[libc++] Fix constructing bitset from non-null-terminated arrays (#143691)

Unconditional evaluation of `char_traits<_CharT>::length(__str)` is problematic, because it causes
UB when `__str` points to a non-null-terminated array. We should only call `length` (currently, in
`basic_string_view`'s constructor) when `__n == npos` per [bitset.cons]/8.

Drive-by change: Reduction of conditional compilation, given that
- both `basic_string_view<_CharT>::size_type` and `basic_string<_CharT>::size_type` must be 
  `size_t`, and thus
- both `basic_string_view<_CharT>::npos` and `basic_string<_CharT>::npos` must be `size_t(-1)`.

For the type sameness in the standard wording, see:
- [string.view.template.general]
- [basic.string.general]
- [allocator.traits.types]/6
- [default.allocator.general]/1

Fixes #143684

Author: frederick-vs-ja

libcxx/include/bitset

@@ -645,16 +645,13 @@ public:
   template <class _CharT, __enable_if_t<_IsCharLikeType<_CharT>::value, int> = 0>
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 explicit bitset(
       const _CharT* __str,
-#  if _LIBCPP_STD_VER >= 26
-      typename basic_string_view<_CharT>::size_type __n = basic_string_view<_CharT>::npos,
-#  else
-      typename basic_string<_CharT>::size_type __n = basic_string<_CharT>::npos,
-#  endif
+      size_t __n    = basic_string<_CharT>::npos,
       _CharT __zero = _CharT('0'),
       _CharT __one  = _CharT('1')) {
-
-    size_t __rlen = std::min(__n, char_traits<_CharT>::length(__str));
-    __init_from_string_view(basic_string_view<_CharT>(__str, __rlen), __zero, __one);
+    if (__n == basic_string<_CharT>::npos)
+      __init_from_string_view(basic_string_view<_CharT>(__str), __zero, __one);
+    else
+      __init_from_string_view(basic_string_view<_CharT>(__str, __n), __zero, __one);
   }
 #  if _LIBCPP_STD_VER >= 26
   template <class _CharT, class _Traits>

libcxx/test/std/utilities/template.bitset/bitset.cons/char_ptr_ctor.pass.cpp

@@ -72,6 +72,35 @@ TEST_CONSTEXPR_CXX23 void test_char_pointer_ctor()
     for (std::size_t i = 10; i < v.size(); ++i)
         assert(v[i] == false);
   }
+  // Verify that this constructor doesn't read over the given bound.
+  // See https://github.com/llvm/llvm-project/issues/143684
+  {
+    const char not_null_terminated[] = {'1', '0', '1', '0', '1', '0', '1', '0', '1', '0'};
+    std::bitset<N> v(not_null_terminated, 10);
+    std::size_t M = std::min<std::size_t>(v.size(), 10);
+    for (std::size_t i = 0; i < M; ++i)
+      assert(v[i] == (not_null_terminated[M - 1 - i] == '1'));
+    for (std::size_t i = 10; i < v.size(); ++i)
+      assert(!v[i]);
+  }
+  {
+    const char not_null_terminated[] = {'1', 'a', '1', 'a', '1', 'a', '1', 'a', '1', 'a'};
+    std::bitset<N> v(not_null_terminated, 10, 'a');
+    std::size_t M = std::min<std::size_t>(v.size(), 10);
+    for (std::size_t i = 0; i < M; ++i)
+      assert(v[i] == (not_null_terminated[M - 1 - i] == '1'));
+    for (std::size_t i = 10; i < v.size(); ++i)
+      assert(!v[i]);
+  }
+  {
+    const char not_null_terminated[] = {'b', 'a', 'b', 'a', 'b', 'a', 'b', 'a', 'b', 'a'};
+    std::bitset<N> v(not_null_terminated, 10, 'a', 'b');
+    std::size_t M = std::min<std::size_t>(v.size(), 10);
+    for (std::size_t i = 0; i < M; ++i)
+      assert(v[i] == (not_null_terminated[M - 1 - i] == 'b'));
+    for (std::size_t i = 10; i < v.size(); ++i)
+      assert(!v[i]);
+  }
 }
 
 TEST_CONSTEXPR_CXX23 bool test() {