[libc++] Fix no-op shrink_to_fit for vector<bool> (#120495)

This PR addresses an issue where the `shrink_to_fit` function in
`vector<bool>` is effectively a no-op, meaning it will never shrink the
capacity.

Fixes #122502

Author: winner245

libcxx/include/__vector/vector_bool.h

@@ -859,11 +859,13 @@ _LIBCPP_CONSTEXPR_SINCE_CXX20 void vector<bool, _Allocator>::reserve(size_type _
 
 template <class _Allocator>
 _LIBCPP_CONSTEXPR_SINCE_CXX20 void vector<bool, _Allocator>::shrink_to_fit() _NOEXCEPT {
-  if (__external_cap_to_internal(size()) > __cap_) {
+  if (__external_cap_to_internal(size()) < __cap_) {
 #if _LIBCPP_HAS_EXCEPTIONS
     try {
 #endif // _LIBCPP_HAS_EXCEPTIONS
-      vector(*this, allocator_type(__alloc_)).swap(*this);
+      vector __v(*this, allocator_type(__alloc_));
+      if (__v.__cap_ < __cap_)
+        __v.swap(*this);
 #if _LIBCPP_HAS_EXCEPTIONS
     } catch (...) {
     }

libcxx/test/std/containers/sequences/vector.bool/shrink_to_fit.pass.cpp

@@ -11,7 +11,10 @@
 
 // void shrink_to_fit();
 
+// XFAIL: FROZEN-CXX03-HEADERS-FIXME
+
 #include <cassert>
+#include <climits>
 #include <vector>
 
 #include "increasing_allocator.h"
@@ -20,19 +23,56 @@
 
 TEST_CONSTEXPR_CXX20 bool tests() {
   {
-    std::vector<bool> v(100);
+    using C = std::vector<bool>;
+    C v(100);
     v.push_back(1);
+    C::size_type before_cap = v.capacity();
+    v.clear();
     v.shrink_to_fit();
-    assert(v.capacity() >= 101);
-    assert(v.size() >= 101);
+    assert(v.capacity() <= before_cap);
+    LIBCPP_ASSERT(v.capacity() == 0); // libc++ honors the shrink_to_fit request as a QOI matter
+    assert(v.size() == 0);
   }
-#if TEST_STD_VER >= 11
   {
-    std::vector<bool, min_allocator<bool>> v(100);
+    using C = std::vector<bool, min_allocator<bool> >;
+    C v(100);
     v.push_back(1);
+    C::size_type before_cap = v.capacity();
     v.shrink_to_fit();
     assert(v.capacity() >= 101);
-    assert(v.size() >= 101);
+    assert(v.capacity() <= before_cap);
+    assert(v.size() == 101);
+    v.erase(v.begin() + 1, v.end());
+    v.shrink_to_fit();
+    assert(v.capacity() <= before_cap);
+    LIBCPP_ASSERT(v.capacity() == C(1).capacity()); // libc++ honors the shrink_to_fit request as a QOI matter.
+    assert(v.size() == 1);
+  }
+
+#if defined(_LIBCPP_VERSION)
+  {
+    using C                = std::vector<bool>;
+    unsigned bits_per_word = static_cast<unsigned>(sizeof(C::__storage_type) * CHAR_BIT);
+    C v(bits_per_word);
+    v.push_back(1);
+    assert(v.capacity() == bits_per_word * 2);
+    assert(v.size() == bits_per_word + 1);
+    v.pop_back();
+    v.shrink_to_fit();
+    assert(v.capacity() == bits_per_word);
+    assert(v.size() == bits_per_word);
+  }
+  {
+    using C                = std::vector<bool>;
+    unsigned bits_per_word = static_cast<unsigned>(sizeof(C::__storage_type) * CHAR_BIT);
+    C v;
+    v.reserve(bits_per_word * 2);
+    v.push_back(1);
+    assert(v.capacity() == bits_per_word * 2);
+    assert(v.size() == 1);
+    v.shrink_to_fit();
+    assert(v.capacity() == bits_per_word);
+    assert(v.size() == 1);
   }
 #endif