE_VISIBILITY const_reference back() const _NOEXCEPT;
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    reference front() _NOEXCEPT;
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    const_reference front() const _NOEXCEPT;
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    reference back() _NOEXCEPT;
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    const_reference back() const _NOEXCEPT;
 
     template <class _Tp>
-    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
+    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS _LIBCPP_CONSTEXPR_AFTER_CXX17
     __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
             basic_string&
         >
                  assign(const _Tp & __t) { __self_view __sv = __t; return assign(__sv.data(), __sv.size()); }
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string& assign(const basic_string& __str) { return *this = __str; }
 #ifndef _LIBCPP_CXX03_LANG
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string& assign(basic_string&& __str)
         _NOEXCEPT_((__noexcept_move_assign_container<_Allocator, __alloc_traits>::value))
         {*this = _VSTD::move(__str); return *this;}
 #endif
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string& assign(const basic_string& __str, size_type __pos, size_type __n=npos);
     template <class _Tp>
-    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
+    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS _LIBCPP_CONSTEXPR_AFTER_CXX17
     __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value
             && !__is_same_uncvref<_Tp, basic_string>::value,
             basic_string&
         >
                   assign(const _Tp & __t, size_type __pos, size_type __n=npos);
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string& assign(const value_type* __s, size_type __n);
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string& assign(const value_type* __s);
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string& assign(size_type __n, value_type __c);
     template<class _InputIterator>
-    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
+    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS _LIBCPP_CONSTEXPR_AFTER_CXX17
     __enable_if_t
         <
             __is_exactly_cpp17_input_iterator<_InputIterator>::value,
             basic_string&
         >
         assign(_InputIterator __first, _InputIterator __last);
     template<class _ForwardIterator>
-    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
+    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS _LIBCPP_CONSTEXPR_AFTER_CXX17
     __enable_if_t
         <
             __is_cpp17_forward_iterator<_ForwardIterator>::value,
             basic_string&
         >
         assign(_ForwardIterator __first, _ForwardIterator __last);
 #ifndef _LIBCPP_CXX03_LANG
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string& assign(initializer_list<value_type> __il) {return assign(__il.begin(), __il.size());}
 #endif // _LIBCPP_CXX03_LANG
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string& insert(size_type __pos1, const basic_string& __str);
 
     template <class _Tp>
-    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
+    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS _LIBCPP_CONSTEXPR_AFTER_CXX17
     __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
             basic_string&
         >
                  insert(size_type __pos1, const _Tp& __t)
     { __self_view __sv = __t; return insert(__pos1, __sv.data(), __sv.size()); }
 
     template <class _Tp>
-    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
+    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS _LIBCPP_CONSTEXPR_AFTER_CXX17
     __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value && !__is_same_uncvref<_Tp, basic_string>::value,
             basic_string&
         >
                   insert(size_type __pos1, const _Tp& __t, size_type __pos2, size_type __n=npos);
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string& insert(size_type __pos1, const basic_string& __str, size_type __pos2, size_type __n=npos);
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string& insert(size_type __pos, const value_type* __s, size_type __n);
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string& insert(size_type __pos, const value_type* __s);
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string& insert(size_type __pos, size_type __n, value_type __c);
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     iterator      insert(const_iterator __pos, value_type __c);
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     iterator      insert(const_iterator __pos, size_type __n, value_type __c);
     template<class _InputIterator>
-    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
+    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS _LIBCPP_CONSTEXPR_AFTER_CXX17
     __enable_if_t
         <
             __is_exactly_cpp17_input_iterator<_InputIterator>::value,
             iterator
         >
         insert(const_iterator __pos, _InputIterator __first, _InputIterator __last);
     template<class _ForwardIterator>
-    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
+    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS _LIBCPP_CONSTEXPR_AFTER_CXX17
     __enable_if_t
         <
             __is_cpp17_forward_iterator<_ForwardIterator>::value,
             iterator
         >
         insert(const_iterator __pos, _ForwardIterator __first, _ForwardIterator __last);
 #ifndef _LIBCPP_CXX03_LANG
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     iterator insert(const_iterator __pos, initializer_list<value_type> __il)
                     {return insert(__pos, __il.begin(), __il.end());}
 #endif // _LIBCPP_CXX03_LANG
-
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string& erase(size_type __pos = 0, size_type __n = npos);
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     iterator      erase(const_iterator __pos);
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     iterator      erase(const_iterator __first, const_iterator __last);
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string& replace(size_type __pos1, size_type __n1, const basic_string& __str);
 
     template <class _Tp>
-    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
+    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS _LIBCPP_CONSTEXPR_AFTER_CXX17
     __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
             basic_string&
         >
                   replace(size_type __pos1, size_type __n1, const _Tp& __t) { __self_view __sv = __t; return replace(__pos1, __n1, __sv.data(), __sv.size()); }
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string& replace(size_type __pos1, size_type __n1, const basic_string& __str, size_type __pos2, size_type __n2=npos);
     template <class _Tp>
-    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
+    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS _LIBCPP_CONSTEXPR_AFTER_CXX17
     __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value  && !__is_same_uncvref<_Tp, basic_string>::value,
             basic_string&
         >
                   replace(size_type __pos1, size_type __n1, const _Tp& __t, size_type __pos2, size_type __n2=npos);
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string& replace(size_type __pos, size_type __n1, const value_type* __s, size_type __n2);
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string& replace(size_type __pos, size_type __n1, const value_type* __s);
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string& replace(size_type __pos, size_type __n1, size_type __n2, value_type __c);
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string& replace(const_iterator __i1, const_iterator __i2, const basic_string& __str);
 
     template <class _Tp>
-    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
+    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS _LIBCPP_CONSTEXPR_AFTER_CXX17
     __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
             basic_string&
         >
                   replace(const_iterator __i1, const_iterator __i2, const _Tp& __t) { __self_view __sv = __t; return replace(__i1 - begin(), __i2 - __i1, __sv); }
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string& replace(const_iterator __i1, const_iterator __i2, const value_type* __s, size_type __n);
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string& replace(const_iterator __i1, const_iterator __i2, const value_type* __s);
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string& replace(const_iterator __i1, const_iterator __i2, size_type __n, value_type __c);
     template<class _InputIterator>
-    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
+    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS _LIBCPP_CONSTEXPR_AFTER_CXX17
     __enable_if_t
         <
             __is_cpp17_input_iterator<_InputIterator>::value,
             basic_string&
         >
         replace(const_iterator __i1, const_iterator __i2, _InputIterator __j1, _InputIterator __j2);
 #ifndef _LIBCPP_CXX03_LANG
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string& replace(const_iterator __i1, const_iterator __i2, initializer_list<value_type> __il)
         {return replace(__i1, __i2, __il.begin(), __il.end());}
 #endif // _LIBCPP_CXX03_LANG
 
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     size_type copy(value_type* __s, size_type __n, size_type __pos = 0) const;
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string substr(size_type __pos = 0, size_type __n = npos) const;
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     void swap(basic_string& __str)
 #if _LIBCPP_STD_VER >= 14
         _NOEXCEPT;
 #else
         _NOEXCEPT_(!__alloc_traits::propagate_on_container_swap::value ||
                     __is_nothrow_swappable<allocator_type>::value);
 #endif
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     const value_type* c_str() const _NOEXCEPT {return data();}
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     const value_type* data() const _NOEXCEPT  {return _VSTD::__to_address(__get_pointer());}
 #if _LIBCPP_STD_VER > 14 || defined(_LIBCPP_BUILDING_LIBRARY)
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     value_type* data()             _NOEXCEPT  {return _VSTD::__to_address(__get_pointer());}
 #endif
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     allocator_type get_allocator() const _NOEXCEPT {return __alloc();}
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     size_type find(const basic_string& __str, size_type __pos = 0) const _NOEXCEPT;
 
     template <class _Tp>
-    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
+    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS _LIBCPP_CONSTEXPR_AFTER_CXX17
     __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
             size_type
         >
               find(const _Tp& __t, size_type __pos = 0) const _NOEXCEPT;
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     size_type find(const value_type* __s, size_type __pos, size_type __n) const _NOEXCEPT;
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     size_type find(const value_type* __s, size_type __pos = 0) const _NOEXCEPT;
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     size_type find(value_type __c, size_type __pos = 0) const _NOEXCEPT;
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     size_type rfind(const basic_string& __str, size_type __pos = npos) const _NOEXCEPT;
 
     template <class _Tp>
-    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
+    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS _LIBCPP_CONSTEXPR_AFTER_CXX17
     __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
             size_type
         >
               rfind(const _Tp& __t, size_type __pos = npos) const _NOEXCEPT;
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     size_type rfind(const value_type* __s, size_type __pos, size_type __n) const _NOEXCEPT;
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     size_type rfind(const value_type* __s, size_type __pos = npos) const _NOEXCEPT;
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     size_type rfind(value_type __c, size_type __pos = npos) const _NOEXCEPT;
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     size_type find_first_of(const basic_string& __str, size_type __pos = 0) const _NOEXCEPT;
 
     template <class _Tp>
-    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
+    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS _LIBCPP_CONSTEXPR_AFTER_CXX17
     __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
             size_type
         >
               find_first_of(const _Tp& __t, size_type __pos = 0) const _NOEXCEPT;
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     size_type find_first_of(const value_type* __s, size_type __pos, size_type __n) const _NOEXCEPT;
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     size_type find_first_of(const value_type* __s, size_type __pos = 0) const _NOEXCEPT;
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     size_type find_first_of(value_type __c, size_type __pos = 0) const _NOEXCEPT;
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     size_type find_last_of(const basic_string& __str, size_type __pos = npos) const _NOEXCEPT;
 
     template <class _Tp>
-    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
+    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS _LIBCPP_CONSTEXPR_AFTER_CXX17
     __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
             size_type
         >
               find_last_of(const _Tp& __t, size_type __pos = npos) const _NOEXCEPT;
+              _LIBCPP_CONSTEXPR_AFTER_CXX17
     size_type find_last_of(const value_type* __s, size_type __pos, size_type __n) const _NOEXCEPT;
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     size_type find_last_of(const value_type* __s, size_type __pos = npos) const _NOEXCEPT;
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     size_type find_last_of(value_type __c, size_type __pos = npos) const _NOEXCEPT;
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     size_type find_first_not_of(const basic_string& __str, size_type __pos = 0) const _NOEXCEPT;
 
     template <class _Tp>
-    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
+    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS _LIBCPP_CONSTEXPR_AFTER_CXX17
     __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
             size_type
         >
               find_first_not_of(const _Tp &__t, size_type __pos = 0) const _NOEXCEPT;
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     size_type find_first_not_of(const value_type* __s, size_type __pos, size_type __n) const _NOEXCEPT;
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     size_type find_first_not_of(const value_type* __s, size_type __pos = 0) const _NOEXCEPT;
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     size_type find_first_not_of(value_type __c, size_type __pos = 0) const _NOEXCEPT;
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     size_type find_last_not_of(const basic_string& __str, size_type __pos = npos) const _NOEXCEPT;
 
     template <class _Tp>
-    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
+    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS _LIBCPP_CONSTEXPR_AFTER_CXX17
     __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
             size_type
         >
               find_last_not_of(const _Tp& __t, size_type __pos = npos) const _NOEXCEPT;
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     size_type find_last_not_of(const value_type* __s, size_type __pos, size_type __n) const _NOEXCEPT;
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     size_type find_last_not_of(const value_type* __s, size_type __pos = npos) const _NOEXCEPT;
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     size_type find_last_not_of(value_type __c, size_type __pos = npos) const _NOEXCEPT;
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     int compare(const basic_string& __str) const _NOEXCEPT;
 
     template <class _Tp>
-    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
+    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS _LIBCPP_CONSTEXPR_AFTER_CXX17
     __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
             int
         >
         compare(const _Tp &__t) const _NOEXCEPT;
 
     template <class _Tp>
-    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
+    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS _LIBCPP_CONSTEXPR_AFTER_CXX17
     __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
             int
         >
          compare(size_type __pos1, size_type __n1, const _Tp& __t) const;
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     int compare(size_type __pos1, size_type __n1, const basic_string& __str) const;
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     int compare(size_type __pos1, size_type __n1, const basic_string& __str, size_type __pos2, size_type __n2=npos) const;
 
     template <class _Tp>
-    inline _LIBCPP_INLINE_VISIBILITY
+    inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
         __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value  && !__is_same_uncvref<_Tp, basic_string>::value,
             int
         >
         compare(size_type __pos1, size_type __n1, const _Tp& __t, size_type __pos2, size_type __n2=npos) const;
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     int compare(const value_type* __s) const _NOEXCEPT;
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     int compare(size_type __pos1, size_type __n1, const value_type* __s) const;
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     int compare(size_type __pos1, size_type __n1, const value_type* __s, size_type __n2) const;
 
 #if _LIBCPP_STD_VER > 17
     _LIBCPP_CONSTEXPR_AFTER_CXX11 _LIBCPP_INLINE_VISIBILITY
     bool starts_with(__self_view __sv) const _NOEXCEPT
     { return __self_view(data(), size()).starts_with(__sv); }
 
     _LIBCPP_CONSTEXPR_AFTER_CXX11 _LIBCPP_INLINE_VISIBILITY
     bool starts_with(value_type __c) const _NOEXCEPT
     { return !empty() && _Traits::eq(front(), __c); }
 
     _LIBCPP_CONSTEXPR_AFTER_CXX11 _LIBCPP_INLINE_VISIBILITY
#if _LIBCPP_STD_VER > 20
 
     constexpr _LIBCPP_INLINE_VISIBILITY
     bool contains(const value_type* __s) const
     { return __self_view(data(), size()).contains(__s); }
 #endif
 
     _LIBCPP_INLINE_VISIBILITY bool __invariants() const;
 
     _LIBCPP_INLINE_VISIBILITY void __clear_and_shrink() _NOEXCEPT;
 
-    _LIBCPP_INLINE_VISIBILITY void __shrink_or_extend(size_type __target_capacity);
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    void __shrink_or_extend(size_type __target_capacity);
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     bool __is_long() const _NOEXCEPT
-        {return bool(__r_.first().__s.__size_ & __short_mask);}
+    { 
+        if (__libcpp_is_constant_evaluated())
+            return true;
+        return bool(__r_.first().__s.__size_ & __short_mask);
+    }
 
 #if _LIBCPP_DEBUG_LEVEL == 2
 
     bool __dereferenceable(const const_iterator* __i) const;
     bool __decrementable(const const_iterator* __i) const;
     bool __addable(const const_iterator* __i, ptrdiff_t __n) const;
     bool __subscriptable(const const_iterator* __i, ptrdiff_t __n) const;
 
 #endif // _LIBCPP_DEBUG_LEVEL == 2
 
 private:
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     allocator_type& __alloc() _NOEXCEPT
         {return __r_.second();}
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     const allocator_type& __alloc() const _NOEXCEPT
         {return __r_.second();}
 
 #ifdef _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
 
     _LIBCPP_INLINE_VISIBILITY
     void __set_short_size(size_type __s) _NOEXCEPT
 #   ifdef _LIBCPP_BIG_ENDIAN

 #   ifdef _LIBCPP_BIG_ENDIAN
         {return __r_.first().__s.__size_;}
 #   else
         {return __r_.first().__s.__size_ >> 1;}
 #   endif
 
