libstdc++: Make __normal_iterator constexpr, always_inline, nodiscard

The __gnu_cxx::__normal_iterator type we use for std::vector::iterator
is not specified by the standard, it's an implementation detail. This
means it's not constrained by the rule that forbids strengthening
constexpr. We can make it meet the constexpr iterator requirements for
older standards, not only when it's required to be for C++20.

For the non-const member functions they can't be constexpr in C++11, so
use _GLIBCXX14_CONSTEXPR for those. For all constructors, const members
and non-member operator overloads, use _GLIBCXX_CONSTEXPR or just
constexpr.

We can also liberally add [[nodiscard]] and [[gnu::always_inline]]
attributes to those functions.

Also change some internal helpers for std::move_iterator which can be
unconditionally constexpr and marked nodiscard.

libstdc++-v3/ChangeLog:

	* include/bits/stl_iterator.h (__normal_iterator): Make all
	members and overloaded operators constexpr before C++20, and add
	always_inline attribute
	(__to_address): Add nodiscard and always_inline attributes.
	(__make_move_if_noexcept_iterator): Add nodiscard
	and make unconditionally constexpr.
	(__niter_base(__normal_iterator), __niter_base(Iter)):
	Add nodiscard and always_inline attributes.
	(__niter_base(reverse_iterator), __niter_base(move_iterator))
	(__miter_base): Add inline.
	(__niter_wrap(From, To)): Add nodiscard attribute.
	(__niter_wrap(const Iter&, Iter)): Add nodiscard and
	always_inline attributes.

Reviewed-by: Patrick Palka <ppalka@redhat.com>

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

@@ -1046,24 +1046,29 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       using iterator_concept = std::__detail::__iter_concept<_Iterator>;
 #endif
 
-      _GLIBCXX_CONSTEXPR __normal_iterator() _GLIBCXX_NOEXCEPT
-      : _M_current(_Iterator()) { }
+      __attribute__((__always_inline__))
+      _GLIBCXX_CONSTEXPR
+      __normal_iterator() _GLIBCXX_NOEXCEPT
+      : _M_current() { }
 
-      explicit _GLIBCXX20_CONSTEXPR
+      __attribute__((__always_inline__))
+      explicit _GLIBCXX_CONSTEXPR
       __normal_iterator(const _Iterator& __i) _GLIBCXX_NOEXCEPT
       : _M_current(__i) { }
 
       // Allow iterator to const_iterator conversion
 #if __cplusplus >= 201103L
       template<typename _Iter, typename = __convertible_from<_Iter>>
-	_GLIBCXX20_CONSTEXPR
+	[[__gnu__::__always_inline__]]
+	constexpr
 	__normal_iterator(const __normal_iterator<_Iter, _Container>& __i)
 	noexcept
 #else
       // N.B. _Container::pointer is not actually in container requirements,
       // but is present in std::vector and std::basic_string.
       template<typename _Iter>
-        __normal_iterator(const __normal_iterator<_Iter,
+	__attribute__((__always_inline__))
+	__normal_iterator(const __normal_iterator<_Iter,
 			  typename __enable_if<
 	       (std::__are_same<_Iter, typename _Container::pointer>::__value),
 		      _Container>::__type>& __i)
@@ -1071,17 +1076,21 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
         : _M_current(__i.base()) { }
 
       // Forward iterator requirements
-      _GLIBCXX20_CONSTEXPR
+
+      __attribute__((__always_inline__)) _GLIBCXX_NODISCARD
+      _GLIBCXX_CONSTEXPR
       reference
       operator*() const _GLIBCXX_NOEXCEPT
       { return *_M_current; }
 
-      _GLIBCXX20_CONSTEXPR
+      __attribute__((__always_inline__)) _GLIBCXX_NODISCARD
+      _GLIBCXX_CONSTEXPR
       pointer
       operator->() const _GLIBCXX_NOEXCEPT
       { return _M_current; }
 
-      _GLIBCXX20_CONSTEXPR
+      __attribute__((__always_inline__))
+      _GLIBCXX14_CONSTEXPR
       __normal_iterator&
       operator++() _GLIBCXX_NOEXCEPT
       {
@@ -1089,13 +1098,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	return *this;
       }
 
