libstdc++: Constrain std::vector default constructor [PR113841]

This is needed to avoid errors outside the immediate context when
evaluating is_default_constructible_v<vector<T, A>> when A is not
default constructible.

To avoid diagnostic regressions for 23_containers/vector/48101_neg.cc we
need to make the std::allocator<cv T> partial specializations default
constructible, which they probably should have been anyway.

libstdc++-v3/ChangeLog:

	PR libstdc++/113841
	* include/bits/allocator.h (allocator<cv T>): Add default
	constructor to partial specializations for cv-qualified types.
	* include/bits/stl_vector.h (_Vector_impl::_Vector_impl()):
	Constrain so that it's only present if the allocator is default
	constructible.
	* include/bits/stl_bvector.h (_Bvector_impl::_Bvector_impl()):
	Likewise.
	* testsuite/23_containers/vector/cons/113841.cc: New test.

libstdc++-v3/include/bits/allocator.h

@@ -254,6 +254,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     {
     public:
       typedef _Tp value_type;
+      allocator() { }
       template<typename _Up> allocator(const allocator<_Up>&) { }
     };
 
@@ -262,6 +263,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     {
     public:
       typedef _Tp value_type;
+      allocator() { }
       template<typename _Up> allocator(const allocator<_Up>&) { }
     };
 
@@ -270,6 +272,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     {
     public:
       typedef _Tp value_type;
+      allocator() { }
       template<typename _Up> allocator(const allocator<_Up>&) { }
     };
   /// @endcond

libstdc++-v3/include/bits/stl_bvector.h

@@ -593,6 +593,9 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
 	_GLIBCXX20_CONSTEXPR
 	_Bvector_impl() _GLIBCXX_NOEXCEPT_IF(
 	  is_nothrow_default_constructible<_Bit_alloc_type>::value)
+#if __cpp_concepts
+	requires is_default_constructible_v<_Bit_alloc_type>
+#endif
 	: _Bit_alloc_type()
 	{ }
 

libstdc++-v3/include/bits/stl_vector.h

@@ -135,6 +135,9 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
 	_GLIBCXX20_CONSTEXPR
 	_Vector_impl() _GLIBCXX_NOEXCEPT_IF(
 	    is_nothrow_default_constructible<_Tp_alloc_type>::value)
+#if __cpp_lib_concepts
+	requires is_default_constructible_v<_Tp_alloc_type>
+#endif
 	: _Tp_alloc_type()
 	{ }
 

libstdc++-v3/testsuite/23_containers/vector/cons/113841.cc

@@ -0,0 +1,34 @@
+// { dg-do compile { target c++20 } }
+
+#include <vector>
+
+template<typename T>
+struct Alloc
+{
+  using value_type = T;
+
+  Alloc(int) { } // not default constructible
+
+  template<typename U> Alloc(const Alloc<U>&) { }
+
+  T* allocate(std::size_t n) { return std::allocator<T>().allocate(n); }
+  void deallocate(T* p, std::size_t n) { std::allocator<T>().deallocate(p, n); }
+};
+
+template<typename T> struct wrap { T t; };
+
+template<typename T> void do_adl(T&) { }
+
+void test_pr113841()
+{
+  using test_type = std::vector<int, Alloc<int>>;
+  std::pair<const int, wrap<test_type>>* h = nullptr;
+  do_adl(h);
+}
+
+void test_pr113841_bool()
+{
+  using test_type = std::vector<bool, Alloc<bool>>;
+  std::pair<const int, wrap<test_type>>* h = nullptr;
+  do_adl(h);
+}