[gcc r12-11042] libstdc++: Constrain std::vector default constructor [PR113841]

[Jonathan Wakely via Gcc-cvs]

https://gcc.gnu.org/g:9357f7e0fbfbf0d7d5adb2cd1588a765f40b6162

commit r12-11042-g9357f7e0fbfbf0d7d5adb2cd1588a765f40b6162
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Fri Feb 9 17:06:20 2024 +0000

    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.
    
    (cherry picked from commit 142cc4c223d695e515ed2504501b91d8a7ac6eb8)

Diff:
---
 libstdc++-v3/include/bits/allocator.h              |  3 ++
 libstdc++-v3/include/bits/stl_bvector.h            |  3 ++
 libstdc++-v3/include/bits/stl_vector.h             |  3 ++
 .../testsuite/23_containers/vector/cons/113841.cc  | 34 ++++++++++++++++++++++
 4 files changed, 43 insertions(+)

diff --git a/libstdc++-v3/include/bits/allocator.h 
b/libstdc++-v3/include/bits/allocator.h
index aec0b374fd18..a27b7d92503e 100644
--- a/libstdc++-v3/include/bits/allocator.h
+++ b/libstdc++-v3/include/bits/allocator.h
@@ -247,6 +247,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     {
     public:
       typedef _Tp value_type;
+      allocator() { }
       template<typename _Up> allocator(const allocator<_Up>&) { }
     };
 
@@ -255,6 +256,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     {
     public:
       typedef _Tp value_type;
+      allocator() { }
       template<typename _Up> allocator(const allocator<_Up>&) { }
     };
 
@@ -263,6 +265,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     {
     public:
       typedef _Tp value_type;
+      allocator() { }
       template<typename _Up> allocator(const allocator<_Up>&) { }
     };
 
diff --git a/libstdc++-v3/include/bits/stl_bvector.h 
b/libstdc++-v3/include/bits/stl_bvector.h
index e1559c253d81..aa8c4026d235 100644
--- a/libstdc++-v3/include/bits/stl_bvector.h
+++ b/libstdc++-v3/include/bits/stl_bvector.h
@@ -548,6 +548,9 @@ _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()
        { }
 
diff --git a/libstdc++-v3/include/bits/stl_vector.h 
b/libstdc++-v3/include/bits/stl_vector.h
index 977474acd7f0..df1010c07afa 100644
--- a/libstdc++-v3/include/bits/stl_vector.h
+++ b/libstdc++-v3/include/bits/stl_vector.h
@@ -136,6 +136,9 @@ _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()
        { }
 
diff --git a/libstdc++-v3/testsuite/23_containers/vector/cons/113841.cc 
b/libstdc++-v3/testsuite/23_containers/vector/cons/113841.cc
new file mode 100644
index 000000000000..a7721d27f796
--- /dev/null
+++ b/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);
+}