[libc++] Add missed constexpr to erase(_if) in <string> (#129666)

`std::erase(_if)` for `basic_string` were made `constexpr` in C++20 by
cplusplus/draft@2c1ab97 as follow-up
changes of P0980R1.

This patch implements the missed changes that were not tracked in a
specific paper.

Author: frederick-vs-ja

libcxx/include/string

@@ -516,10 +516,10 @@ basic_istream<charT, traits>&
 getline(basic_istream<charT, traits>& is, basic_string<charT, traits, Allocator>& str);
 
 template<class charT, class traits, class Allocator, class U>
-typename basic_string<charT, traits, Allocator>::size_type
+constexpr typename basic_string<charT, traits, Allocator>::size_type
 erase(basic_string<charT, traits, Allocator>& c, const U& value);    // C++20
 template<class charT, class traits, class Allocator, class Predicate>
-typename basic_string<charT, traits, Allocator>::size_type
+constexpr typename basic_string<charT, traits, Allocator>::size_type
 erase_if(basic_string<charT, traits, Allocator>& c, Predicate pred); // C++20
 
 typedef basic_string<char>    string;
@@ -4022,15 +4022,15 @@ getline(basic_istream<_CharT, _Traits>&& __is, basic_string<_CharT, _Traits, _Al
 
 #  if _LIBCPP_STD_VER >= 20
 template <class _CharT, class _Traits, class _Allocator, class _Up>
-inline _LIBCPP_HIDE_FROM_ABI typename basic_string<_CharT, _Traits, _Allocator>::size_type
+inline _LIBCPP_HIDE_FROM_ABI constexpr typename basic_string<_CharT, _Traits, _Allocator>::size_type
 erase(basic_string<_CharT, _Traits, _Allocator>& __str, const _Up& __v) {
   auto __old_size = __str.size();
   __str.erase(std::remove(__str.begin(), __str.end(), __v), __str.end());
   return __old_size - __str.size();
 }
 
 template <class _CharT, class _Traits, class _Allocator, class _Predicate>
-inline _LIBCPP_HIDE_FROM_ABI typename basic_string<_CharT, _Traits, _Allocator>::size_type
+inline _LIBCPP_HIDE_FROM_ABI constexpr typename basic_string<_CharT, _Traits, _Allocator>::size_type
 erase_if(basic_string<_CharT, _Traits, _Allocator>& __str, _Predicate __pred) {
   auto __old_size = __str.size();
   __str.erase(std::remove_if(__str.begin(), __str.end(), __pred), __str.end());

libcxx/test/std/strings/strings.erasure/erase.pass.cpp

@@ -11,7 +11,7 @@
 // <string>
 
 // template <class charT, class traits, class Allocator, class U>
-//   typename basic_string<charT, traits, Allocator>::size_type
+//   constexpr typename basic_string<charT, traits, Allocator>::size_type
 //   erase(basic_string<charT, traits, Allocator>& c, const U& value);
 
 #include <string>
@@ -22,15 +22,15 @@
 #include "min_allocator.h"
 
 template <class S, class U>
-void test0(S s, U val, S expected, std::size_t expected_erased_count) {
+constexpr void test0(S s, U val, S expected, std::size_t expected_erased_count) {
   ASSERT_SAME_TYPE(typename S::size_type, decltype(std::erase(s, val)));
   assert(expected_erased_count == std::erase(s, val));
   LIBCPP_ASSERT(s.__invariants());
   assert(s == expected);
 }
 
 template <class S>
-void test() {
+constexpr void test() {
   test0(S(""), 'a', S(""), 0);
 
   test0(S("a"), 'a', S(""), 1);
@@ -64,10 +64,17 @@ void test() {
   test0(S("aba"), opt('c'), S("aba"), 0);
 }
 
-int main(int, char**) {
+constexpr bool test() {
   test<std::string>();
   test<std::basic_string<char, std::char_traits<char>, min_allocator<char>>>();
   test<std::basic_string<char, std::char_traits<char>, test_allocator<char>>>();
 
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
   return 0;
 }

libcxx/test/std/strings/strings.erasure/erase_if.pass.cpp

@@ -11,7 +11,7 @@
 // <string>
 
 // template <class charT, class traits, class Allocator, class Predicate>
-//   typename basic_string<charT, traits, Allocator>::size_type
+//   constexpr typename basic_string<charT, traits, Allocator>::size_type
 //   erase_if(basic_string<charT, traits, Allocator>& c, Predicate pred);
 
 #include <string>
@@ -21,15 +21,15 @@
 #include "min_allocator.h"
 
 template <class S, class Pred>
-void test0(S s, Pred p, S expected, std::size_t expected_erased_count) {
+constexpr void test0(S s, Pred p, S expected, std::size_t expected_erased_count) {
   ASSERT_SAME_TYPE(typename S::size_type, decltype(std::erase_if(s, p)));
   assert(expected_erased_count == std::erase_if(s, p));
   LIBCPP_ASSERT(s.__invariants());
   assert(s == expected);
 }
 
 template <typename S>
-void test() {
+constexpr void test() {
   auto isA   = [](auto ch) { return ch == 'a'; };
   auto isB   = [](auto ch) { return ch == 'b'; };
   auto isC   = [](auto ch) { return ch == 'c'; };
@@ -66,10 +66,17 @@ void test() {
   test0(S("aba"), True, S(""), 3);
 }
 
-int main(int, char**) {
+constexpr bool test() {
   test<std::string>();
   test<std::basic_string<char, std::char_traits<char>, min_allocator<char>>>();
   test<std::basic_string<char, std::char_traits<char>, test_allocator<char>>>();
 
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
   return 0;
 }