 #endif // _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     void __set_long_size(size_type __s) _NOEXCEPT
         {__r_.first().__l.__size_ = __s;}
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     size_type __get_long_size() const _NOEXCEPT
         {return __r_.first().__l.__size_;}
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     void __set_size(size_type __s) _NOEXCEPT
         {if (__is_long()) __set_long_size(__s); else __set_short_size(__s);}
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     void __set_long_cap(size_type __s) _NOEXCEPT
         {__r_.first().__l.__cap_  = __long_mask | __s;}
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     size_type __get_long_cap() const _NOEXCEPT
         {return __r_.first().__l.__cap_ & size_type(~__long_mask);}
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     void __set_long_pointer(pointer __p) _NOEXCEPT
         {__r_.first().__l.__data_ = __p;}
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     pointer __get_long_pointer() _NOEXCEPT
         {return __r_.first().__l.__data_;}
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     const_pointer __get_long_pointer() const _NOEXCEPT
         {return __r_.first().__l.__data_;}
     _LIBCPP_INLINE_VISIBILITY
     pointer __get_short_pointer() _NOEXCEPT
         {return pointer_traits<pointer>::pointer_to(__r_.first().__s.__data_[0]);}
     _LIBCPP_INLINE_VISIBILITY
     const_pointer __get_short_pointer() const _NOEXCEPT
         {return pointer_traits<const_pointer>::pointer_to(__r_.first().__s.__data_[0]);}
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     pointer __get_pointer() _NOEXCEPT
         {return __is_long() ? __get_long_pointer() : __get_short_pointer();}
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     const_pointer __get_pointer() const _NOEXCEPT
         {return __is_long() ? __get_long_pointer() : __get_short_pointer();}
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     void __zero() _NOEXCEPT
-        {
-            size_type (&__a)[__n_words] = __r_.first().__r.__words;
-            for (unsigned __i = 0; __i < __n_words; ++__i)
-                __a[__i] = 0;
-        }
+        if (__libcpp_is_constant_evaluated())
+        {
+            __r_.first() = __rep();
+            return;
+        }
 
     template <size_type __a> static
-        _LIBCPP_INLINE_VISIBILITY
+        _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
         size_type __align_it(size_type __s) _NOEXCEPT
             {return (__s + (__a-1)) & ~(__a-1);}
     enum {__alignment = 16};
-    static _LIBCPP_INLINE_VISIBILITY
+    static _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     size_type __recommend(size_type __s) _NOEXCEPT
-        {
+        if (__libcpp_is_constant_evaluated())
+            return __s + 1;
         if (__s < __min_cap) return static_cast<size_type>(__min_cap) - 1;
         size_type __guess = __align_it<sizeof(value_type) < __alignment ?
                      __alignment/sizeof(value_type) : 1 > (__s+1) - 1;
         if (__guess == __min_cap) ++__guess;
         return __guess;
-        }
 
-    inline
+    inline _LIBCPP_CONSTEXPR_AFTER_CXX17
     void __init(const value_type* __s, size_type __sz, size_type __reserve);
-    inline
+    inline _LIBCPP_CONSTEXPR_AFTER_CXX17
     void __init(const value_type* __s, size_type __sz);
-    inline
+    inline _LIBCPP_CONSTEXPR_AFTER_CXX17
     void __init(size_type __n, value_type __c);
 
     // Slow path for the (inlined) copy constructor for 'long' strings.
     // Always externally instantiated and not inlined.
     // Requires that __s is zero terminated.
     // The main reason for this function to exist is because for unstable, we
     // want to allow inlining of the copy constructor. However, we don't want
     // to call the __init() functions as those are marked as inline which may
     // result in over-aggressive inlining by the compiler, where our aim is
     // to only inline the fast path code directly in the ctor.
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     void __init_copy_ctor_external(const value_type* __s, size_type __sz);
 
     template <class _InputIterator>
-    inline
+    inline _LIBCPP_CONSTEXPR_AFTER_CXX17
     __enable_if_t
     <
         __is_exactly_cpp17_input_iterator<_InputIterator>::value
     >
     __init(_InputIterator __first, _InputIterator __last);
 
     template <class _ForwardIterator>
-    inline
+    inline _LIBCPP_CONSTEXPR_AFTER_CXX17
     __enable_if_t
     <
         __is_cpp17_forward_iterator<_ForwardIterator>::value
     >
     __init(_ForwardIterator __first, _ForwardIterator __last);
 
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     void __grow_by(size_type __old_cap, size_type __delta_cap, size_type __old_sz,
                    size_type __n_copy,  size_type __n_del,     size_type __n_add = 0);
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     void __grow_by_and_replace(size_type __old_cap, size_type __delta_cap, size_type __old_sz,
                                size_type __n_copy,  size_type __n_del,
                                size_type __n_add, const value_type* __p_new_stuff);
 
     // __assign_no_alias is invoked for assignment operations where we
     // have proof that the input does not alias the current instance.
     // For example, operator=(basic_string) performs a 'self' check.
     template <bool __is_short>
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string& __assign_no_alias(const value_type* __s, size_type __n);
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     void __erase_to_end(size_type __pos);
 
+    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17
+    void __finish_replace(size_type& __sz, size_type& __n1, size_type& __n2, value_type*& __p);
+
     // __erase_external_with_move is invoked for erase() invocations where
     // `n ~= npos`, likely requiring memory moves on the string data.
     void __erase_external_with_move(size_type __pos, size_type __n);
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     void __copy_assign_alloc(const basic_string& __str)
         {__copy_assign_alloc(__str, integral_constant<bool,
                       __alloc_traits::propagate_on_container_copy_assignment::value>());}
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     void __copy_assign_alloc(const basic_string& __str, true_type)
         {
             if (__alloc() == __str.__alloc())
                 __alloc() = __str.__alloc();
             else
             {
                 if (!__str.__is_long())
                 {
                     __clear_and_shrink();
                     __alloc() = __str.__alloc();
                 }
                 else
                 {
                     allocator_type __a = __str.__alloc();
                     pointer __p = __alloc_traits::allocate(__a, __str.__get_long_cap());
                     __clear_and_shrink();
                     __alloc() = _VSTD::move(__a);
                     __set_long_pointer(__p);
                     __set_long_cap(__str.__get_long_cap());
                     __set_long_size(__str.size());
                 }
             }
         }
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     void __copy_assign_alloc(const basic_string&, false_type) _NOEXCEPT
         {}
 
 #ifndef _LIBCPP_CXX03_LANG
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     void __move_assign(basic_string& __str, false_type)
         _NOEXCEPT_(__alloc_traits::is_always_equal::value);
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     void __move_assign(basic_string& __str, true_type)
 #if _LIBCPP_STD_VER > 14
         _NOEXCEPT;
 #else
         _NOEXCEPT_(is_nothrow_move_assignable<allocator_type>::value);
 #endif
 #endif
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     void
     __move_assign_alloc(basic_string& __str)
         _NOEXCEPT_(
             !__alloc_traits::propagate_on_container_move_assignment::value ||
             is_nothrow_move_assignable<allocator_type>::value)
     {__move_assign_alloc(__str, integral_constant<bool,
                       __alloc_traits::propagate_on_container_move_assignment::value>());}
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     void __move_assign_alloc(basic_string& __c, true_type)
         _NOEXCEPT_(is_nothrow_move_assignable<allocator_type>::value)
         {
             __alloc() = _VSTD::move(__c.__alloc());
         }
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     void __move_assign_alloc(basic_string&, false_type)
         _NOEXCEPT
         {}
 
     basic_string& __assign_external(const value_type* __s);
+    _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string& __assign_external(const value_type* __s, size_type __n);
 
     // Assigns the value in __s, guaranteed to be __n < __min_cap in length.
     inline basic_string& __assign_short(const value_type* __s, size_type __n) {
       pointer __p = __is_long()
                         ? (__set_long_size(__n), __get_long_pointer())
                         : (__set_short_size(__n), __get_short_pointer());
       traits_type::move(_VSTD::__to_address(__p), __s, __n);
       traits_type::assign(__p[__n], value_type());
       return *this;
     }
 
-    _LIBCPP_INLINE_VISIBILITY void __invalidate_all_iterators();
-    _LIBCPP_INLINE_VISIBILITY void __invalidate_iterators_past(size_type);
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    void __invalidate_all_iterators();
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    void __invalidate_iterators_past(size_type);
 
     template<class _Tp>
     _LIBCPP_INLINE_VISIBILITY
     bool __addr_in_range(_Tp&& __t) const {
         const volatile void *__p = _VSTD::addressof(__t);
         return data() <= __p && __p <= data() + size();
     }
 
     _LIBCPP_NORETURN _LIBCPP_HIDE_FROM_ABI
     void __throw_length_error() const {
 #ifndef _LIBCPP_NO_EXCEPTIONS
         __basic_string_common<true>::__throw_length_error();
 #else
         _VSTD::abort();
 #endif
     }
 
-    _LIBCPP_NORETURN _LIBCPP_HIDE_FROM_ABI
+    _LIBCPP_NORETURN _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17
     void __throw_out_of_range() const {
 #ifndef _LIBCPP_NO_EXCEPTIONS
         __basic_string_common<true>::__throw_out_of_range();
 #else
         _VSTD::abort();
 #endif
     }
 
-    friend basic_string operator+<>(const basic_string&, const basic_string&);
-    friend basic_string operator+<>(const value_type*, const basic_string&);
-    friend basic_string operator+<>(value_type, const basic_string&);
-    friend basic_string operator+<>(const basic_string&, const value_type*);
-    friend basic_string operator+<>(const basic_string&, value_type);
+    _LIBCPP_HIDE_FROM_ABI
+    _LIBCPP_CONSTEXPR void __begin_lifetime(pointer __p, size_t __count)
+    {
+#if _LIBCPP_STD_VER > 17
+        if (__libcpp_is_constant_evaluated())
+        {
+            for (size_t __i = 0; __i < __count; ++__i)
+                construct_at(&__p[__i], value_type());
+        }
+#endif
+    }
+
+    friend _LIBCPP_CONSTEXPR_AFTER_CXX17 basic_string operator+<>(const basic_string&, const basic_string&);
+    friend _LIBCPP_CONSTEXPR_AFTER_CXX17 basic_string operator+<>(const value_type*, const basic_string&);
+    friend _LIBCPP_CONSTEXPR_AFTER_CXX17 basic_string operator+<>(value_type, const basic_string&);
+    friend _LIBCPP_CONSTEXPR_AFTER_CXX17 basic_string operator+<>(const basic_string&, const value_type*);
+    friend _LIBCPP_CONSTEXPR_AFTER_CXX17 basic_string operator+<>(const basic_string&, value_type);
 };
 
 // These declarations must appear before any functions are implicitly used
 // so that they have the correct visibility specifier.
 #ifdef _LIBCPP_ABI_STRING_OPTIMIZED_EXTERNAL_INSTANTIATION
 _LIBCPP_STRING_UNSTABLE_EXTERN_TEMPLATE_LIST(_LIBCPP_EXTERN_TEMPLATE, char)
 _LIBCPP_STRING_UNSTABLE_EXTERN_TEMPLATE_LIST(_LIBCPP_EXTERN_TEMPLATE, wchar_t)
 #else
template<class _CharT,
          class = enable_if_t<__is_allocator<_Allocator>::value>,
          class _Sz = typename allocator_traits<_Allocator>::size_type
          >
 basic_string(basic_string_view<_CharT, _Traits>, _Sz, _Sz, const _Allocator& = _Allocator())
   -> basic_string<_CharT, _Traits, _Allocator>;
 #endif
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 void
 basic_string<_CharT, _Traits, _Allocator>::__invalidate_all_iterators()
 {
 #if _LIBCPP_DEBUG_LEVEL == 2
     __get_db()->__invalidate_all(this);
 #endif
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 void
 basic_string<_CharT, _Traits, _Allocator>::__invalidate_iterators_past(size_type __pos)
 {
 #if _LIBCPP_DEBUG_LEVEL == 2
     __c_node* __c = __get_db()->__find_c_and_lock(this);
     if (__c)
     {
         const_pointer __new_last = __get_pointer() + __pos;
if (__c)
         __get_db()->unlock();
     }
 #else
     (void)__pos;
 #endif // _LIBCPP_DEBUG_LEVEL == 2
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>::basic_string()
     _NOEXCEPT_(is_nothrow_default_constructible<allocator_type>::value)
      : __r_(__default_init_tag(), __default_init_tag())
 {
 #if _LIBCPP_DEBUG_LEVEL == 2
     __get_db()->__insert_c(this);
 #endif
+    if (__libcpp_is_constant_evaluated())
+        __init(size_type(), value_type());
+    else
         __zero();
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>::basic_string(const allocator_type& __a)
 #if _LIBCPP_STD_VER <= 14
         _NOEXCEPT_(is_nothrow_copy_constructible<allocator_type>::value)
 #else
         _NOEXCEPT
 #endif
 : __r_(__default_init_tag(), __a)
 {
 #if _LIBCPP_DEBUG_LEVEL == 2
     __get_db()->__insert_c(this);
 #endif
+    if (__libcpp_is_constant_evaluated())
+        __init(size_type(0), value_type());
+    else
         __zero();
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 void basic_string<_CharT, _Traits, _Allocator>::__init(const value_type* __s,
                                                        size_type __sz,
                                                        size_type __reserve)
 {
+    if (__libcpp_is_constant_evaluated())
+        __zero();
     if (__reserve > max_size())
         this->__throw_length_error();
     pointer __p;
-    if (__reserve < __min_cap)
+    if (__reserve < __min_cap && !__libcpp_is_constant_evaluated())
     {
         __set_short_size(__sz);
         __p = __get_short_pointer();
     }
     else
     {
         size_type __cap = __recommend(__reserve);
         __p = __alloc_traits::allocate(__alloc(), __cap+1);
+        __begin_lifetime(__p, __cap + 1);
         __set_long_pointer(__p);
         __set_long_cap(__cap+1);
         __set_long_size(__sz);
     }
     traits_type::copy(_VSTD::__to_address(__p), __s, __sz);
     traits_type::assign(__p[__sz], value_type());
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 void
 basic_string<_CharT, _Traits, _Allocator>::__init(const value_type* __s, size_type __sz)
 {
-    if (__sz > max_size())
-        this->__throw_length_error();
-    pointer __p;
-    if (__sz < __min_cap)
-    {
-        __set_short_size(__sz);
-        __p = __get_short_pointer();
-    }
-    else
-    {
-        size_type __cap = __recommend(__sz);
-        __p = __alloc_traits::allocate(__alloc(), __cap+1);
-        __set_long_pointer(__p);
-        __set_long_cap(__cap+1);
-        __set_long_size(__sz);
-    }
-    traits_type::copy(_VSTD::__to_address(__p), __s, __sz);
-    traits_type::assign(__p[__sz], value_type());
+    __init(__s, __sz, __sz);
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 template <class>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>::basic_string(const _CharT* __s, const _Allocator& __a)
     : __r_(__default_init_tag(), __a)
 {
     _LIBCPP_ASSERT(__s != nullptr, "basic_string(const char*, allocator) detected nullptr");
     __init(__s, traits_type::length(__s));
 #if _LIBCPP_DEBUG_LEVEL == 2
     __get_db()->__insert_c(this);
 #endif
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>::basic_string(const _CharT* __s, size_type __n)
      : __r_(__default_init_tag(), __default_init_tag())
 {
     _LIBCPP_ASSERT(__n == 0 || __s != nullptr, "basic_string(const char*, n) detected nullptr");
     __init(__s, __n);
 #if _LIBCPP_DEBUG_LEVEL == 2
     __get_db()->__insert_c(this);
 #endif
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>::basic_string(const _CharT* __s, size_type __n, const _Allocator& __a)
     : __r_(__default_init_tag(), __a)
 {
     _LIBCPP_ASSERT(__n == 0 || __s != nullptr, "basic_string(const char*, n, allocator) detected nullptr");
     __init(__s, __n);
 #if _LIBCPP_DEBUG_LEVEL == 2
     __get_db()->__insert_c(this);
 #endif
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>::basic_string(const basic_string& __str)
     : __r_(__default_init_tag(), __alloc_traits::select_on_container_copy_construction(__str.__alloc()))
 {
     if (!__str.__is_long())
         __r_.first().__r = __str.__r_.first().__r;
     else
         __init_copy_ctor_external(_VSTD::__to_address(__str.__get_long_pointer()),
                                   __str.__get_long_size());
 