-      _GLIBCXX20_CONSTEXPR
+      __attribute__((__always_inline__))
+      _GLIBCXX14_CONSTEXPR
       __normal_iterator
       operator++(int) _GLIBCXX_NOEXCEPT
       { return __normal_iterator(_M_current++); }
 
       // Bidirectional iterator requirements
-      _GLIBCXX20_CONSTEXPR
+
+      __attribute__((__always_inline__))
+      _GLIBCXX14_CONSTEXPR
       __normal_iterator&
       operator--() _GLIBCXX_NOEXCEPT
       {
@@ -1103,38 +1115,46 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	return *this;
       }
 
-      _GLIBCXX20_CONSTEXPR
+      __attribute__((__always_inline__))
+      _GLIBCXX14_CONSTEXPR
       __normal_iterator
       operator--(int) _GLIBCXX_NOEXCEPT
       { return __normal_iterator(_M_current--); }
 
       // Random access iterator requirements
-      _GLIBCXX20_CONSTEXPR
+
+      __attribute__((__always_inline__)) _GLIBCXX_NODISCARD
+      _GLIBCXX_CONSTEXPR
       reference
       operator[](difference_type __n) const _GLIBCXX_NOEXCEPT
       { return _M_current[__n]; }
 
-      _GLIBCXX20_CONSTEXPR
+      __attribute__((__always_inline__))
+      _GLIBCXX14_CONSTEXPR
       __normal_iterator&
       operator+=(difference_type __n) _GLIBCXX_NOEXCEPT
       { _M_current += __n; return *this; }
 
-      _GLIBCXX20_CONSTEXPR
+      __attribute__((__always_inline__)) _GLIBCXX_NODISCARD
+      _GLIBCXX_CONSTEXPR
       __normal_iterator
       operator+(difference_type __n) const _GLIBCXX_NOEXCEPT
       { return __normal_iterator(_M_current + __n); }
 
-      _GLIBCXX20_CONSTEXPR
+      __attribute__((__always_inline__))
+      _GLIBCXX14_CONSTEXPR
       __normal_iterator&
       operator-=(difference_type __n) _GLIBCXX_NOEXCEPT
       { _M_current -= __n; return *this; }
 
-      _GLIBCXX20_CONSTEXPR
+      __attribute__((__always_inline__)) _GLIBCXX_NODISCARD
+      _GLIBCXX_CONSTEXPR
       __normal_iterator
       operator-(difference_type __n) const _GLIBCXX_NOEXCEPT
       { return __normal_iterator(_M_current - __n); }
 
-      _GLIBCXX20_CONSTEXPR
+      __attribute__((__always_inline__)) _GLIBCXX_NODISCARD
+      _GLIBCXX_CONSTEXPR
       const _Iterator&
       base() const _GLIBCXX_NOEXCEPT
       { return _M_current; }
@@ -1150,7 +1170,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
 #if __cpp_lib_three_way_comparison
   template<typename _IteratorL, typename _IteratorR, typename _Container>
-    [[nodiscard]]
+    [[nodiscard, __gnu__::__always_inline__]]
     constexpr bool
     operator==(const __normal_iterator<_IteratorL, _Container>& __lhs,
 	       const __normal_iterator<_IteratorR, _Container>& __rhs)
@@ -1161,7 +1181,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     { return __lhs.base() == __rhs.base(); }
 
   template<typename _IteratorL, typename _IteratorR, typename _Container>
-    [[nodiscard]]
+    [[nodiscard, __gnu__::__always_inline__]]
     constexpr std::__detail::__synth3way_t<_IteratorR, _IteratorL>
     operator<=>(const __normal_iterator<_IteratorL, _Container>& __lhs,
 		const __normal_iterator<_IteratorR, _Container>& __rhs)
@@ -1169,7 +1189,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     { return std::__detail::__synth3way(__lhs.base(), __rhs.base()); }
 
