libstdc++/debug: Fix constexpr _Safe_iterator in C++20 mode [PR109536]

Some _Safe_iterator member functions define a variable of non-literal
type __gnu_cxx::__scoped_lock, which automatically disqualifies them from
being constexpr in C++20 mode even if that code path is never constant
evaluated.  This restriction was lifted by P2242R3 for C++23, but we
need to work around it in C++20 mode.  To that end this patch defines
a pair of macros that encapsulate the lambda-based workaround mentioned
in that paper and uses it to make these functions valid C++20 constexpr
functions.  The augmented std::vector test element_access/constexpr.cc
now successfully compiles in C++20 mode with -D_GLIBCXX_DEBUG (and it
should test all member functions modified by this patch).

	PR libstdc++/109536

libstdc++-v3/ChangeLog:

	* include/debug/safe_base.h (_Safe_sequence_base::_M_swap):
	Remove _GLIBCXX20_CONSTEXPR from non-inline member function.
	* include/debug/safe_iterator.h
	(_GLIBCXX20_CONSTEXPR_NON_LITERAL_SCOPE_BEGIN): Define.
	(_GLIBCXX20_CONSTEXPR_NON_LITERAL_SCOPE_END): Define.
	(_Safe_iterator::operator=): Use them around the code path that
	defines a variable of type __gnu_cxx::__scoped_lock.
	(_Safe_iterator::operator++): Likewise.
	(_Safe_iterator::operator--): Likewise.
	(_Safe_iterator::operator+=): Likewise.
	(_Safe_iterator::operator-=): Likewise.
	* testsuite/23_containers/vector/element_access/constexpr.cc
	(test_iterators): Test more iterator operations.
	* testsuite/23_containers/vector/bool/element_access/constexpr.cc
	(test_iterators): Likewise.
	* testsuite/std/ranges/adaptors/all.cc (test08) [_GLIBCXX_DEBUG]:
	Remove.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/include/debug/safe_base.h

@@ -268,7 +268,6 @@ namespace __gnu_debug
      *  operation is complete all iterators that originally referenced
      *  one container now reference the other container.
      */
-    _GLIBCXX20_CONSTEXPR
     void
     _M_swap(_Safe_sequence_base& __x) _GLIBCXX_USE_NOEXCEPT;
 

libstdc++-v3/include/debug/safe_iterator.h

@@ -65,6 +65,20 @@
   _GLIBCXX_DEBUG_VERIFY_OPERANDS(_Lhs, _Rhs, __msg_distance_bad,	\
 				 __msg_distance_different)
 
+// This pair of macros helps with writing valid C++20 constexpr functions that
+// contain a non-constexpr code path that defines a non-literal variable, which
+// was otherwise disallowed until P2242R3 for C++23.  We use them below around
+// __gnu_cxx::__scoped_lock variables so that the containing functions are still
+// considered valid C++20 constexpr functions.
+
+#if __cplusplus >= 202002L && __cpp_constexpr < 202110L
+# define _GLIBCXX20_CONSTEXPR_NON_LITERAL_SCOPE_BEGIN [&]() -> void
+# define _GLIBCXX20_CONSTEXPR_NON_LITERAL_SCOPE_END ();
+#else
+# define _GLIBCXX20_CONSTEXPR_NON_LITERAL_SCOPE_BEGIN
+# define _GLIBCXX20_CONSTEXPR_NON_LITERAL_SCOPE_END
+#endif
+
 namespace __gnu_debug
 {
   /** Helper struct to deal with sequence offering a before_begin
@@ -266,11 +280,11 @@ namespace __gnu_debug
 			      ._M_iterator(__x, "other"));
 
 	if (this->_M_sequence && this->_M_sequence == __x._M_sequence)
-	  {
+	  _GLIBCXX20_CONSTEXPR_NON_LITERAL_SCOPE_BEGIN {
 	    __gnu_cxx::__scoped_lock __l(this->_M_get_mutex());
 	    base() = __x.base();
 	    _M_version = __x._M_sequence->_M_version;
-	  }
+	  } _GLIBCXX20_CONSTEXPR_NON_LITERAL_SCOPE_END
 	else
 	  {
 	    _M_detach();
@@ -306,11 +320,11 @@ namespace __gnu_debug
 	  return *this;
 