 #if _LIBCPP_DEBUG_LEVEL == 2
     __get_db()->__insert_c(this);
 #endif
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>::basic_string(
     const basic_string& __str, const allocator_type& __a)
     : __r_(__default_init_tag(), __a)
 {
     if (!__str.__is_long())
         __r_.first().__r = __str.__r_.first().__r;
     else
         __init_copy_ctor_external(_VSTD::__to_address(__str.__get_long_pointer()),
                                   __str.__get_long_size());
 #if _LIBCPP_DEBUG_LEVEL == 2
     __get_db()->__insert_c(this);
 #endif
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 void basic_string<_CharT, _Traits, _Allocator>::__init_copy_ctor_external(
     const value_type* __s, size_type __sz) {
+  if (__libcpp_is_constant_evaluated())
+    __zero();
   pointer __p;
-  if (__sz < __min_cap) {
+  if (__sz < __min_cap && !__libcpp_is_constant_evaluated()) {
     __p = __get_short_pointer();
     __set_short_size(__sz);
   } else {
     if (__sz > max_size())
       this->__throw_length_error();
     size_t __cap = __recommend(__sz);
     __p = __alloc_traits::allocate(__alloc(), __cap + 1);
+    __begin_lifetime(__p, __cap + 1);
     __set_long_pointer(__p);
     __set_long_cap(__cap + 1);
     __set_long_size(__sz);
   }
   traits_type::copy(_VSTD::__to_address(__p), __s, __sz + 1);
 }
 
 #ifndef _LIBCPP_CXX03_LANG
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>::basic_string(basic_string&& __str)
 #if _LIBCPP_STD_VER <= 14
         _NOEXCEPT_(is_nothrow_move_constructible<allocator_type>::value)
 #else
         _NOEXCEPT
 #endif
     : __r_(_VSTD::move(__str.__r_))
 {
     __str.__zero();
 #if _LIBCPP_DEBUG_LEVEL == 2
     __get_db()->__insert_c(this);
     if (__is_long())
         __get_db()->swap(this, &__str);
 #endif
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>::basic_string(basic_string&& __str, const allocator_type& __a)
     : __r_(__default_init_tag(), __a)
 {
     if (__str.__is_long() && __a != __str.__alloc()) // copy, not move
         __init(_VSTD::__to_address(__str.__get_long_pointer()), __str.__get_long_size());
     else
     {
+        if (__libcpp_is_constant_evaluated())
+        {
+            __r_.first() = __rep();
+            __r_.first().__l = __str.__r_.first().__l;
+        }
+        else
             __r_.first().__r = __str.__r_.first().__r;
         __str.__zero();
     }
 #if _LIBCPP_DEBUG_LEVEL == 2
     __get_db()->__insert_c(this);
     if (__is_long())
         __get_db()->swap(this, &__str);
 #endif
 }
 
 #endif // _LIBCPP_CXX03_LANG
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 void
 basic_string<_CharT, _Traits, _Allocator>::__init(size_type __n, value_type __c)
 {
+    if (__libcpp_is_constant_evaluated())
+        __zero();
     if (__n > max_size())
         this->__throw_length_error();
     pointer __p;
-    if (__n < __min_cap)
+    if (__n < __min_cap && !__libcpp_is_constant_evaluated())
     {
         __set_short_size(__n);
         __p = __get_short_pointer();
     }
     else
     {
         size_type __cap = __recommend(__n);
         __p = __alloc_traits::allocate(__alloc(), __cap+1);
+        __begin_lifetime(__p, __cap + 1);
         __set_long_pointer(__p);
         __set_long_cap(__cap+1);
         __set_long_size(__n);
     }
     traits_type::assign(_VSTD::__to_address(__p), __n, __c);
     traits_type::assign(__p[__n], value_type());
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>::basic_string(size_type __n, _CharT __c)
      : __r_(__default_init_tag(), __default_init_tag())
 {
     __init(__n, __c);
 #if _LIBCPP_DEBUG_LEVEL == 2
     __get_db()->__insert_c(this);
 #endif
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-template <class>
+template <class> _LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>::basic_string(size_type __n, _CharT __c, const _Allocator& __a)
     : __r_(__default_init_tag(), __a)
 {
     __init(__n, __c);
 #if _LIBCPP_DEBUG_LEVEL == 2
     __get_db()->__insert_c(this);
 #endif
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>::basic_string(const basic_string& __str,
                                                         size_type __pos, size_type __n,
                                                         const _Allocator& __a)
     : __r_(__default_init_tag(), __a)
 {
     size_type __str_sz = __str.size();
     if (__pos > __str_sz)
         this->__throw_out_of_range();
     __init(__str.data() + __pos, _VSTD::min(__n, __str_sz - __pos));
 #if _LIBCPP_DEBUG_LEVEL == 2
     __get_db()->__insert_c(this);
 #endif
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>::basic_string(const basic_string& __str, size_type __pos,
                                                         const _Allocator& __a)
     : __r_(__default_init_tag(), __a)
 {
     size_type __str_sz = __str.size();
     if (__pos > __str_sz)
         this->__throw_out_of_range();
     __init(__str.data() + __pos, __str_sz - __pos);
 #if _LIBCPP_DEBUG_LEVEL == 2
     __get_db()->__insert_c(this);
 #endif
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 template <class _Tp, class>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>::basic_string(
              const _Tp& __t, size_type __pos, size_type __n, const allocator_type& __a)
     : __r_(__default_init_tag(), __a)
 {
     __self_view __sv0 = __t;
     __self_view __sv = __sv0.substr(__pos, __n);
     __init(__sv.data(), __sv.size());
 #if _LIBCPP_DEBUG_LEVEL == 2
     __get_db()->__insert_c(this);
 #endif
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 template <class _Tp, class>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>::basic_string(const _Tp & __t)
      : __r_(__default_init_tag(), __default_init_tag())
 {
     __self_view __sv = __t;
     __init(__sv.data(), __sv.size());
 #if _LIBCPP_DEBUG_LEVEL == 2
     __get_db()->__insert_c(this);
 #endif
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 template <class _Tp, class>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>::basic_string(const _Tp & __t, const _Allocator& __a)
     : __r_(__default_init_tag(), __a)
 {
     __self_view __sv = __t;
     __init(__sv.data(), __sv.size());
 #if _LIBCPP_DEBUG_LEVEL == 2
     __get_db()->__insert_c(this);
 #endif
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 template <class _InputIterator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 __enable_if_t
 <
     __is_exactly_cpp17_input_iterator<_InputIterator>::value
 >
 basic_string<_CharT, _Traits, _Allocator>::__init(_InputIterator __first, _InputIterator __last)
 {
     __zero();
 #ifndef _LIBCPP_NO_EXCEPTIONS
catch (...)
             __alloc_traits::deallocate(__alloc(), __get_long_pointer(), __get_long_cap());
         throw;
     }
 #endif // _LIBCPP_NO_EXCEPTIONS
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 template <class _ForwardIterator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 __enable_if_t
 <
     __is_cpp17_forward_iterator<_ForwardIterator>::value
 >
 basic_string<_CharT, _Traits, _Allocator>::__init(_ForwardIterator __first, _ForwardIterator __last)
 {
     size_type __sz = static_cast<size_type>(_VSTD::distance(__first, __last));
     if (__sz > max_size())
         this->__throw_length_error();
     pointer __p;
-    if (__sz < __min_cap)
+    if (__sz < __min_cap && !__libcpp_is_constant_evaluated())
     {
         __set_short_size(__sz);
         __p = __get_short_pointer();
     }
     else
     {
         size_type __cap = __recommend(__sz);
         __p = __alloc_traits::allocate(__alloc(), __cap+1);
+        __begin_lifetime(__p, __cap + 1);
         __set_long_pointer(__p);
         __set_long_cap(__cap+1);
         __set_long_size(__sz);
     }
 
 #ifndef _LIBCPP_NO_EXCEPTIONS
     try
     {
catch (...)
             __alloc_traits::deallocate(__alloc(), __get_long_pointer(), __get_long_cap());
         throw;
     }
 #endif  // _LIBCPP_NO_EXCEPTIONS
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 template<class _InputIterator, class>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>::basic_string(_InputIterator __first, _InputIterator __last)
      : __r_(__default_init_tag(), __default_init_tag())
 {
     __init(__first, __last);
 #if _LIBCPP_DEBUG_LEVEL == 2
     __get_db()->__insert_c(this);
 #endif
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 template<class _InputIterator, class>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>::basic_string(_InputIterator __first, _InputIterator __last,
                                                         const allocator_type& __a)
     : __r_(__default_init_tag(), __a)
 {
     __init(__first, __last);
 #if _LIBCPP_DEBUG_LEVEL == 2
     __get_db()->__insert_c(this);
 #endif
 }
 
 #ifndef _LIBCPP_CXX03_LANG
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>::basic_string(
     initializer_list<_CharT> __il)
      : __r_(__default_init_tag(), __default_init_tag())
 {
     __init(__il.begin(), __il.end());
 #if _LIBCPP_DEBUG_LEVEL == 2
     __get_db()->__insert_c(this);
 #endif
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
-
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>::basic_string(
     initializer_list<_CharT> __il, const _Allocator& __a)
     : __r_(__default_init_tag(), __a)
 {
     __init(__il.begin(), __il.end());
 #if _LIBCPP_DEBUG_LEVEL == 2
     __get_db()->__insert_c(this);
 #endif
 }
 
 #endif // _LIBCPP_CXX03_LANG
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>::~basic_string()
 {
 #if _LIBCPP_DEBUG_LEVEL == 2
     __get_db()->__erase_c(this);
 #endif
-    if (__is_long())
+    if (__libcpp_is_constant_evaluated())
+    {
+        if (__get_long_pointer() != nullptr)
+            __alloc_traits::deallocate(__alloc(), __get_long_pointer(), __get_long_cap());
+    }
+    else if (__is_long())
         __alloc_traits::deallocate(__alloc(), __get_long_pointer(), __get_long_cap());
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 void
 basic_string<_CharT, _Traits, _Allocator>::__grow_by_and_replace
     (size_type __old_cap, size_type __delta_cap, size_type __old_sz,
      size_type __n_copy,  size_type __n_del,     size_type __n_add, const value_type* __p_new_stuff)
 {
     size_type __ms = max_size();
     if (__delta_cap > __ms - __old_cap - 1)
         this->__throw_length_error();
     pointer __old_p = __get_pointer();
     size_type __cap = __old_cap < __ms / 2 - __alignment ?
                           __recommend(_VSTD::max(__old_cap + __delta_cap, 2 * __old_cap)) :
                           __ms - 1;
-    pointer __p = __alloc_traits::allocate(__alloc(), __cap+1);
+    pointer __p = __alloc_traits::allocate(__alloc(), __cap + 1);
+    __begin_lifetime(__p, __cap + 1);
     __invalidate_all_iterators();
     if (__n_copy != 0)
         traits_type::copy(_VSTD::__to_address(__p),
                           _VSTD::__to_address(__old_p), __n_copy);
     if (__n_add != 0)
         traits_type::copy(_VSTD::__to_address(__p) + __n_copy, __p_new_stuff, __n_add);
     size_type __sec_cp_sz = __old_sz - __n_del - __n_copy;
     if (__sec_cp_sz != 0)
         traits_type::copy(_VSTD::__to_address(__p) + __n_copy + __n_add,
                           _VSTD::__to_address(__old_p) + __n_copy + __n_del, __sec_cp_sz);
     if (__old_cap+1 != __min_cap)
         __alloc_traits::deallocate(__alloc(), __old_p, __old_cap+1);
     __set_long_pointer(__p);
     __set_long_cap(__cap+1);
     __old_sz = __n_copy + __n_add + __sec_cp_sz;
     __set_long_size(__old_sz);
     traits_type::assign(__p[__old_sz], value_type());
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 void
 basic_string<_CharT, _Traits, _Allocator>::__grow_by(size_type __old_cap, size_type __delta_cap, size_type __old_sz,
                                                      size_type __n_copy,  size_type __n_del,     size_type __n_add)
 {
     size_type __ms = max_size();
     if (__delta_cap > __ms - __old_cap)
         this->__throw_length_error();
     pointer __old_p = __get_pointer();
     size_type __cap = __old_cap < __ms / 2 - __alignment ?
                           __recommend(_VSTD::max(__old_cap + __delta_cap, 2 * __old_cap)) :
                           __ms - 1;
     pointer __p = __alloc_traits::allocate(__alloc(), __cap+1);
+    __begin_lifetime(__p, __cap + 1);
     __invalidate_all_iterators();
     if (__n_copy != 0)
         traits_type::copy(_VSTD::__to_address(__p),
                           _VSTD::__to_address(__old_p), __n_copy);
     size_type __sec_cp_sz = __old_sz - __n_del - __n_copy;
     if (__sec_cp_sz != 0)
         traits_type::copy(_VSTD::__to_address(__p) + __n_copy + __n_add,
                           _VSTD::__to_address(__old_p) + __n_copy + __n_del,
                           __sec_cp_sz);
     if (__old_cap+1 != __min_cap)
         __alloc_traits::deallocate(__alloc(), __old_p, __old_cap+1);
     __set_long_pointer(__p);
     __set_long_cap(__cap+1);
 }
 