   template<typename _Iterator, typename _Container>
-    [[nodiscard]]
+    [[nodiscard, __gnu__::__always_inline__]]
     constexpr bool
     operator==(const __normal_iterator<_Iterator, _Container>& __lhs,
 	       const __normal_iterator<_Iterator, _Container>& __rhs)
@@ -1180,7 +1200,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     { return __lhs.base() == __rhs.base(); }
 
   template<typename _Iterator, typename _Container>
-    [[nodiscard]]
+    [[nodiscard, __gnu__::__always_inline__]]
     constexpr std::__detail::__synth3way_t<_Iterator>
     operator<=>(const __normal_iterator<_Iterator, _Container>& __lhs,
 		const __normal_iterator<_Iterator, _Container>& __rhs)
@@ -1189,7 +1209,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #else
    // Forward iterator requirements
   template<typename _IteratorL, typename _IteratorR, typename _Container>
-    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
+    __attribute__((__always_inline__)) _GLIBCXX_NODISCARD _GLIBCXX_CONSTEXPR
     inline bool
     operator==(const __normal_iterator<_IteratorL, _Container>& __lhs,
 	       const __normal_iterator<_IteratorR, _Container>& __rhs)
@@ -1197,7 +1217,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     { return __lhs.base() == __rhs.base(); }
 
   template<typename _Iterator, typename _Container>
-    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
+    __attribute__((__always_inline__)) _GLIBCXX_NODISCARD _GLIBCXX_CONSTEXPR
     inline bool
     operator==(const __normal_iterator<_Iterator, _Container>& __lhs,
 	       const __normal_iterator<_Iterator, _Container>& __rhs)
@@ -1205,7 +1225,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     { return __lhs.base() == __rhs.base(); }
 
   template<typename _IteratorL, typename _IteratorR, typename _Container>
-    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
+    __attribute__((__always_inline__)) _GLIBCXX_NODISCARD _GLIBCXX_CONSTEXPR
     inline bool
     operator!=(const __normal_iterator<_IteratorL, _Container>& __lhs,
 	       const __normal_iterator<_IteratorR, _Container>& __rhs)
@@ -1213,7 +1233,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     { return __lhs.base() != __rhs.base(); }
 
   template<typename _Iterator, typename _Container>
-    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
+    __attribute__((__always_inline__)) _GLIBCXX_NODISCARD _GLIBCXX_CONSTEXPR
     inline bool
     operator!=(const __normal_iterator<_Iterator, _Container>& __lhs,
 	       const __normal_iterator<_Iterator, _Container>& __rhs)
@@ -1222,7 +1242,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   // Random access iterator requirements
   template<typename _IteratorL, typename _IteratorR, typename _Container>
-    _GLIBCXX_NODISCARD
+    __attribute__((__always_inline__)) _GLIBCXX_NODISCARD _GLIBCXX_CONSTEXPR
     inline bool
     operator<(const __normal_iterator<_IteratorL, _Container>& __lhs,
 	      const __normal_iterator<_IteratorR, _Container>& __rhs)
@@ -1230,7 +1250,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     { return __lhs.base() < __rhs.base(); }
 
   template<typename _Iterator, typename _Container>
-    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
+    __attribute__((__always_inline__)) _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
     inline bool
     operator<(const __normal_iterator<_Iterator, _Container>& __lhs,
 	      const __normal_iterator<_Iterator, _Container>& __rhs)
@@ -1238,7 +1258,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     { return __lhs.base() < __rhs.base(); }
 
   template<typename _IteratorL, typename _IteratorR, typename _Container>
-    _GLIBCXX_NODISCARD
+    __attribute__((__always_inline__)) _GLIBCXX_NODISCARD _GLIBCXX_CONSTEXPR
     inline bool
     operator>(const __normal_iterator<_IteratorL, _Container>& __lhs,
 	      const __normal_iterator<_IteratorR, _Container>& __rhs)
@@ -1246,7 +1266,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     { return __lhs.base() > __rhs.base(); }
 
   template<typename _Iterator, typename _Container>
-    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
+    __attribute__((__always_inline__)) _GLIBCXX_NODISCARD _GLIBCXX_CONSTEXPR
     inline bool
     operator>(const __normal_iterator<_Iterator, _Container>& __lhs,
 	      const __normal_iterator<_Iterator, _Container>& __rhs)
@@ -1254,7 +1274,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     { return __lhs.base() > __rhs.base(); }
 
