libstdc++: Fix std::ranges::to errors

Fix some errors that Patrick noticed, and remove a #if 0 group that I
didn't mean to leave in the file.

libstdc++-v3/ChangeLog:

	* include/std/ranges (__detail::__toable): Fix incorrect use of
	_Range instead of _Cont.
	(__detail::_ToClosure, __detail::_ToClosure2): Add missing
	constexpr specifier on constructors.
	* testsuite/std/ranges/conv/1.cc (_Cont, _Cont2, _Cont3): Remove
	unnecessary begin() and end() members.
	(test_constexpr): New function to check range adaptors are
	usable in constant expressions.

libstdc++-v3/include/std/ranges

@@ -9251,7 +9251,7 @@ namespace __detail
 
   template<typename _Cont, typename _Range>
     constexpr bool __toable = requires {
-      requires (!input_range<_Range>
+      requires (!input_range<_Cont>
 		  || convertible_to<range_reference_t<_Range>,
 				    range_value_t<_Cont>>);
     };
@@ -9290,7 +9290,7 @@ namespace __detail
 	  else if constexpr (constructible_from<_Cont, from_range_t, _Rg, _Args...>)
 	    return _Cont(from_range, std::forward<_Rg>(__r),
 			 std::forward<_Args>(__args)...);
-	  else if constexpr (requires { common_range<_Rg>;
+	  else if constexpr (requires { requires common_range<_Rg>;
 		typename __iter_category_t<iterator_t<_Rg>>;
 		requires derived_from<__iter_category_t<iterator_t<_Rg>>,
 				      input_iterator_tag>;
@@ -9346,26 +9346,6 @@ namespace __detail
       bool operator==(const _InputIter&) const;
     };
 
-#if 0
-  template<template<typename...> typename _Cont, typename _Rg,
-	   typename... _Args>
-    concept __deduce_expr_1 = requires {
-      _Cont(std::declval<_Rg>(), std::declval<_Args>()...);
-    };
-
-  template<template<typename...> typename _Cont, typename _Rg,
-	   typename... _Args>
-    concept __deduce_expr_2 = requires {
-      _Cont(from_range, std::declval<_Rg>(), std::declval<_Args>()...);
-    };
-
-  template<template<typename...> typename _Cont, typename _Rg,
-	   typename... _Args>
-    concept __deduce_expr_3 = requires(_InputIter<_Rg> __i) {
-      _Cont(std::move(__i), std::move(__i), std::declval<_Args>()...);
-    };
-#endif
-
   template<template<typename...> typename _Cont, input_range _Rg,
 	   typename... _Args>
     using _DeduceExpr1
@@ -9418,6 +9398,7 @@ namespace __detail
       tuple<decay_t<_Args>...> _M_bound_args;
 
     public:
+      constexpr
       _ToClosure(_Args&&... __args)
       : _M_bound_args(std::forward<_Args>(__args)...)
       { }
@@ -9498,6 +9479,7 @@ namespace __detail
       tuple<decay_t<_Args>...> _M_bound_args;
 
     public:
+      constexpr
       _ToClosure2(_Args&&... __args)
       : _M_bound_args(std::forward<_Args>(__args)...)
       { }

libstdc++-v3/testsuite/std/ranges/conv/1.cc

@@ -89,9 +89,6 @@ struct Cont1
     : c(r, args...)
     { }
 
-  typename C::iterator begin();
-  typename C::iterator end();
-
   C c;
 };
 
@@ -153,9 +150,6 @@ struct Cont2
     : c(r, args...)
     { }
 
-  typename C::iterator begin();
-  typename C::iterator end();
-
   C c;
 };
 
@@ -186,9 +180,6 @@ struct Cont3
     : c(first, last, args...)
     { }
 
-  typename C::iterator begin();
-  typename C::iterator end();
-
   C c;
 };
 
@@ -222,10 +213,6 @@ struct Cont4
   Cont4() { }
   Cont4(typename C::allocator_type a) : c(a) { }
 
-  // Required to satisfy range
-  typename C::iterator begin() { return c.begin(); }
-  typename C::iterator end() { return c.end(); }
-
   // Satisfying container-insertable requires either this ...
   template<typename T>
     requires UsePushBack
@@ -254,7 +241,9 @@ struct Cont4
     used_reserve = true;
   }
 
-  // Required to satisfy reservable-container
+  // Satisfying sized_range is required to satisfy reservable-container
+  typename C::iterator begin() { return c.begin(); }
+  typename C::iterator end() { return c.end(); }
   auto size() const { return c.size(); }
 
   // Required to satisfy reservable-container
@@ -355,6 +344,17 @@ test_nodiscard()
   std::ranges::to<std::vector>();        // { dg-warning "ignoring return" }
 }
 
+void
+test_constexpr()
+{
+  constexpr int x = [](int i) {
+    auto c1 = std::views::iota(1, i) | std::ranges::to<std::vector<int>>();
+    auto c2 = std::views::iota(i, 10) | std::ranges::to<std::vector>();
+    return c1[0] + c2[0];
+  }(5);
+  static_assert(x == 6);
+}
+
 int main()
 {
   test_p1206r7_examples();
@@ -366,4 +366,5 @@ int main()
   test_2_2();
   test_lwg3984();
   test_nodiscard();
+  test_constexpr();
 }