 // assign
 
 template <class _CharT, class _Traits, class _Allocator>
 template <bool __is_short>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::__assign_no_alias(
     const value_type* __s, size_type __n) {
   size_type __cap = __is_short ? __min_cap : __get_long_cap();
   if (__n < __cap) {
     pointer __p = __is_short ? __get_short_pointer() : __get_long_pointer();
     __is_short ? __set_short_size(__n) : __set_long_size(__n);
     traits_type::copy(_VSTD::__to_address(__p), __s, __n);
     traits_type::assign(__p[__n], value_type());
     __invalidate_iterators_past(__n);
   } else {
     size_type __sz = __is_short ? __get_short_size() : __get_long_size();
     __grow_by_and_replace(__cap - 1, __n - __cap + 1, __sz, 0, __sz, __n, __s);
   }
   return *this;
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::__assign_external(
     const value_type* __s, size_type __n) {
   size_type __cap = capacity();
   if (__cap >= __n) {
     value_type* __p = _VSTD::__to_address(__get_pointer());
     traits_type::move(__p, __s, __n);
     traits_type::assign(__p[__n], value_type());
     __set_size(__n);
     __invalidate_iterators_past(__n);
   } else {
     size_type __sz = size();
     __grow_by_and_replace(__cap, __n - __cap, __sz, 0, __sz, __n, __s);
   }
   return *this;
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::assign(const value_type* __s, size_type __n)
 {
     _LIBCPP_ASSERT(__n == 0 || __s != nullptr, "string::assign received nullptr");
-    return (__builtin_constant_p(__n) && __n < __min_cap)
+    return (__builtin_constant_p(__n) && __n < __min_cap && !__libcpp_is_constant_evaluated())
                ? __assign_short(__s, __n)
                : __assign_external(__s, __n);
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::assign(size_type __n, value_type __c)
 {
     size_type __cap = capacity();
     if (__cap < __n)
     {
         size_type __sz = size();
         __grow_by(__cap, __n - __cap, __sz, 0, __sz);
     }
     value_type* __p = _VSTD::__to_address(__get_pointer());
     traits_type::assign(__p, __n, __c);
     traits_type::assign(__p[__n], value_type());
     __set_size(__n);
     __invalidate_iterators_past(__n);
     return *this;
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::operator=(value_type __c)
 {
     pointer __p;
     if (__is_long())
     {
         __p = __get_long_pointer();
         __set_long_size(1);
     }
     else
     {
         __p = __get_short_pointer();
         __set_short_size(1);
     }
     traits_type::assign(*__p, __c);
     traits_type::assign(*++__p, value_type());
     __invalidate_iterators_past(1);
     return *this;
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::operator=(const basic_string& __str)
 {
   if (this != &__str) {
     __copy_assign_alloc(__str);
     if (!__is_long()) {
       if (!__str.__is_long()) {
         __r_.first().__r = __str.__r_.first().__r;
       } else {
         return __assign_no_alias<true>(__str.data(), __str.size());
       }
     } else {
       return __assign_no_alias<false>(__str.data(), __str.size());
     }
   }
   return *this;
 }
 
 #ifndef _LIBCPP_CXX03_LANG
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 void
 basic_string<_CharT, _Traits, _Allocator>::__move_assign(basic_string& __str, false_type)
     _NOEXCEPT_(__alloc_traits::is_always_equal::value)
 {
     if (__alloc() != __str.__alloc())
         assign(__str);
     else
         __move_assign(__str, true_type());
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 void
 basic_string<_CharT, _Traits, _Allocator>::__move_assign(basic_string& __str, true_type)
 #if _LIBCPP_STD_VER > 14
     _NOEXCEPT
 #else
     _NOEXCEPT_(is_nothrow_move_assignable<allocator_type>::value)
 #endif
 {
   if (__is_long()) {
     __alloc_traits::deallocate(__alloc(), __get_long_pointer(),
                                __get_long_cap());
 #if _LIBCPP_STD_VER <= 14
     if (!is_nothrow_move_assignable<allocator_type>::value) {
       __set_short_size(0);
       traits_type::assign(__get_short_pointer()[0], value_type());
     }
 #endif
   }
   __move_assign_alloc(__str);
   __r_.first() = __str.__r_.first();
+  if (__libcpp_is_constant_evaluated())
+      __str.__zero();
+  else
+  {
   __str.__set_short_size(0);
   traits_type::assign(__str.__get_short_pointer()[0], value_type());
   }
+}
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::operator=(basic_string&& __str)
     _NOEXCEPT_((__noexcept_move_assign_container<_Allocator, __alloc_traits>::value))
 {
     __move_assign(__str, integral_constant<bool,
           __alloc_traits::propagate_on_container_move_assignment::value>());
     return *this;
 }
 
 #endif
 
 template <class _CharT, class _Traits, class _Allocator>
 template<class _InputIterator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 __enable_if_t
 <
      __is_exactly_cpp17_input_iterator<_InputIterator>::value,
     basic_string<_CharT, _Traits, _Allocator>&
 >
 basic_string<_CharT, _Traits, _Allocator>::assign(_InputIterator __first, _InputIterator __last)
 {
     const basic_string __temp(__first, __last, __alloc());
     assign(__temp.data(), __temp.size());
     return *this;
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 template<class _ForwardIterator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 __enable_if_t
 <
     __is_cpp17_forward_iterator<_ForwardIterator>::value,
     basic_string<_CharT, _Traits, _Allocator>&
 >
 basic_string<_CharT, _Traits, _Allocator>::assign(_ForwardIterator __first, _ForwardIterator __last)
 {
     size_type __cap = capacity();
basic_string<_CharT, _Traits, _Allocator>::assign(_ForwardIterator __first, _ForwardIterator __last)
     {
         const basic_string __temp(__first, __last, __alloc());
         assign(__temp.data(), __temp.size());
     }
     return *this;
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::assign(const basic_string& __str, size_type __pos, size_type __n)
 {
     size_type __sz = __str.size();
     if (__pos > __sz)
         this->__throw_out_of_range();
     return assign(__str.data() + __pos, _VSTD::min(__n, __sz - __pos));
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 template <class _Tp>
 __enable_if_t
 <
     __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value
     && !__is_same_uncvref<_Tp, basic_string<_CharT, _Traits, _Allocator> >::value,
     basic_string<_CharT, _Traits, _Allocator>&
 >
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>::assign(const _Tp & __t, size_type __pos, size_type __n)
 {
     __self_view __sv = __t;
     size_type __sz = __sv.size();
     if (__pos > __sz)
         this->__throw_out_of_range();
     return assign(__sv.data() + __pos, _VSTD::min(__n, __sz - __pos));
 }
 
 
 template <class _CharT, class _Traits, class _Allocator>
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::__assign_external(const value_type* __s) {
   return __assign_external(__s, traits_type::length(__s));
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::assign(const value_type* __s)
 {
     _LIBCPP_ASSERT(__s != nullptr, "string::assign received nullptr");
     return __builtin_constant_p(*__s)
                ? (traits_type::length(__s) < __min_cap
                       ? __assign_short(__s, traits_type::length(__s))
                       : __assign_external(__s, traits_type::length(__s)))
                : __assign_external(__s);
 }
 // append
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::append(const value_type* __s, size_type __n)
 {
     _LIBCPP_ASSERT(__n == 0 || __s != nullptr, "string::append received nullptr");
     size_type __cap = capacity();
     size_type __sz = size();
     if (__cap - __sz >= __n)
     {
         if (__n)
         {
             value_type* __p = _VSTD::__to_address(__get_pointer());
             traits_type::copy(__p + __sz, __s, __n);
             __sz += __n;
             __set_size(__sz);
             traits_type::assign(__p[__sz], value_type());
         }
     }
     else
         __grow_by_and_replace(__cap, __sz + __n - __cap, __sz, __sz, 0, __n, __s);
     return *this;
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::append(size_type __n, value_type __c)
 {
     if (__n)
     {
         size_type __cap = capacity();
         size_type __sz = size();
         if (__cap - __sz < __n)
if (__n)
         pointer __p = __get_pointer();
         __sz += __n;
         __set_size(__sz);
         traits_type::assign(__p[__sz], value_type());
     }
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 void
 basic_string<_CharT, _Traits, _Allocator>::push_back(value_type __c)
 {
     bool __is_short = !__is_long();
     size_type __cap;
     size_type __sz;
     if (__is_short)
     {
else
         __set_long_size(__sz+1);
     }
     traits_type::assign(*__p, __c);
     traits_type::assign(*++__p, value_type());
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 template<class _ForwardIterator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 __enable_if_t
 <
     __is_cpp17_forward_iterator<_ForwardIterator>::value,
     basic_string<_CharT, _Traits, _Allocator>&
 >
 basic_string<_CharT, _Traits, _Allocator>::append(
     _ForwardIterator __first, _ForwardIterator __last)
 {
if (__n)
             const basic_string __temp(__first, __last, __alloc());
             append(__temp.data(), __temp.size());
         }
     }
     return *this;
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::append(const basic_string& __str)
 {
     return append(__str.data(), __str.size());
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::append(const basic_string& __str, size_type __pos, size_type __n)
 {
     size_type __sz = __str.size();
     if (__pos > __sz)
         this->__throw_out_of_range();
     return append(__str.data() + __pos, _VSTD::min(__n, __sz - __pos));
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 template <class _Tp>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
     __enable_if_t
     <
         __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value  && !__is_same_uncvref<_Tp, basic_string<_CharT, _Traits, _Allocator> >::value,
         basic_string<_CharT, _Traits, _Allocator>&
     >
 basic_string<_CharT, _Traits, _Allocator>::append(const _Tp & __t, size_type __pos, size_type __n)
 {
     __self_view __sv = __t;
     size_type __sz = __sv.size();
     if (__pos > __sz)
         this->__throw_out_of_range();
     return append(__sv.data() + __pos, _VSTD::min(__n, __sz - __pos));
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::append(const value_type* __s)
 {
     _LIBCPP_ASSERT(__s != nullptr, "string::append received nullptr");
     return append(__s, traits_type::length(__s));
 }
 
 // insert
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::insert(size_type __pos, const value_type* __s, size_type __n)
 {
     _LIBCPP_ASSERT(__n == 0 || __s != nullptr, "string::insert received nullptr");
     size_type __sz = size();
     if (__pos > __sz)
         this->__throw_out_of_range();
     size_type __cap = capacity();
     if (__cap - __sz >= __n)
     {
         if (__n)
         {
             value_type* __p = _VSTD::__to_address(__get_pointer());
             size_type __n_move = __sz - __pos;
             if (__n_move != 0)
             {
                 if (__p + __pos <= __s && __s < __p + __sz)
                     __s += __n;
                 traits_type::move(__p + __pos + __n, __p + __pos, __n_move);
             }
             traits_type::move(__p + __pos, __s, __n);
             __sz += __n;
             __set_size(__sz);
             traits_type::assign(__p[__sz], value_type());
         }
     }
     else
         __grow_by_and_replace(__cap, __sz + __n - __cap, __sz, __pos, 0, __n, __s);
     return *this;
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::insert(size_type __pos, size_type __n, value_type __c)
 {
     size_type __sz = size();
     if (__pos > __sz)
         this->__throw_out_of_range();
     if (__n)
     {
if (__n)
         __set_size(__sz);
         traits_type::assign(__p[__sz], value_type());
     }
     return *this;
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 template<class _InputIterator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 __enable_if_t
 <
    __is_exactly_cpp17_input_iterator<_InputIterator>::value,
    typename basic_string<_CharT, _Traits, _Allocator>::iterator
 >
 basic_string<_CharT, _Traits, _Allocator>::insert(const_iterator __pos, _InputIterator __first, _InputIterator __last)
 {
 #if _LIBCPP_DEBUG_LEVEL == 2
     _LIBCPP_ASSERT(__get_const_db()->__find_c_from_i(&__pos) == this,
         "string::insert(iterator, range) called with an iterator not"
         " referring to this string");
 #endif
     const basic_string __temp(__first, __last, __alloc());
     return insert(__pos, __temp.data(), __temp.data() + __temp.size());
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 template<class _ForwardIterator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 __enable_if_t
 <
     __is_cpp17_forward_iterator<_ForwardIterator>::value,
     typename basic_string<_CharT, _Traits, _Allocator>::iterator
 >
 basic_string<_CharT, _Traits, _Allocator>::insert(const_iterator __pos, _ForwardIterator __first, _ForwardIterator __last)
 {
 #if _LIBCPP_DEBUG_LEVEL == 2
if (__n)
             const basic_string __temp(__first, __last, __alloc());
             return insert(__pos, __temp.data(), __temp.data() + __temp.size());
         }
     }
     return begin() + __ip;
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::insert(size_type __pos1, const basic_string& __str)
 {
     return insert(__pos1, __str.data(), __str.size());
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::insert(size_type __pos1, const basic_string& __str,
                                                   size_type __pos2, size_type __n)
 {
     size_type __str_sz = __str.size();
     if (__pos2 > __str_sz)
         this->__throw_out_of_range();
     return insert(__pos1, __str.data() + __pos2, _VSTD::min(__n, __str_sz - __pos2));
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 template <class _Tp>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 __enable_if_t
 <
     __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value  && !__is_same_uncvref<_Tp, basic_string<_CharT, _Traits, _Allocator> >::value,
     basic_string<_CharT, _Traits, _Allocator>&
 >
 basic_string<_CharT, _Traits, _Allocator>::insert(size_type __pos1, const _Tp& __t,
                                                   size_type __pos2, size_type __n)
 {
     __self_view __sv = __t;
     size_type __str_sz = __sv.size();
     if (__pos2 > __str_sz)
         this->__throw_out_of_range();
     return insert(__pos1, __sv.data() + __pos2, _VSTD::min(__n, __str_sz - __pos2));
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::insert(size_type __pos, const value_type* __s)
 {
     _LIBCPP_ASSERT(__s != nullptr, "string::insert received nullptr");
     return insert(__pos, __s, traits_type::length(__s));
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::iterator
 basic_string<_CharT, _Traits, _Allocator>::insert(const_iterator __pos, value_type __c)
 {
     size_type __ip = static_cast<size_type>(__pos - begin());
     size_type __sz = size();
     size_type __cap = capacity();
     value_type* __p;
     if (__cap == __sz)
basic_string<_CharT, _Traits, _Allocator>::insert(const_iterator __pos, value_type __c)
     }
     traits_type::assign(__p[__ip], __c);
     traits_type::assign(__p[++__sz], value_type());
     __set_size(__sz);
     return begin() + static_cast<difference_type>(__ip);
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::iterator
 basic_string<_CharT, _Traits, _Allocator>::insert(const_iterator __pos, size_type __n, value_type __c)
 {
 #if _LIBCPP_DEBUG_LEVEL == 2
     _LIBCPP_ASSERT(__get_const_db()->__find_c_from_i(&__pos) == this,
         "string::insert(iterator, n, value) called with an iterator not"
         " referring to this string");
 #endif
     difference_type __p = __pos - begin();
     insert(static_cast<size_type>(__p), __n, __c);
     return begin() + __p;
 }
 