   template<typename _IteratorL, typename _IteratorR, typename _Container>
-    _GLIBCXX_NODISCARD
+    __attribute__((__always_inline__)) _GLIBCXX_NODISCARD _GLIBCXX_CONSTEXPR
     inline bool
     operator<=(const __normal_iterator<_IteratorL, _Container>& __lhs,
 	       const __normal_iterator<_IteratorR, _Container>& __rhs)
@@ -1262,7 +1282,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     { return __lhs.base() <= __rhs.base(); }
 
   template<typename _Iterator, typename _Container>
-    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
+    __attribute__((__always_inline__)) _GLIBCXX_NODISCARD _GLIBCXX_CONSTEXPR
     inline bool
     operator<=(const __normal_iterator<_Iterator, _Container>& __lhs,
 	       const __normal_iterator<_Iterator, _Container>& __rhs)
@@ -1270,7 +1290,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     { return __lhs.base() <= __rhs.base(); }
 
   template<typename _IteratorL, typename _IteratorR, typename _Container>
-    _GLIBCXX_NODISCARD
+    __attribute__((__always_inline__)) _GLIBCXX_NODISCARD _GLIBCXX_CONSTEXPR
     inline bool
     operator>=(const __normal_iterator<_IteratorL, _Container>& __lhs,
 	       const __normal_iterator<_IteratorR, _Container>& __rhs)
@@ -1278,7 +1298,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     { return __lhs.base() >= __rhs.base(); }
 
   template<typename _Iterator, typename _Container>
-    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
+    __attribute__((__always_inline__)) _GLIBCXX_NODISCARD _GLIBCXX_CONSTEXPR
     inline bool
     operator>=(const __normal_iterator<_Iterator, _Container>& __lhs,
 	       const __normal_iterator<_Iterator, _Container>& __rhs)
@@ -1293,8 +1313,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<typename _IteratorL, typename _IteratorR, typename _Container>
 #if __cplusplus >= 201103L
     // DR 685.
-    [[__nodiscard__]] _GLIBCXX20_CONSTEXPR
-    inline auto
+    [[__nodiscard__, __gnu__::__always_inline__]]
+    constexpr auto
     operator-(const __normal_iterator<_IteratorL, _Container>& __lhs,
 	      const __normal_iterator<_IteratorR, _Container>& __rhs) noexcept
     -> decltype(__lhs.base() - __rhs.base())
@@ -1306,7 +1326,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     { return __lhs.base() - __rhs.base(); }
 
   template<typename _Iterator, typename _Container>
-    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
+    __attribute__((__always_inline__)) _GLIBCXX_NODISCARD _GLIBCXX_CONSTEXPR
     inline typename __normal_iterator<_Iterator, _Container>::difference_type
     operator-(const __normal_iterator<_Iterator, _Container>& __lhs,
 	      const __normal_iterator<_Iterator, _Container>& __rhs)
@@ -1314,7 +1334,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     { return __lhs.base() - __rhs.base(); }
 
   template<typename _Iterator, typename _Container>
-    _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
+    __attribute__((__always_inline__)) _GLIBCXX_NODISCARD _GLIBCXX_CONSTEXPR
     inline __normal_iterator<_Iterator, _Container>
     operator+(typename __normal_iterator<_Iterator, _Container>::difference_type
 	      __n, const __normal_iterator<_Iterator, _Container>& __i)
@@ -1332,6 +1352,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   // Need to overload __to_address because the pointer_traits primary template
   // will deduce element_type of __normal_iterator<T*, C> as T* rather than T.
   template<typename _Iterator, typename _Container>
+    [[__gnu__::__always_inline__]] _GLIBCXX_NODISCARD
     constexpr auto
     __to_address(const __gnu_cxx::__normal_iterator<_Iterator,
 						    _Container>& __it) noexcept
@@ -1786,7 +1807,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     = __conditional_t<__move_if_noexcept_cond
       <typename iterator_traits<_Iterator>::value_type>::value,
 		_Iterator, move_iterator<_Iterator>>>
-    inline _GLIBCXX17_CONSTEXPR _ReturnType
+    [[__nodiscard__]]
+    constexpr _ReturnType
     __make_move_if_noexcept_iterator(_Iterator __i)
     { return _ReturnType(__i); }
 