 	if (this->_M_sequence && this->_M_sequence == __x._M_sequence)
-	  {
+	  _GLIBCXX20_CONSTEXPR_NON_LITERAL_SCOPE_BEGIN {
 	    __gnu_cxx::__scoped_lock __l(this->_M_get_mutex());
 	    base() = __x.base();
 	    _M_version = __x._M_sequence->_M_version;
-	  }
+	  } _GLIBCXX20_CONSTEXPR_NON_LITERAL_SCOPE_END
 	else
 	  {
 	    _M_detach();
@@ -378,8 +392,10 @@ namespace __gnu_debug
 	_GLIBCXX_DEBUG_VERIFY(this->_M_incrementable(),
 			      _M_message(__msg_bad_inc)
 			      ._M_iterator(*this, "this"));
-	__gnu_cxx::__scoped_lock __l(this->_M_get_mutex());
-	++base();
+	_GLIBCXX20_CONSTEXPR_NON_LITERAL_SCOPE_BEGIN {
+	  __gnu_cxx::__scoped_lock __l(this->_M_get_mutex());
+	  ++base();
+	} _GLIBCXX20_CONSTEXPR_NON_LITERAL_SCOPE_END
 	return *this;
       }
 
@@ -697,8 +713,10 @@ namespace __gnu_debug
 	_GLIBCXX_DEBUG_VERIFY(this->_M_decrementable(),
 			      _M_message(__msg_bad_dec)
 			      ._M_iterator(*this, "this"));
-	__gnu_cxx::__scoped_lock __l(this->_M_get_mutex());
-	--this->base();
+	_GLIBCXX20_CONSTEXPR_NON_LITERAL_SCOPE_BEGIN {
+	  __gnu_cxx::__scoped_lock __l(this->_M_get_mutex());
+	  --this->base();
+	} _GLIBCXX20_CONSTEXPR_NON_LITERAL_SCOPE_END
 	return *this;
       }
 
@@ -912,8 +930,10 @@ namespace __gnu_debug
 	_GLIBCXX_DEBUG_VERIFY(this->_M_can_advance(__n),
 			      _M_message(__msg_advance_oob)
 			      ._M_iterator(*this)._M_integer(__n));
-	__gnu_cxx::__scoped_lock __l(this->_M_get_mutex());
-	this->base() += __n;
+	_GLIBCXX20_CONSTEXPR_NON_LITERAL_SCOPE_BEGIN {
+	  __gnu_cxx::__scoped_lock __l(this->_M_get_mutex());
+	  this->base() += __n;
+	} _GLIBCXX20_CONSTEXPR_NON_LITERAL_SCOPE_END
 	return *this;
       }
 
@@ -930,8 +950,10 @@ namespace __gnu_debug
 	_GLIBCXX_DEBUG_VERIFY(this->_M_can_advance(-__n),
 			      _M_message(__msg_retreat_oob)
 			      ._M_iterator(*this)._M_integer(__n));
-	__gnu_cxx::__scoped_lock __l(this->_M_get_mutex());
-	this->base() -= __n;
+	_GLIBCXX20_CONSTEXPR_NON_LITERAL_SCOPE_BEGIN {
+	  __gnu_cxx::__scoped_lock __l(this->_M_get_mutex());
+	  this->base() -= __n;
+	} _GLIBCXX20_CONSTEXPR_NON_LITERAL_SCOPE_END
 	return *this;
       }
 