 // replace
 
-template <class _CharT, class _Traits, class _Allocator>
+template<class _CharT, class _Traits, class _Allocator>
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
+void
+basic_string<_CharT, _Traits, _Allocator>::__finish_replace(size_type& __sz, size_type& __n1, size_type& __n2, value_type*& __p)
+{
+        __sz += __n2 - __n1;
+        __set_size(__sz);
+        __invalidate_iterators_past(__sz);
+        traits_type::assign(__p[__sz], value_type());
+}
+
+template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::replace(size_type __pos, size_type __n1, const value_type* __s, size_type __n2)
     _LIBCPP_DISABLE_UBSAN_UNSIGNED_INTEGER_CHECK
 {
     _LIBCPP_ASSERT(__n2 == 0 || __s != nullptr, "string::replace received nullptr");
     size_type __sz = size();
     if (__pos > __sz)
         this->__throw_out_of_range();
     __n1 = _VSTD::min(__n1, __sz - __pos);
     size_type __cap = capacity();
     if (__cap - __sz + __n1 >= __n2)
     {
         value_type* __p = _VSTD::__to_address(__get_pointer());
         if (__n1 != __n2)
         {
             size_type __n_move = __sz - __pos - __n1;
             if (__n_move != 0)
             {
                 if (__n1 > __n2)
                 {
                     traits_type::move(__p + __pos, __s, __n2);
                     traits_type::move(__p + __pos + __n2, __p + __pos + __n1, __n_move);
-                    goto __finish;
+                    __finish_replace(__sz, __n1, __n2, __p);
+                    return *this;
                 }
                 if (__p + __pos < __s && __s < __p + __sz)
                 {
                     if (__p + __pos + __n1 <= __s)
                         __s += __n2 - __n1;
                     else // __p + __pos < __s < __p + __pos + __n1
                     {
                         traits_type::move(__p + __pos, __s, __n1);
                         __pos += __n1;
                         __s += __n2;
                         __n2 -= __n1;
                         __n1 = 0;
                     }
                 }
                 traits_type::move(__p + __pos + __n2, __p + __pos + __n1, __n_move);
             }
         }
         traits_type::move(__p + __pos, __s, __n2);
-__finish:
 // __sz += __n2 - __n1; in this and the below function below can cause unsigned
 // integer overflow, but this is a safe operation, so we disable the check.
-        __sz += __n2 - __n1;
-        __set_size(__sz);
-        __invalidate_iterators_past(__sz);
-        traits_type::assign(__p[__sz], value_type());
+        __finish_replace(__sz, __n1, __n2, __p);
     }
     else
         __grow_by_and_replace(__cap, __sz - __n1 + __n2 - __cap, __sz, __pos, __n1, __n2, __s);
     return *this;
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::replace(size_type __pos, size_type __n1, size_type __n2, value_type __c)
     _LIBCPP_DISABLE_UBSAN_UNSIGNED_INTEGER_CHECK
 {
     size_type __sz = size();
     if (__pos > __sz)
         this->__throw_out_of_range();
     __n1 = _VSTD::min(__n1, __sz - __pos);
basic_string<_CharT, _Traits, _Allocator>::replace(size_type __pos, size_type __n1, size_type __n2, value_type __c)
     __set_size(__sz);
     __invalidate_iterators_past(__sz);
     traits_type::assign(__p[__sz], value_type());
     return *this;
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 template<class _InputIterator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 __enable_if_t
 <
     __is_cpp17_input_iterator<_InputIterator>::value,
     basic_string<_CharT, _Traits, _Allocator>&
 >
 basic_string<_CharT, _Traits, _Allocator>::replace(const_iterator __i1, const_iterator __i2,
                                                    _InputIterator __j1, _InputIterator __j2)
 {
     const basic_string __temp(__j1, __j2, __alloc());
     return this->replace(__i1, __i2, __temp);
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::replace(size_type __pos1, size_type __n1, const basic_string& __str)
 {
     return replace(__pos1, __n1, __str.data(), __str.size());
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::replace(size_type __pos1, size_type __n1, const basic_string& __str,
                                                    size_type __pos2, size_type __n2)
 {
     size_type __str_sz = __str.size();
     if (__pos2 > __str_sz)
         this->__throw_out_of_range();
     return replace(__pos1, __n1, __str.data() + __pos2, _VSTD::min(__n2, __str_sz - __pos2));
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 template <class _Tp>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 __enable_if_t
 <
     __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value && !__is_same_uncvref<_Tp, basic_string<_CharT, _Traits, _Allocator> >::value,
     basic_string<_CharT, _Traits, _Allocator>&
 >
 basic_string<_CharT, _Traits, _Allocator>::replace(size_type __pos1, size_type __n1, const _Tp& __t,
                                                    size_type __pos2, size_type __n2)
 {
     __self_view __sv = __t;
     size_type __str_sz = __sv.size();
     if (__pos2 > __str_sz)
         this->__throw_out_of_range();
     return replace(__pos1, __n1, __sv.data() + __pos2, _VSTD::min(__n2, __str_sz - __pos2));
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::replace(size_type __pos, size_type __n1, const value_type* __s)
 {
     _LIBCPP_ASSERT(__s != nullptr, "string::replace received nullptr");
     return replace(__pos, __n1, __s, traits_type::length(__s));
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::replace(const_iterator __i1, const_iterator __i2, const basic_string& __str)
 {
     return replace(static_cast<size_type>(__i1 - begin()), static_cast<size_type>(__i2 - __i1),
                    __str.data(), __str.size());
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::replace(const_iterator __i1, const_iterator __i2, const value_type* __s, size_type __n)
 {
     return replace(static_cast<size_type>(__i1 - begin()), static_cast<size_type>(__i2 - __i1), __s, __n);
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::replace(const_iterator __i1, const_iterator __i2, const value_type* __s)
 {
     return replace(static_cast<size_type>(__i1 - begin()), static_cast<size_type>(__i2 - __i1), __s);
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::replace(const_iterator __i1, const_iterator __i2, size_type __n, value_type __c)
 {
     return replace(static_cast<size_type>(__i1 - begin()), static_cast<size_type>(__i2 - __i1), __n, __c);
 }
 
 // erase
 
if (__n)
         __sz -= __n;
         __set_size(__sz);
         __invalidate_iterators_past(__sz);
         traits_type::assign(__p[__sz], value_type());
     }
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::erase(size_type __pos,
                                                  size_type __n) {
   if (__pos > size()) this->__throw_out_of_range();
   if (__n == npos) {
     __erase_to_end(__pos);
   } else {
     __erase_external_with_move(__pos, __n);
   }
   return *this;
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::iterator
 basic_string<_CharT, _Traits, _Allocator>::erase(const_iterator __pos)
 {
 #if _LIBCPP_DEBUG_LEVEL == 2
     _LIBCPP_ASSERT(__get_const_db()->__find_c_from_i(&__pos) == this,
         "string::erase(iterator) called with an iterator not"
         " referring to this string");
 #endif
     _LIBCPP_ASSERT(__pos != end(),
         "string::erase(iterator) called with a non-dereferenceable iterator");
     iterator __b = begin();
     size_type __r = static_cast<size_type>(__pos - __b);
     erase(__r, 1);
     return __b + static_cast<difference_type>(__r);
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::iterator
 basic_string<_CharT, _Traits, _Allocator>::erase(const_iterator __first, const_iterator __last)
 {
 #if _LIBCPP_DEBUG_LEVEL == 2
     _LIBCPP_ASSERT(__get_const_db()->__find_c_from_i(&__first) == this,
         "string::erase(iterator,  iterator) called with an iterator not"
         " referring to this string");
 #endif
     _LIBCPP_ASSERT(__first <= __last, "string::erase(first, last) called with invalid range");
     iterator __b = begin();
     size_type __r = static_cast<size_type>(__first - __b);
     erase(__r, static_cast<size_type>(__last - __first));
     return __b + static_cast<difference_type>(__r);
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 void
 basic_string<_CharT, _Traits, _Allocator>::pop_back()
 {
     _LIBCPP_ASSERT(!empty(), "string::pop_back(): string is already empty");
     size_type __sz;
     if (__is_long())
     {
         __sz = __get_long_size() - 1;
         __set_long_size(__sz);
         traits_type::assign(*(__get_long_pointer() + __sz), value_type());
     }
     else
     {
         __sz = __get_short_size() - 1;
         __set_short_size(__sz);
         traits_type::assign(*(__get_short_pointer() + __sz), value_type());
     }
     __invalidate_iterators_past(__sz);
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 void
 basic_string<_CharT, _Traits, _Allocator>::clear() _NOEXCEPT
 {
     __invalidate_all_iterators();
     if (__is_long())
     {
         traits_type::assign(*__get_long_pointer(), value_type());
         __set_long_size(0);
     }
     else
     {
         traits_type::assign(*__get_short_pointer(), value_type());
         __set_short_size(0);
     }
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 void
 basic_string<_CharT, _Traits, _Allocator>::__erase_to_end(size_type __pos)
 {
     if (__is_long())
     {
         traits_type::assign(*(__get_long_pointer() + __pos), value_type());
         __set_long_size(__pos);
     }
     else
     {
         traits_type::assign(*(__get_short_pointer() + __pos), value_type());
         __set_short_size(__pos);
     }
     __invalidate_iterators_past(__pos);
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 void
 basic_string<_CharT, _Traits, _Allocator>::resize(size_type __n, value_type __c)
 {
     size_type __sz = size();
     if (__n > __sz)
         append(__n - __sz, __c);
     else
         __erase_to_end(__n);
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 inline void
 basic_string<_CharT, _Traits, _Allocator>::__resize_default_init(size_type __n)
 {
     size_type __sz = size();
     if (__n > __sz) {
        __append_default_init(__n - __sz);
     } else
         __erase_to_end(__n);
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::max_size() const _NOEXCEPT
 {
     size_type __m = __alloc_traits::max_size(__alloc());
 #ifdef _LIBCPP_BIG_ENDIAN
     return (__m <= ~__long_mask ? __m : __m/2) - __alignment;
 #else
     return __m - __alignment;
 #endif
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 void
 basic_string<_CharT, _Traits, _Allocator>::reserve(size_type __requested_capacity)
 {
     if (__requested_capacity > max_size())
         this->__throw_length_error();
 
 #if _LIBCPP_STD_VER > 17
     // Reserve never shrinks as of C++20.
     if (__requested_capacity <= capacity()) return;
 #endif
 
     size_type __target_capacity = _VSTD::max(__requested_capacity, size());
     __target_capacity = __recommend(__target_capacity);
     if (__target_capacity == capacity()) return;
 
     __shrink_or_extend(__target_capacity);
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 void
 basic_string<_CharT, _Traits, _Allocator>::shrink_to_fit() _NOEXCEPT
 {
     size_type __target_capacity = __recommend(size());
     if (__target_capacity == capacity()) return;
 
     __shrink_or_extend(__target_capacity);
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 void
 basic_string<_CharT, _Traits, _Allocator>::__shrink_or_extend(size_type __target_capacity)
 {
     size_type __cap = capacity();
     size_type __sz = size();
 
     pointer __new_data, __p;
     bool __was_long, __now_long;
-    if (__target_capacity == __min_cap - 1)
+    if (__target_capacity == __min_cap - 1 && !__libcpp_is_constant_evaluated())
     {
         __was_long = true;
         __now_long = false;
         __new_data = __get_short_pointer();
         __p = __get_long_pointer();
     }
     else
     {
         if (__target_capacity > __cap)
+        {
             __new_data = __alloc_traits::allocate(__alloc(), __target_capacity+1);
+            __begin_lifetime(__new_data, __target_capacity + 1);
+        }
         else
         {
         #ifndef _LIBCPP_NO_EXCEPTIONS
             try
             {
         #endif // _LIBCPP_NO_EXCEPTIONS
                 __new_data = __alloc_traits::allocate(__alloc(), __target_capacity+1);
+                __begin_lifetime(__new_data, __target_capacity + 1);
         #ifndef _LIBCPP_NO_EXCEPTIONS
             }
             catch (...)
             {
                 return;
             }
         #else  // _LIBCPP_NO_EXCEPTIONS
             if (__new_data == nullptr)
if (__now_long)
         __set_long_pointer(__new_data);
     }
     else
         __set_short_size(__sz);
     __invalidate_all_iterators();
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::const_reference
 basic_string<_CharT, _Traits, _Allocator>::operator[](size_type __pos) const _NOEXCEPT
 {
     _LIBCPP_ASSERT(__pos <= size(), "string index out of bounds");
     return *(data() + __pos);
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::reference
 basic_string<_CharT, _Traits, _Allocator>::operator[](size_type __pos) _NOEXCEPT
 {
     _LIBCPP_ASSERT(__pos <= size(), "string index out of bounds");
     return *(__get_pointer() + __pos);
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::const_reference
 basic_string<_CharT, _Traits, _Allocator>::at(size_type __n) const
 {
     if (__n >= size())
         this->__throw_out_of_range();
     return (*this)[__n];
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::reference
 basic_string<_CharT, _Traits, _Allocator>::at(size_type __n)
 {
     if (__n >= size())
         this->__throw_out_of_range();
     return (*this)[__n];
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::reference
 basic_string<_CharT, _Traits, _Allocator>::front() _NOEXCEPT
 {
     _LIBCPP_ASSERT(!empty(), "string::front(): string is empty");
     return *__get_pointer();
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::const_reference
 basic_string<_CharT, _Traits, _Allocator>::front() const _NOEXCEPT
 {
     _LIBCPP_ASSERT(!empty(), "string::front(): string is empty");
     return *data();
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::reference
 basic_string<_CharT, _Traits, _Allocator>::back() _NOEXCEPT
 {
     _LIBCPP_ASSERT(!empty(), "string::back(): string is empty");
     return *(__get_pointer() + size() - 1);
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::const_reference
 basic_string<_CharT, _Traits, _Allocator>::back() const _NOEXCEPT
 {
     _LIBCPP_ASSERT(!empty(), "string::back(): string is empty");
     return *(data() + size() - 1);
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::copy(value_type* __s, size_type __n, size_type __pos) const
 {
     size_type __sz = size();
     if (__pos > __sz)
         this->__throw_out_of_range();
     size_type __rlen = _VSTD::min(__n, __sz - __pos);
     traits_type::copy(__s, data() + __pos, __rlen);
     return __rlen;
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>
 basic_string<_CharT, _Traits, _Allocator>::substr(size_type __pos, size_type __n) const
 {
     return basic_string(*this, __pos, __n, __alloc());
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 void
 basic_string<_CharT, _Traits, _Allocator>::swap(basic_string& __str)
 #if _LIBCPP_STD_VER >= 14
         _NOEXCEPT
 #else
         _NOEXCEPT_(!__alloc_traits::propagate_on_container_swap::value ||
                     __is_nothrow_swappable<allocator_type>::value)
 #endif

 {
     typedef typename _Traits::char_type char_type;
     _LIBCPP_INLINE_VISIBILITY
     bool operator()(const char_type& __x, const char_type& __y) _NOEXCEPT
         {return _Traits::eq(__x, __y);}
 };
 
 template<class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::find(const value_type* __s,
                                                 size_type __pos,
                                                 size_type __n) const _NOEXCEPT
 {
     _LIBCPP_ASSERT(__n == 0 || __s != nullptr, "string::find(): received nullptr");
     return __str_find<value_type, size_type, traits_type, npos>
         (data(), size(), __s, __pos, __n);
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::find(const basic_string& __str,
                                                 size_type __pos) const _NOEXCEPT
 {
     return __str_find<value_type, size_type, traits_type, npos>
         (data(), size(), __str.data(), __pos, __str.size());
 }
 
 template<class _CharT, class _Traits, class _Allocator>
 template <class _Tp>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 __enable_if_t
 <
     __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
     typename basic_string<_CharT, _Traits, _Allocator>::size_type
 >
 basic_string<_CharT, _Traits, _Allocator>::find(const _Tp &__t,
                                                 size_type __pos) const _NOEXCEPT
 {
     __self_view __sv = __t;
     return __str_find<value_type, size_type, traits_type, npos>
         (data(), size(), __sv.data(), __pos, __sv.size());
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::find(const value_type* __s,
                                                 size_type __pos) const _NOEXCEPT
 {
     _LIBCPP_ASSERT(__s != nullptr, "string::find(): received nullptr");
     return __str_find<value_type, size_type, traits_type, npos>
         (data(), size(), __s, __pos, traits_type::length(__s));
 }
 
 template<class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::find(value_type __c,
                                                 size_type __pos) const _NOEXCEPT
 {
     return __str_find<value_type, size_type, traits_type, npos>
         (data(), size(), __c, __pos);
 }
 