@@ -1795,7 +1817,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<typename _Tp, typename _ReturnType
     = __conditional_t<__move_if_noexcept_cond<_Tp>::value,
 		      const _Tp*, move_iterator<_Tp*>>>
-    inline _GLIBCXX17_CONSTEXPR _ReturnType
+    [[__nodiscard__]]
+    constexpr _ReturnType
     __make_move_if_noexcept_iterator(_Tp* __i)
     { return _ReturnType(__i); }
 
@@ -1806,7 +1829,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       typedef __true_type __type;
     };
 
-
 #define _GLIBCXX_MAKE_MOVE_ITERATOR(_Iter) std::make_move_iterator(_Iter)
 #define _GLIBCXX_MAKE_MOVE_IF_NOEXCEPT_ITERATOR(_Iter) \
   std::__make_move_if_noexcept_iterator(_Iter)
@@ -2968,14 +2990,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   // Unwrap a __normal_iterator to get the underlying iterator
   // (usually a pointer). See uses in std::copy, std::fill, etc.
   template<typename _Iterator, typename _Container>
+    __attribute__((__always_inline__)) _GLIBCXX_NODISCARD
     _GLIBCXX20_CONSTEXPR
-    _Iterator
+    inline _Iterator
     __niter_base(__gnu_cxx::__normal_iterator<_Iterator, _Container> __it)
     _GLIBCXX_NOEXCEPT_IF(std::is_nothrow_copy_constructible<_Iterator>::value)
     { return __it.base(); }
 
   // Fallback implementation used for iterators that can't be unwrapped.
   template<typename _Iterator>
+    __attribute__((__always_inline__)) _GLIBCXX_NODISCARD
     _GLIBCXX20_CONSTEXPR
     inline _Iterator
     __niter_base(_Iterator __it)
@@ -3007,28 +3031,28 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #if __cplusplus >= 201103L
   template<typename _Iterator>
     _GLIBCXX20_CONSTEXPR
-    auto
+    inline auto
     __niter_base(reverse_iterator<_Iterator> __it)
     -> decltype(__make_reverse_iterator(__niter_base(__it.base())))
     { return __make_reverse_iterator(__niter_base(__it.base())); }
 
   template<typename _Iterator>
     _GLIBCXX20_CONSTEXPR
-    auto
+    inline auto
     __niter_base(move_iterator<_Iterator> __it)
     -> decltype(make_move_iterator(__niter_base(__it.base())))
     { return make_move_iterator(__niter_base(__it.base())); }
 
   template<typename _Iterator>
     _GLIBCXX20_CONSTEXPR
-    auto
+    inline auto
     __miter_base(reverse_iterator<_Iterator> __it)
     -> decltype(__make_reverse_iterator(__miter_base(__it.base())))
     { return __make_reverse_iterator(__miter_base(__it.base())); }
 
   template<typename _Iterator>
     _GLIBCXX20_CONSTEXPR
-    auto
+    inline auto
     __miter_base(move_iterator<_Iterator> __it)
     -> decltype(__miter_base(__it.base()))
     { return __miter_base(__it.base()); }
@@ -3039,6 +3063,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   // random access iterators, like pointers).
   // All overloads of std::__niter_base must be declared before this.
   template<typename _From, typename _To>
+    _GLIBCXX_NODISCARD
     _GLIBCXX20_CONSTEXPR
     inline _From
     __niter_wrap(_From __from, _To __res)
@@ -3046,6 +3071,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   // No need to wrap, iterator already has the right type.
   template<typename _Iterator>
+    __attribute__((__always_inline__)) _GLIBCXX_NODISCARD
     _GLIBCXX20_CONSTEXPR
     inline _Iterator
     __niter_wrap(const _Iterator&, _Iterator __res)