@@ -1156,6 +1178,8 @@ _GLIBCXX_END_NAMESPACE_VERSION
 }
 #endif
 
+#undef _GLIBCXX20_CONSTEXPR_NON_LITERAL_SCOPE_END
+#undef _GLIBCXX20_CONSTEXPR_NON_LITERAL_SCOPE_BEGIN
 #undef _GLIBCXX_DEBUG_VERIFY_DIST_OPERANDS
 #undef _GLIBCXX_DEBUG_VERIFY_REL_OPERANDS
 #undef _GLIBCXX_DEBUG_VERIFY_EQ_OPERANDS

libstdc++-v3/testsuite/23_containers/vector/bool/element_access/constexpr.cc

@@ -18,22 +18,40 @@ test_iterators()
   VERIFY( v.crend() == v.rend() );
 
   auto it = v.begin();
+  VERIFY( it[0] == 0 );
   VERIFY( *it == v.front() );
+  VERIFY( it[1] == v[1] );
   VERIFY( it++ == v.begin() );
   VERIFY( ++it == v.end() );
   VERIFY( (it - 2) == v.begin() );
+  VERIFY( (it - v.begin()) == 2 );
   it -= 2;
   it += 1;
   VERIFY( (it + 1) == v.end() );
+  VERIFY( (1 + it) == v.end() );
+  it = it + 1;
+  auto it2 = v.begin();
+  std::swap(it, it2);
+  VERIFY( it == v.begin() );
+  VERIFY( it2 == v.end() );
 
   auto rit = v.rbegin();
+  VERIFY( rit[0] == 0 );
   VERIFY( *rit == v.back() );
+  VERIFY( rit[1] == v[0] );
   VERIFY( rit++ == v.rbegin() );
   VERIFY( ++rit == v.rend() );
   VERIFY( (rit - 2) == v.rbegin() );
+  VERIFY( (rit - v.rbegin()) == 2 );
   rit -= 2;
   rit += 1;
   VERIFY( (rit + 1) == v.rend() );
+  VERIFY( (1 + rit) == v.rend() );
+  rit = rit + 1;
+  auto rit2 = v.rbegin();
+  std::swap(rit, rit2);
+  VERIFY( rit == v.rbegin() );
+  VERIFY( rit2 == v.rend() );
 
   return true;
 }

libstdc++-v3/testsuite/23_containers/vector/element_access/constexpr.cc

@@ -18,22 +18,40 @@ test_iterators()
   VERIFY( v.crend() == v.rend() );
 
   auto it = v.begin();
+  VERIFY( it[0] == 0 );
   VERIFY( &*it == &v.front() );
+  VERIFY( &it[1] == &v[1] );
   VERIFY( it++ == v.begin() );
   VERIFY( ++it == v.end() );
   VERIFY( (it - 2) == v.begin() );
+  VERIFY( (it - v.begin()) == 2 );
   it -= 2;
   it += 1;
   VERIFY( (it + 1) == v.end() );
+  VERIFY( (1 + it) == v.end() );
+  it = it + 1;
+  auto it2 = v.begin();
+  std::swap(it, it2);
+  VERIFY( it == v.begin() );
+  VERIFY( it2 == v.end() );
 
   auto rit = v.rbegin();
+  VERIFY( rit[0] == 0 );
   VERIFY( &*rit == &v.back() );
+  VERIFY( &rit[1] == &v[0] );
   VERIFY( rit++ == v.rbegin() );
   VERIFY( ++rit == v.rend() );
   VERIFY( (rit - 2) == v.rbegin() );
+  VERIFY( (rit - v.rbegin()) == 2 );
   rit -= 2;
   rit += 1;
   VERIFY( (rit + 1) == v.rend() );
+  VERIFY( (1 + rit) == v.rend() );
+  rit = rit + 1;
+  auto rit2 = v.rbegin();
+  std::swap(rit, rit2);
+  VERIFY( rit == v.rbegin() );
+  VERIFY( rit2 == v.rend() );
 
   return true;
 }

libstdc++-v3/testsuite/std/ranges/adaptors/all.cc

@@ -156,11 +156,7 @@ test07()
 constexpr bool
 test08()
 {
-#ifdef _GLIBCXX_DEBUG
-  using std::_GLIBCXX_STD_C::vector;
-#else
   using std::vector;
-#endif
 
   // Verify P2415R2 "What is a view?" changes.
   // In particular, rvalue non-view non-borrowed ranges are now viewable.