 // rfind
 
 template<class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::rfind(const value_type* __s,
                                                  size_type __pos,
                                                  size_type __n) const _NOEXCEPT
 {
     _LIBCPP_ASSERT(__n == 0 || __s != nullptr, "string::rfind(): received nullptr");
     return __str_rfind<value_type, size_type, traits_type, npos>
         (data(), size(), __s, __pos, __n);
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::rfind(const basic_string& __str,
                                                  size_type __pos) const _NOEXCEPT
 {
     return __str_rfind<value_type, size_type, traits_type, npos>
         (data(), size(), __str.data(), __pos, __str.size());
 }
 
 template<class _CharT, class _Traits, class _Allocator>
 template <class _Tp>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 __enable_if_t
 <
     __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
     typename basic_string<_CharT, _Traits, _Allocator>::size_type
 >
 basic_string<_CharT, _Traits, _Allocator>::rfind(const _Tp& __t,
                                                 size_type __pos) const _NOEXCEPT
 {
     __self_view __sv = __t;
     return __str_rfind<value_type, size_type, traits_type, npos>
         (data(), size(), __sv.data(), __pos, __sv.size());
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::rfind(const value_type* __s,
                                                  size_type __pos) const _NOEXCEPT
 {
     _LIBCPP_ASSERT(__s != nullptr, "string::rfind(): received nullptr");
     return __str_rfind<value_type, size_type, traits_type, npos>
         (data(), size(), __s, __pos, traits_type::length(__s));
 }
 
 template<class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::rfind(value_type __c,
                                                  size_type __pos) const _NOEXCEPT
 {
     return __str_rfind<value_type, size_type, traits_type, npos>
         (data(), size(), __c, __pos);
 }
 
 // find_first_of
 
 template<class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::find_first_of(const value_type* __s,
                                                          size_type __pos,
                                                          size_type __n) const _NOEXCEPT
 {
     _LIBCPP_ASSERT(__n == 0 || __s != nullptr, "string::find_first_of(): received nullptr");
     return __str_find_first_of<value_type, size_type, traits_type, npos>
         (data(), size(), __s, __pos, __n);
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::find_first_of(const basic_string& __str,
                                                          size_type __pos) const _NOEXCEPT
 {
     return __str_find_first_of<value_type, size_type, traits_type, npos>
         (data(), size(), __str.data(), __pos, __str.size());
 }
 
 template<class _CharT, class _Traits, class _Allocator>
 template <class _Tp>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 __enable_if_t
 <
     __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
     typename basic_string<_CharT, _Traits, _Allocator>::size_type
 >
 basic_string<_CharT, _Traits, _Allocator>::find_first_of(const _Tp& __t,
                                                 size_type __pos) const _NOEXCEPT
 {
     __self_view __sv = __t;
     return __str_find_first_of<value_type, size_type, traits_type, npos>
         (data(), size(), __sv.data(), __pos, __sv.size());
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::find_first_of(const value_type* __s,
                                                          size_type __pos) const _NOEXCEPT
 {
     _LIBCPP_ASSERT(__s != nullptr, "string::find_first_of(): received nullptr");
     return __str_find_first_of<value_type, size_type, traits_type, npos>
         (data(), size(), __s, __pos, traits_type::length(__s));
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::find_first_of(value_type __c,
                                                          size_type __pos) const _NOEXCEPT
 {
     return find(__c, __pos);
 }
 
 // find_last_of
 
 template<class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::find_last_of(const value_type* __s,
                                                         size_type __pos,
                                                         size_type __n) const _NOEXCEPT
 {
     _LIBCPP_ASSERT(__n == 0 || __s != nullptr, "string::find_last_of(): received nullptr");
     return __str_find_last_of<value_type, size_type, traits_type, npos>
         (data(), size(), __s, __pos, __n);
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::find_last_of(const basic_string& __str,
                                                         size_type __pos) const _NOEXCEPT
 {
     return __str_find_last_of<value_type, size_type, traits_type, npos>
         (data(), size(), __str.data(), __pos, __str.size());
 }
 
 template<class _CharT, class _Traits, class _Allocator>
 template <class _Tp>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 __enable_if_t
 <
     __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
     typename basic_string<_CharT, _Traits, _Allocator>::size_type
 >
 basic_string<_CharT, _Traits, _Allocator>::find_last_of(const _Tp& __t,
                                                 size_type __pos) const _NOEXCEPT
 {
     __self_view __sv = __t;
     return __str_find_last_of<value_type, size_type, traits_type, npos>
         (data(), size(), __sv.data(), __pos, __sv.size());
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::find_last_of(const value_type* __s,
                                                         size_type __pos) const _NOEXCEPT
 {
     _LIBCPP_ASSERT(__s != nullptr, "string::find_last_of(): received nullptr");
     return __str_find_last_of<value_type, size_type, traits_type, npos>
         (data(), size(), __s, __pos, traits_type::length(__s));
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::find_last_of(value_type __c,
                                                         size_type __pos) const _NOEXCEPT
 {
     return rfind(__c, __pos);
 }
 
 // find_first_not_of
 
 template<class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::find_first_not_of(const value_type* __s,
                                                              size_type __pos,
                                                              size_type __n) const _NOEXCEPT
 {
     _LIBCPP_ASSERT(__n == 0 || __s != nullptr, "string::find_first_not_of(): received nullptr");
     return __str_find_first_not_of<value_type, size_type, traits_type, npos>
         (data(), size(), __s, __pos, __n);
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::find_first_not_of(const basic_string& __str,
                                                              size_type __pos) const _NOEXCEPT
 {
     return __str_find_first_not_of<value_type, size_type, traits_type, npos>
         (data(), size(), __str.data(), __pos, __str.size());
 }
 
 template<class _CharT, class _Traits, class _Allocator>
 template <class _Tp>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 __enable_if_t
 <
     __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
     typename basic_string<_CharT, _Traits, _Allocator>::size_type
 >
 basic_string<_CharT, _Traits, _Allocator>::find_first_not_of(const _Tp& __t,
                                                 size_type __pos) const _NOEXCEPT
 {
     __self_view __sv = __t;
     return __str_find_first_not_of<value_type, size_type, traits_type, npos>
         (data(), size(), __sv.data(), __pos, __sv.size());
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::find_first_not_of(const value_type* __s,
                                                              size_type __pos) const _NOEXCEPT
 {
     _LIBCPP_ASSERT(__s != nullptr, "string::find_first_not_of(): received nullptr");
     return __str_find_first_not_of<value_type, size_type, traits_type, npos>
         (data(), size(), __s, __pos, traits_type::length(__s));
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::find_first_not_of(value_type __c,
                                                              size_type __pos) const _NOEXCEPT
 {
     return __str_find_first_not_of<value_type, size_type, traits_type, npos>
         (data(), size(), __c, __pos);
 }
 
 // find_last_not_of
 
 template<class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::find_last_not_of(const value_type* __s,
                                                             size_type __pos,
                                                             size_type __n) const _NOEXCEPT
 {
     _LIBCPP_ASSERT(__n == 0 || __s != nullptr, "string::find_last_not_of(): received nullptr");
     return __str_find_last_not_of<value_type, size_type, traits_type, npos>
         (data(), size(), __s, __pos, __n);
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::find_last_not_of(const basic_string& __str,
                                                             size_type __pos) const _NOEXCEPT
 {
     return __str_find_last_not_of<value_type, size_type, traits_type, npos>
         (data(), size(), __str.data(), __pos, __str.size());
 }
 
 template<class _CharT, class _Traits, class _Allocator>
 template <class _Tp>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 __enable_if_t
 <
     __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
     typename basic_string<_CharT, _Traits, _Allocator>::size_type
 >
 basic_string<_CharT, _Traits, _Allocator>::find_last_not_of(const _Tp& __t,
                                                 size_type __pos) const _NOEXCEPT
 {
     __self_view __sv = __t;
     return __str_find_last_not_of<value_type, size_type, traits_type, npos>
         (data(), size(), __sv.data(), __pos, __sv.size());
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::find_last_not_of(const value_type* __s,
                                                             size_type __pos) const _NOEXCEPT
 {
     _LIBCPP_ASSERT(__s != nullptr, "string::find_last_not_of(): received nullptr");
     return __str_find_last_not_of<value_type, size_type, traits_type, npos>
         (data(), size(), __s, __pos, traits_type::length(__s));
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 typename basic_string<_CharT, _Traits, _Allocator>::size_type
 basic_string<_CharT, _Traits, _Allocator>::find_last_not_of(value_type __c,
                                                             size_type __pos) const _NOEXCEPT
 {
     return __str_find_last_not_of<value_type, size_type, traits_type, npos>
         (data(), size(), __c, __pos);
 }
 
 // compare
 
 template <class _CharT, class _Traits, class _Allocator>
 template <class _Tp>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 __enable_if_t
 <
     __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
     int
 >
 basic_string<_CharT, _Traits, _Allocator>::compare(const _Tp& __t) const _NOEXCEPT
 {
     __self_view __sv = __t;
     size_t __lhs_sz = size();
     size_t __rhs_sz = __sv.size();
     int __result = traits_type::compare(data(), __sv.data(),
                                         _VSTD::min(__lhs_sz, __rhs_sz));
     if (__result != 0)
         return __result;
     if (__lhs_sz < __rhs_sz)
         return -1;
     if (__lhs_sz > __rhs_sz)
         return 1;
     return 0;
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 int
 basic_string<_CharT, _Traits, _Allocator>::compare(const basic_string& __str) const _NOEXCEPT
 {
     return compare(__self_view(__str));
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 int
 basic_string<_CharT, _Traits, _Allocator>::compare(size_type __pos1,
                                                    size_type __n1,
                                                    const value_type* __s,
                                                    size_type __n2) const
 {
     _LIBCPP_ASSERT(__n2 == 0 || __s != nullptr, "string::compare(): received nullptr");
     size_type __sz = size();
     if (__pos1 > __sz || __n2 == npos)
         this->__throw_out_of_range();
     size_type __rlen = _VSTD::min(__n1, __sz - __pos1);
     int __r = traits_type::compare(data() + __pos1, __s, _VSTD::min(__rlen, __n2));
     if (__r == 0)
     {
         if (__rlen < __n2)
             __r = -1;
         else if (__rlen > __n2)
             __r = 1;
     }
     return __r;
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 template <class _Tp>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 __enable_if_t
 <
     __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
     int
 >
 basic_string<_CharT, _Traits, _Allocator>::compare(size_type __pos1,
                                                    size_type __n1,
                                                    const _Tp& __t) const
 {
     __self_view __sv = __t;
     return compare(__pos1, __n1, __sv.data(), __sv.size());
 }
 
 template <class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 int
 basic_string<_CharT, _Traits, _Allocator>::compare(size_type __pos1,
                                                    size_type __n1,
                                                    const basic_string& __str) const
 {
     return compare(__pos1, __n1, __str.data(), __str.size());
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 template <class _Tp>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 __enable_if_t
 <
     __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value
     && !__is_same_uncvref<_Tp, basic_string<_CharT, _Traits, _Allocator> >::value,
     int
 >
 basic_string<_CharT, _Traits, _Allocator>::compare(size_type __pos1,
                                                    size_type __n1,
                                                    const _Tp& __t,
                                                    size_type __pos2,
                                                    size_type __n2) const
 {
     __self_view __sv = __t;
     return __self_view(*this).substr(__pos1, __n1).compare(__sv.substr(__pos2, __n2));
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 int
 basic_string<_CharT, _Traits, _Allocator>::compare(size_type __pos1,
                                                    size_type __n1,
                                                    const basic_string& __str,
                                                    size_type __pos2,
                                                    size_type __n2) const
 {
         return compare(__pos1, __n1, __self_view(__str), __pos2, __n2);
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 int
 basic_string<_CharT, _Traits, _Allocator>::compare(const value_type* __s) const _NOEXCEPT
 {
     _LIBCPP_ASSERT(__s != nullptr, "string::compare(): received nullptr");
     return compare(0, npos, __s, traits_type::length(__s));
 }
 
 template <class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 int
 basic_string<_CharT, _Traits, _Allocator>::compare(size_type __pos1,
                                                    size_type __n1,
                                                    const value_type* __s) const
 {
     _LIBCPP_ASSERT(__s != nullptr, "string::compare(): received nullptr");
     return compare(__pos1, __n1, __s, traits_type::length(__s));
 }
 
 // __invariants
 
 template<class _CharT, class _Traits, class _Allocator>
 inline
 bool
 basic_string<_CharT, _Traits, _Allocator>::__invariants() const
 {
     if (size() > capacity())
         return false;
     if (capacity() < __min_cap - 1)
         return false;
     if (data() == nullptr)
         return false;
     if (data()[size()] != value_type())
         return false;
     return true;
 }
 
 // __clear_and_shrink
 
 template<class _CharT, class _Traits, class _Allocator>
 inline
 void
 basic_string<_CharT, _Traits, _Allocator>::__clear_and_shrink() _NOEXCEPT
 {
     clear();
     if(__is_long())
     {
         __alloc_traits::deallocate(__alloc(), __get_long_pointer(), capacity() + 1);
         __set_long_cap(0);
         __set_short_size(0);
         traits_type::assign(*__get_short_pointer(), value_type());
     }
 }
 
 // operator==
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 bool
 operator==(const basic_string<_CharT, _Traits, _Allocator>& __lhs,
            const basic_string<_CharT, _Traits, _Allocator>& __rhs) _NOEXCEPT
 {
     size_t __lhs_sz = __lhs.size();
     return __lhs_sz == __rhs.size() && _Traits::compare(__lhs.data(),
                                                         __rhs.data(),
                                                         __lhs_sz) == 0;
 }
 
 template<class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 bool
 operator==(const basic_string<char, char_traits<char>, _Allocator>& __lhs,
            const basic_string<char, char_traits<char>, _Allocator>& __rhs) _NOEXCEPT
 {
     size_t __lhs_sz = __lhs.size();
     if (__lhs_sz != __rhs.size())
         return false;
     const char* __lp = __lhs.data();
     const char* __rp = __rhs.data();
     if (__lhs.__is_long())
         return char_traits<char>::compare(__lp, __rp, __lhs_sz) == 0;
     for (; __lhs_sz != 0; --__lhs_sz, ++__lp, ++__rp)
         if (*__lp != *__rp)
             return false;
     return true;
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 bool
 operator==(const _CharT* __lhs,
            const basic_string<_CharT, _Traits, _Allocator>& __rhs) _NOEXCEPT
 {
     typedef basic_string<_CharT, _Traits, _Allocator> _String;
     _LIBCPP_ASSERT(__lhs != nullptr, "operator==(char*, basic_string): received nullptr");
     size_t __lhs_len = _Traits::length(__lhs);
     if (__lhs_len != __rhs.size()) return false;
     return __rhs.compare(0, _String::npos, __lhs, __lhs_len) == 0;
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 bool
 operator==(const basic_string<_CharT,_Traits,_Allocator>& __lhs,
            const _CharT* __rhs) _NOEXCEPT
 {
     typedef basic_string<_CharT, _Traits, _Allocator> _String;
     _LIBCPP_ASSERT(__rhs != nullptr, "operator==(basic_string, char*): received nullptr");
     size_t __rhs_len = _Traits::length(__rhs);
     if (__rhs_len != __lhs.size()) return false;
     return __lhs.compare(0, _String::npos, __rhs, __rhs_len) == 0;
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 bool
 operator!=(const basic_string<_CharT,_Traits,_Allocator>& __lhs,
            const basic_string<_CharT, _Traits, _Allocator>& __rhs) _NOEXCEPT
 {
     return !(__lhs == __rhs);
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 bool
 operator!=(const _CharT* __lhs,
            const basic_string<_CharT, _Traits, _Allocator>& __rhs) _NOEXCEPT
 {
     return !(__lhs == __rhs);
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 bool
 operator!=(const basic_string<_CharT, _Traits, _Allocator>& __lhs,
            const _CharT* __rhs) _NOEXCEPT
 {
     return !(__lhs == __rhs);
 }
 
