[PATCH 2/2] libstdc++: More user-friendly failed assertions from shared_ptr dereference

[Jonathan Wakely]

Currently dereferencing an empty shared_ptr prints a complicated
internal type in the assertion message:

include/bits/shared_ptr_base.h:1377: std::__shared_ptr_access<_Tp, _Lp, 
<anonymous>, <anonymous> >::element_type& std::__shared_ptr_access<_Tp, _Lp, 
<anonymous>, <anonymous> >::operator*() const [with _Tp = 
std::filesystem::__cxx11::recursive_directory_iterator::_Dir_stack; 
__gnu_cxx::_Lock_policy _Lp = __gnu_cxx::_S_atomic; bool <anonymous> = false; 
bool <anonymous> = false; element_type = 
std::filesystem::__cxx11::recursive_directory_iterator::_Dir_stack]: Assertion 
'_M_get() != nullptr' failed.

Users don't care about any of the _Lp and <anonymous> template
parameters, so this is unnecessarily verbose.

We can simplify it to something that only mentions "shared_ptr_deref"
and the element type:

include/bits/shared_ptr_base.h:1371: _Tp* std::__shared_ptr_deref(_Tp*) [with 
_Tp = filesystem::__cxx11::recursive_directory_iterator::_Dir_stack]: Assertion 
'__p != nullptr' failed.

libstdc++-v3/ChangeLog:

        * include/bits/shared_ptr_base.h (__shared_ptr_deref): New
        function template.
        (__shared_ptr_access, __shared_ptr_access<>): Use it.
---
Tested x86_64-linux.

Also available for review at:
https://forge.sourceware.org/gcc/gcc-TEST/pulls/13

 libstdc++-v3/include/bits/shared_ptr_base.h | 28 +++++++++++++--------
 1 file changed, 17 insertions(+), 11 deletions(-)

diff --git a/libstdc++-v3/include/bits/shared_ptr_base.h 
b/libstdc++-v3/include/bits/shared_ptr_base.h
index 9a7617e7014..ee01594ce0c 100644
--- a/libstdc++-v3/include/bits/shared_ptr_base.h
+++ b/libstdc++-v3/include/bits/shared_ptr_base.h
@@ -1337,6 +1337,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     { };
 
 
+  template<typename _Tp>
+    [[__gnu__::__always_inline__]]
+    inline _Tp*
+    __shared_ptr_deref(_Tp* __p)
+    {
+      __glibcxx_assert(__p != nullptr);
+      return __p;
+    }
+
   // Define operator* and operator-> for shared_ptr<T>.
   template<typename _Tp, _Lock_policy _Lp,
           bool = is_array<_Tp>::value, bool = is_void<_Tp>::value>
@@ -1347,10 +1356,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       element_type&
       operator*() const noexcept
-      {
-       __glibcxx_assert(_M_get() != nullptr);
-       return *_M_get();
-      }
+      { return *std::__shared_ptr_deref(_M_get()); }
 
       element_type*
       operator->() const noexcept
@@ -1392,10 +1398,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       [[__deprecated__("shared_ptr<T[]>::operator* is absent from C++17")]]
       element_type&
       operator*() const noexcept
-      {
-       __glibcxx_assert(_M_get() != nullptr);
-       return *_M_get();
-      }
+      { return *std::__shared_ptr_deref(_M_get()); }
 
       [[__deprecated__("shared_ptr<T[]>::operator-> is absent from C++17")]]
       element_type*
@@ -1406,13 +1409,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       }
 #endif
 
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wc++17-extensions"
       element_type&
       operator[](ptrdiff_t __i) const noexcept
       {
-       __glibcxx_assert(_M_get() != nullptr);
-       __glibcxx_assert(!extent<_Tp>::value || __i < extent<_Tp>::value);
-       return _M_get()[__i];
+       if constexpr (extent<_Tp>::value)
+         __glibcxx_assert(__i < extent<_Tp>::value);
+       return std::__shared_ptr_deref(_M_get())[__i];
       }
+#pragma GCC diagnostic pop
 
     private:
       element_type*
-- 
2.47.0