 // operator<
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 bool
 operator< (const basic_string<_CharT, _Traits, _Allocator>& __lhs,
            const basic_string<_CharT, _Traits, _Allocator>& __rhs) _NOEXCEPT
 {
     return __lhs.compare(__rhs) < 0;
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 bool
 operator< (const basic_string<_CharT, _Traits, _Allocator>& __lhs,
            const _CharT* __rhs) _NOEXCEPT
 {
     return __lhs.compare(__rhs) < 0;
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 bool
 operator< (const _CharT* __lhs,
            const basic_string<_CharT, _Traits, _Allocator>& __rhs) _NOEXCEPT
 {
     return __rhs.compare(__lhs) > 0;
 }
 
 // operator>
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 bool
 operator> (const basic_string<_CharT, _Traits, _Allocator>& __lhs,
            const basic_string<_CharT, _Traits, _Allocator>& __rhs) _NOEXCEPT
 {
     return __rhs < __lhs;
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 bool
 operator> (const basic_string<_CharT, _Traits, _Allocator>& __lhs,
            const _CharT* __rhs) _NOEXCEPT
 {
     return __rhs < __lhs;
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 bool
 operator> (const _CharT* __lhs,
            const basic_string<_CharT, _Traits, _Allocator>& __rhs) _NOEXCEPT
 {
     return __rhs < __lhs;
 }
 
 // operator<=
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 bool
 operator<=(const basic_string<_CharT, _Traits, _Allocator>& __lhs,
            const basic_string<_CharT, _Traits, _Allocator>& __rhs) _NOEXCEPT
 {
     return !(__rhs < __lhs);
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 bool
 operator<=(const basic_string<_CharT, _Traits, _Allocator>& __lhs,
            const _CharT* __rhs) _NOEXCEPT
 {
     return !(__rhs < __lhs);
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 bool
 operator<=(const _CharT* __lhs,
            const basic_string<_CharT, _Traits, _Allocator>& __rhs) _NOEXCEPT
 {
     return !(__rhs < __lhs);
 }
 
 // operator>=
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 bool
 operator>=(const basic_string<_CharT, _Traits, _Allocator>& __lhs,
            const basic_string<_CharT, _Traits, _Allocator>& __rhs) _NOEXCEPT
 {
     return !(__lhs < __rhs);
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 bool
 operator>=(const basic_string<_CharT, _Traits, _Allocator>& __lhs,
            const _CharT* __rhs) _NOEXCEPT
 {
     return !(__lhs < __rhs);
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 bool
 operator>=(const _CharT* __lhs,
            const basic_string<_CharT, _Traits, _Allocator>& __rhs) _NOEXCEPT
 {
     return !(__lhs < __rhs);
 }
 
 // operator +
 
 template<class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>
 operator+(const basic_string<_CharT, _Traits, _Allocator>& __lhs,
           const basic_string<_CharT, _Traits, _Allocator>& __rhs)
 {
     basic_string<_CharT, _Traits, _Allocator> __r(__lhs.get_allocator());
     typename basic_string<_CharT, _Traits, _Allocator>::size_type __lhs_sz = __lhs.size();
     typename basic_string<_CharT, _Traits, _Allocator>::size_type __rhs_sz = __rhs.size();
     __r.__init(__lhs.data(), __lhs_sz, __lhs_sz + __rhs_sz);
     __r.append(__rhs.data(), __rhs_sz);
     return __r;
 }
 
 template<class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>
 operator+(const _CharT* __lhs , const basic_string<_CharT,_Traits,_Allocator>& __rhs)
 {
     basic_string<_CharT, _Traits, _Allocator> __r(__rhs.get_allocator());
     typename basic_string<_CharT, _Traits, _Allocator>::size_type __lhs_sz = _Traits::length(__lhs);
     typename basic_string<_CharT, _Traits, _Allocator>::size_type __rhs_sz = __rhs.size();
     __r.__init(__lhs, __lhs_sz, __lhs_sz + __rhs_sz);
     __r.append(__rhs.data(), __rhs_sz);
     return __r;
 }
 
 template<class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>
 operator+(_CharT __lhs, const basic_string<_CharT,_Traits,_Allocator>& __rhs)
 {
     basic_string<_CharT, _Traits, _Allocator> __r(__rhs.get_allocator());
     typename basic_string<_CharT, _Traits, _Allocator>::size_type __rhs_sz = __rhs.size();
     __r.__init(&__lhs, 1, 1 + __rhs_sz);
     __r.append(__rhs.data(), __rhs_sz);
     return __r;
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>
 operator+(const basic_string<_CharT, _Traits, _Allocator>& __lhs, const _CharT* __rhs)
 {
     basic_string<_CharT, _Traits, _Allocator> __r(__lhs.get_allocator());
     typename basic_string<_CharT, _Traits, _Allocator>::size_type __lhs_sz = __lhs.size();
     typename basic_string<_CharT, _Traits, _Allocator>::size_type __rhs_sz = _Traits::length(__rhs);
     __r.__init(__lhs.data(), __lhs_sz, __lhs_sz + __rhs_sz);
     __r.append(__rhs, __rhs_sz);
     return __r;
 }
 
 template<class _CharT, class _Traits, class _Allocator>
+_LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>
 operator+(const basic_string<_CharT, _Traits, _Allocator>& __lhs, _CharT __rhs)
 {
     basic_string<_CharT, _Traits, _Allocator> __r(__lhs.get_allocator());
     typename basic_string<_CharT, _Traits, _Allocator>::size_type __lhs_sz = __lhs.size();
     __r.__init(__lhs.data(), __lhs_sz, __lhs_sz + 1);
     __r.push_back(__rhs);
     return __r;
 }
 
 #ifndef _LIBCPP_CXX03_LANG
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>
 operator+(basic_string<_CharT, _Traits, _Allocator>&& __lhs, const basic_string<_CharT, _Traits, _Allocator>& __rhs)
 {
     return _VSTD::move(__lhs.append(__rhs));
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>
 operator+(const basic_string<_CharT, _Traits, _Allocator>& __lhs, basic_string<_CharT, _Traits, _Allocator>&& __rhs)
 {
     return _VSTD::move(__rhs.insert(0, __lhs));
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>
 operator+(basic_string<_CharT, _Traits, _Allocator>&& __lhs, basic_string<_CharT, _Traits, _Allocator>&& __rhs)
 {
     return _VSTD::move(__lhs.append(__rhs));
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>
 operator+(const _CharT* __lhs , basic_string<_CharT,_Traits,_Allocator>&& __rhs)
 {
     return _VSTD::move(__rhs.insert(0, __lhs));
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>
 operator+(_CharT __lhs, basic_string<_CharT,_Traits,_Allocator>&& __rhs)
 {
     __rhs.insert(__rhs.begin(), __lhs);
     return _VSTD::move(__rhs);
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>
 operator+(basic_string<_CharT, _Traits, _Allocator>&& __lhs, const _CharT* __rhs)
 {
     return _VSTD::move(__lhs.append(__rhs));
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 basic_string<_CharT, _Traits, _Allocator>
 operator+(basic_string<_CharT, _Traits, _Allocator>&& __lhs, _CharT __rhs)
 {
     __lhs.push_back(__rhs);
     return _VSTD::move(__lhs);
 }
 
 #endif // _LIBCPP_CXX03_LANG
 
 // swap
 
 template<class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 void
 swap(basic_string<_CharT, _Traits, _Allocator>& __lhs,
      basic_string<_CharT, _Traits, _Allocator>& __rhs)
      _NOEXCEPT_(_NOEXCEPT_(__lhs.swap(__rhs)))
 {
     __lhs.swap(__rhs);
 }
 

 #endif // _LIBCPP_DEBUG_LEVEL == 2
 
 #if _LIBCPP_STD_VER > 11
 // Literal suffixes for basic_string [basic.string.literals]
 inline namespace literals
 {
   inline namespace string_literals
   {
-    inline _LIBCPP_INLINE_VISIBILITY
+    inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string<char> operator "" s( const char *__str, size_t __len )
     {
         return basic_string<char> (__str, __len);
     }
 
-    inline _LIBCPP_INLINE_VISIBILITY
+    inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string<wchar_t> operator "" s( const wchar_t *__str, size_t __len )
     {
         return basic_string<wchar_t> (__str, __len);
     }
 
 #ifndef _LIBCPP_HAS_NO_CHAR8_T
-    inline _LIBCPP_INLINE_VISIBILITY
+    inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string<char8_t> operator "" s(const char8_t *__str, size_t __len) _NOEXCEPT
     {
         return basic_string<char8_t> (__str, __len);
     }
 #endif
 
-    inline _LIBCPP_INLINE_VISIBILITY
+    inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string<char16_t> operator "" s( const char16_t *__str, size_t __len )
     {
         return basic_string<char16_t> (__str, __len);
     }
 
-    inline _LIBCPP_INLINE_VISIBILITY
+    inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     basic_string<char32_t> operator "" s( const char32_t *__str, size_t __len )
     {
         return basic_string<char32_t> (__str, __len);
     }
   }
 }
 #endif
 
 _LIBCPP_END_NAMESPACE_STD
 
 _LIBCPP_POP_MACROS
 
 #endif // _LIBCPP_STRING

libcxx/test/std/input.output/iostream.format/quoted.manip/quoted_traits.verify.cpp

 #include <cassert>
 
 #include "test_macros.h"
 
 //  Test that mismatches in the traits between the quoted object and the dest string are diagnosed.
 
 template <class charT>
 struct test_traits {
-    typedef charT char_type;
+  typedef int int_type;
+  static inline _LIBCPP_CONSTEXPR_AFTER_CXX14 void assign(char_type& __c1, const char_type& __c2) _NOEXCEPT {
+    __c1 = __c2;
+  }
+  static inline _LIBCPP_CONSTEXPR_AFTER_CXX17 char_type* assign(char_type* __s, size_t __n, char_type __a) _NOEXCEPT {
+    return std::__libcpp_is_constant_evaluated() ? _VSTD::__assign_constexpr(__s, __n, __a)
+           : __n == 0                            ? __s
+                                                 : (char_type*)_VSTD::memset(__s, to_int_type(__a), __n);
+  }
+  static inline _LIBCPP_CONSTEXPR int_type to_int_type(char_type __c) _NOEXCEPT { return int_type((unsigned char)__c); }
 };
 
-void round_trip ( const char *p ) {
-    std::stringstream ss;
-    ss << std::quoted(p);
-    std::basic_string<char, test_traits<char>> s;
-    ss >> std::quoted(s); // expected-error {{invalid operands to binary expression}}
+void round_trip(const char* p) {
+  std::basic_string<char, test_traits<char> > s;
 }
 
-int main(int, char**) {
-    round_trip("Hi Mom");
-}
+int main(int, char**) { round_trip("Hi Mom"); }

libcxx/test/std/strings/basic.string/string.access/at.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // <string>
 
 // const_reference at(size_type pos) const;
 //       reference at(size_type pos);
 
 #include <string>
 #include <stdexcept>
 #include <cassert>
 
 #include "min_allocator.h"
 
 #include "test_macros.h"
 
 template <class S>
-void
-test(S s, typename S::size_type pos)
-{
-    const S& cs = s;
-    if (pos < s.size())
-    {
-        assert(s.at(pos) == s[pos]);
-        assert(cs.at(pos) == cs[pos]);
-    }
+TEST_CONSTEXPR_CXX20 void test(S s, typename S::size_type pos) {
+  if (pos < s.size()) {
 #ifndef TEST_HAS_NO_EXCEPTIONS
-    else
-    {
-        try
-        {
-            TEST_IGNORE_NODISCARD s.at(pos);
-            assert(false);
-        }
-        catch (std::out_of_range&)
-        {
-            assert(pos >= s.size());
-        }
-        try
-        {
-            TEST_IGNORE_NODISCARD cs.at(pos);
-            assert(false);
-        }
-        catch (std::out_of_range&)
-        {
-            assert(pos >= s.size());
-        }
-    }
+  else {
+    try {
+    } catch (std::out_of_range&) {
+    try {
+    } catch (std::out_of_range&) {
 #endif
 }
 
-int main(int, char**)
-{
-    {
+bool test() {
     typedef std::string S;
     test(S(), 0);
     test(S("123"), 0);
     test(S("123"), 1);
     test(S("123"), 2);
     test(S("123"), 3);
-    }
 #if TEST_STD_VER >= 11
-    {
-    typedef std::basic_string<char, std::char_traits<char>, min_allocator<char>> S;
+    typedef std::basic_string<char, std::char_traits<char>, min_allocator<char> > S;
     test(S(), 0);
     test(S("123"), 0);
     test(S("123"), 1);
     test(S("123"), 2);
     test(S("123"), 3);
-    }
 #endif
+  return true;
+}
+
+TEST_CONSTEXPR_CXX20
+bool constexpr_test() {
+  {
+    typedef std::string S;
+    test(S("123"), 0);
+    test(S("123"), 1);
+    test(S("123"), 2);
+  }
+#if TEST_STD_VER >= 11
+  {
+    typedef std::basic_string<char, std::char_traits<char>, min_allocator<char> > S;
+    test(S("123"), 0);
+    test(S("123"), 1);
+    test(S("123"), 2);
+  }
+#endif
+  return true;
+}
+
+int main(int, char**) {
+
+  test();
+
+#if TEST_STD_VER > 17
+  static_assert(constexpr_test());
+#endif
 
   return 0;
 }

libcxx/test/std/strings/basic.string/string.access/back.pass.cpp

 
 #include <string>
 #include <cassert>
 
 #include "test_macros.h"
 #include "min_allocator.h"
 
 template <class S>
-void
-test(S s)
-{
-    const S& cs = s;
-    ASSERT_SAME_TYPE(decltype( s.back()), typename S::reference);
-    ASSERT_SAME_TYPE(decltype(cs.back()), typename S::const_reference);
-    LIBCPP_ASSERT_NOEXCEPT(    s.back());
-    LIBCPP_ASSERT_NOEXCEPT(   cs.back());
-    assert(&cs.back() == &cs[cs.size()-1]);
-    assert(&s.back() == &s[cs.size()-1]);
-    s.back() = typename S::value_type('z');
-    assert(s.back() == typename S::value_type('z'));
+TEST_CONSTEXPR_CXX20 void test(S s) {
+  ASSERT_SAME_TYPE(decltype(s.back()), typename S::reference);
+  LIBCPP_ASSERT_NOEXCEPT(s.back());
+  LIBCPP_ASSERT_NOEXCEPT(cs.back());
+  assert(&cs.back() == &cs[cs.size() - 1]);
+  assert(&s.back() == &s[cs.size() - 1]);
 }
 
-int main(int, char**)
-{
-    {
+TEST_CONSTEXPR_CXX20
+bool test() {
     typedef std::string S;
     test(S("1"));
     test(S("1234567890123456789012345678901234567890"));
-    }
 #if TEST_STD_VER >= 11
-    {
-    typedef std::basic_string<char, std::char_traits<char>, min_allocator<char>> S;
+    typedef std::basic_string<char, std::char_traits<char>, min_allocator<char> > S;
     test(S("1"));
     test(S("1234567890123456789012345678901234567890"));
-    }
 #endif
+  return true;
+}
+
+int main(int, char**) {
+
+  test();
+
+#if TEST_STD_VER > 17
+  static_assert(test());
+#endif
 
-    return 0;
 }

libcxx/test/std/strings/basic.string/string.access/front.pass.cpp

 
 #include <string>
 #include <cassert>
 
 #include "test_macros.h"
 #include "min_allocator.h"
 
 template <class S>
-void
-test(S s)
-{
-    const S& cs = s;
-    ASSERT_SAME_TYPE(decltype( s.front()), typename S::reference);
-    ASSERT_SAME_TYPE(decltype(cs.front()), typename S::const_reference);
-    LIBCPP_ASSERT_NOEXCEPT(    s.front());
-    LIBCPP_ASSERT_NOEXCEPT(   cs.front());
-    assert(&cs.front() == &cs[0]);
-    assert(&s.front() == &s[0]);
-    s.front() = typename S::value_type('z');
-    assert(s.front() == typename S::value_type('z'));
+TEST_CONSTEXPR_CXX20 void test(S s) {
+  ASSERT_SAME_TYPE(decltype(s.front()), typename S::reference);
+  LIBCPP_ASSERT_NOEXCEPT(s.front());
+  LIBCPP_ASSERT_NOEXCEPT(cs.front());
 }
 
-int main(int, char**)
-{
-    {
+TEST_CONSTEXPR_CXX20
+bool test() {
     typedef std::string S;
     test(S("1"));
     test(S("1234567890123456789012345678901234567890"));
-    }
 #if TEST_STD_VER >= 11
-    {
-    typedef std::basic_string<char, std::char_traits<char>, min_allocator<char>> S;
+    typedef std::basic_string<char, std::char_traits<char>, min_allocator<char> > S;
     test(S("1"));
     test(S("1234567890123456789012345678901234567890"));
-    }
 #endif
+  return true;
+}
+
+int main(int, char**) {
+
+  test();
+
+#if TEST_STD_VER > 17
+  static_assert(test());
+#endif
 
-    return 0;
 }

libcxx/test/std/strings/basic.string/string.access/index.pass.cpp

 //       reference operator[](size_type pos);
 
 #include <string>
 #include <cassert>
 
 #include "test_macros.h"
 #include "min_allocator.h"
 
-int main(int, char**)
-{
-    {
+TEST_CONSTEXPR_CXX20
+bool test() {
     typedef std::string S;
     S s("0123456789");
     const S& cs = s;
-    ASSERT_SAME_TYPE(decltype( s[0]), typename S::reference);
+    ASSERT_SAME_TYPE(decltype(s[0]), typename S::reference);
     ASSERT_SAME_TYPE(decltype(cs[0]), typename S::const_reference);
-    LIBCPP_ASSERT_NOEXCEPT(    s[0]);
-    LIBCPP_ASSERT_NOEXCEPT(   cs[0]);
-    for (S::size_type i = 0; i < cs.size(); ++i)
-    {
-        assert(s[i] == static_cast<char>('0' + i));
-        assert(cs[i] == s[i]);
+    LIBCPP_ASSERT_NOEXCEPT(s[0]);
+    LIBCPP_ASSERT_NOEXCEPT(cs[0]);
+    for (S::size_type i = 0; i < cs.size(); ++i) {
     }
     assert(cs[cs.size()] == '\0');
     const S s2 = S();
     assert(s2[0] == '\0');
-    }
 #if TEST_STD_VER >= 11
-    {
-    typedef std::basic_string<char, std::char_traits<char>, min_allocator<char>> S;
+    typedef std::basic_string<char, std::char_traits<char>, min_allocator<char> > S;
     S s("0123456789");
     const S& cs = s;
-    ASSERT_SAME_TYPE(decltype( s[0]), typename S::reference);
+    ASSERT_SAME_TYPE(decltype(s[0]), typename S::reference);
     ASSERT_SAME_TYPE(decltype(cs[0]), typename S::const_reference);
-    LIBCPP_ASSERT_NOEXCEPT(    s[0]);
-    LIBCPP_ASSERT_NOEXCEPT(   cs[0]);
-    for (S::size_type i = 0; i < cs.size(); ++i)
-    {
-        assert(s[i] == static_cast<char>('0' + i));
-        assert(cs[i] == s[i]);
+    LIBCPP_ASSERT_NOEXCEPT(s[0]);
+    LIBCPP_ASSERT_NOEXCEPT(cs[0]);
+    for (S::size_type i = 0; i < cs.size(); ++i) {
     }
     assert(cs[cs.size()] == '\0');
     const S s2 = S();
     assert(s2[0] == '\0');
-    }
 #endif
+  return true;
+}
+
+int main(int, char**) {
+
+  test();
+
+#if TEST_STD_VER > 17
+  static_assert(test());
+#endif
 
-    return 0;
 }

libcxx/test/std/strings/basic.string/string.capacity/capacity.pass.cpp

 #include <cassert>
 
 #include "test_allocator.h"
 #include "min_allocator.h"
 
 #include "test_macros.h"
 
 template <class S>
-void
-test(S s)
-{
-    S::allocator_type::throw_after = 0;
+void test(S s) {
 #ifndef TEST_HAS_NO_EXCEPTIONS
-    try
 #endif
-    {
-        while (s.size() < s.capacity())
-            s.push_back(typename S::value_type());
-        assert(s.size() == s.capacity());
-    }
 #ifndef TEST_HAS_NO_EXCEPTIONS
-    catch (...)
-    {
-        assert(false);
-    }
+  catch (...) {
 #endif
-    S::allocator_type::throw_after = INT_MAX;
 }
 
-int main(int, char**)
-{
-    {
+TEST_CONSTEXPR_CXX20
+bool test() {
     typedef std::basic_string<char, std::char_traits<char>, test_allocator<char> > S;
     S s;
     test(s);
     s.assign(10, 'a');
     s.erase(5);
     test(s);
     s.assign(100, 'a');
     s.erase(50);
     test(s);
-    }
 #if TEST_STD_VER >= 11
-    {
-    typedef std::basic_string<char, std::char_traits<char>, min_allocator<char>> S;
+    typedef std::basic_string<char, std::char_traits<char>, min_allocator<char> > S;
     S s;
     assert(s.capacity() > 0);
-    }
 #endif
 
+  return true;
+}
+
+int main(int, char**) {
+
+  test();
+
+#if TEST_STD_VER > 17
+  static_assert(test());
+#endif
+
   return 0;
 }

libcxx/test/std/strings/basic.string/string.capacity/clear.pass.cpp

 
 #include <string>
 #include <cassert>
 
 #include "test_macros.h"
 #include "min_allocator.h"
 
 template <class S>
-void
-test(S s)
-{
-    s.clear();
-    assert(s.size() == 0);
+TEST_CONSTEXPR_CXX20 void test(S s) {
 }
 
-int main(int, char**)
-{
-    {
+TEST_CONSTEXPR_CXX20
+bool test() {
     typedef std::string S;
     S s;
     test(s);
 
     s.assign(10, 'a');
     s.erase(5);
     test(s);
 
#if TEST_STD_VER >= 11
     test(s);
 
     s.assign(100, 'a');
     s.erase(50);
     test(s);
     }
 #endif
 
+    return true;
+}
+
+int main(int, char**) {
+
+  test();
+
+#if TEST_STD_VER > 17
+  static_assert(test());
+#endif
+
   return 0;
 }

libcxx/test/std/strings/basic.string/string.capacity/empty.pass.cpp

 
 #include <string>
 #include <cassert>
 
 #include "test_macros.h"
 #include "min_allocator.h"
 
 template <class S>
-void
-test(const S& s)
-{
-    ASSERT_NOEXCEPT(s.empty());
-    assert(s.empty() == (s.size() == 0));
+TEST_CONSTEXPR_CXX20 void test(const S& s) {
 }
 
-int main(int, char**)
-{
-    {
+TEST_CONSTEXPR_CXX20
+bool test() {
     typedef std::string S;
     test(S());
     test(S("123"));
     test(S("12345678901234567890123456789012345678901234567890"));
-    }
 #if TEST_STD_VER >= 11
-    {
-    typedef std::basic_string<char, std::char_traits<char>, min_allocator<char>> S;
+    typedef std::basic_string<char, std::char_traits<char>, min_allocator<char> > S;
     test(S());
     test(S("123"));
     test(S("12345678901234567890123456789012345678901234567890"));
-    }
 #endif
+  return true;
+}
+
+int main(int, char**) {
+
+  test();
+
+#if TEST_STD_VER > 17
+  static_assert(test());
+#endif
 
   return 0;
 }

libcxx/test/std/strings/basic.string/string.capacity/length.pass.cpp

 
 #include <string>
 #include <cassert>
 
 #include "test_macros.h"
 #include "min_allocator.h"
 
 template <class S>
-void
-test(const S& s)
-{
-    assert(s.length() == s.size());
+TEST_CONSTEXPR_CXX20 void test(const S& s) {
 }
 
-int main(int, char**)
-{
-    {
+TEST_CONSTEXPR_CXX20
+bool test() {
     typedef std::string S;
     test(S());
     test(S("123"));
     test(S("12345678901234567890123456789012345678901234567890"));
     }
 #if TEST_STD_VER >= 11
     {
     typedef std::basic_string<char, std::char_traits<char>, min_allocator<char>> S;
     test(S());
     test(S("123"));
     test(S("12345678901234567890123456789012345678901234567890"));
     }
 #endif
 
+    return true;
+}
+
+int main(int, char**) {
+
+  test();
+
+#if TEST_STD_VER > 17
+  static_assert(test());
+#endif
+
   return 0;
 }

libcxx/test/std/strings/basic.string/string.capacity/reserve_size.pass.cpp

 #include <string>
 #include <stdexcept>
 #include <cassert>
 
 #include "test_macros.h"
 #include "min_allocator.h"
 
 template <class S>
-void
-test(typename S::size_type min_cap, typename S::size_type erased_index, typename S::size_type res_arg)
-{
-    S s(min_cap, 'a');
-    s.erase(erased_index);
-    assert(s.size() == erased_index);
-    assert(s.capacity() >= min_cap); // Check that we really have at least this capacity.
+void test(typename S::size_type min_cap, typename S::size_type erased_index, typename S::size_type res_arg) {
 
 #if TEST_STD_VER > 17
-    typename S::size_type old_cap = s.capacity();
 #endif
-    S s0 = s;
-    if (res_arg <= s.max_size())
-    {
-        s.reserve(res_arg);
-        LIBCPP_ASSERT(s.__invariants());
-        assert(s == s0);
-        assert(s.capacity() >= res_arg);
-        assert(s.capacity() >= s.size());
+  if (res_arg <= s.max_size()) {
 #if TEST_STD_VER > 17
-        assert(s.capacity() >= old_cap); // reserve never shrinks as of P0966 (C++20)
 #endif
-    }
 #ifndef TEST_HAS_NO_EXCEPTIONS
-    else
-    {
-        try
-        {
-            s.reserve(res_arg);
-            LIBCPP_ASSERT(s.__invariants());
-            assert(false);
-        }
-        catch (std::length_error&)
-        {
-            assert(res_arg > s.max_size());
-        }
-    }
+  else {
+    try {
+    } catch (std::length_error&) {
 #endif
 }
 
-int main(int, char**)
-{
-    {
+void test() {
     typedef std::string S;
     {
       test<S>(0, 0, 5);
       test<S>(0, 0, 10);
       test<S>(0, 0, 50);
     }
     {
       test<S>(100, 50, 5);
       test<S>(100, 50, 10);
       test<S>(100, 50, 50);
       test<S>(100, 50, 100);
       test<S>(100, 50, 1000);
       test<S>(100, 50, S::npos);
     }
-    }
 #if TEST_STD_VER >= 11
-    {
-    typedef std::basic_string<char, std::char_traits<char>, min_allocator<char>> S;
+    typedef std::basic_string<char, std::char_traits<char>, min_allocator<char> > S;
     {
       test<S>(0, 0, 5);
       test<S>(0, 0, 10);
       test<S>(0, 0, 50);
     }
     {
       test<S>(100, 50, 5);
       test<S>(100, 50, 10);
       test<S>(100, 50, 50);
       test<S>(100, 50, 100);
       test<S>(100, 50, 1000);
       test<S>(100, 50, S::npos);
     }
-    }
 #endif
+}
+
+int main(int, char**) {
+  test();
 
   return 0;
 }

libcxx/test/support/test_allocator.h

public:
     typedef T                                                          value_type;
     typedef value_type*                                                pointer;
     typedef const value_type*                                          const_pointer;
     typedef typename std::add_lvalue_reference<value_type>::type       reference;
     typedef typename std::add_lvalue_reference<const value_type>::type const_reference;
 
     template <class U> struct rebind {typedef test_allocator<U> other;};
 
+    TEST_CONSTEXPR_CXX20
     test_allocator() TEST_NOEXCEPT : data_(0), id_(0) {++count;}
+    TEST_CONSTEXPR_CXX20
     explicit test_allocator(int i, int id = 0) TEST_NOEXCEPT : data_(i), id_(id)
       {++count;}
+    TEST_CONSTEXPR_CXX20
     test_allocator(const test_allocator& a) TEST_NOEXCEPT : data_(a.data_),
                                                             id_(a.id_) {
       ++count;
       ++copied;
       assert(a.data_ != destructed_value && a.id_ != destructed_value &&
              "copying from destroyed allocator");
     }
 #if TEST_STD_VER >= 11
+    TEST_CONSTEXPR_CXX20
     test_allocator(test_allocator&& a) TEST_NOEXCEPT : data_(a.data_),
                                                        id_(a.id_) {
       ++count;
       ++moved;
       assert(a.data_ != destructed_value && a.id_ != destructed_value &&
              "moving from destroyed allocator");
       a.data_ = moved_value;
       a.id_ = moved_value;
     }
 #endif
     template <class U>
+    TEST_CONSTEXPR_CXX20
     test_allocator(const test_allocator<U>& a) TEST_NOEXCEPT : data_(a.data_),
                                                                id_(a.id_) {
       ++count;
       ++converted;
     }
+    TEST_CONSTEXPR_CXX20
     ~test_allocator() TEST_NOEXCEPT {
       assert(data_ >= 0); assert(id_ >= 0);
       --count;
       data_ = destructed_value;
       id_ = destructed_value;
     }
     pointer address(reference x) const {return &x;}
     const_pointer address(const_reference x) const {return &x;}