[gcc r12-1566] libstdc++: Refine range adaptors' "simple extra args" mechanism [PR100940]

[gcc r12-1566] libstdc++: Refine range adaptors' "simple extra args" mechanism [PR100940]

[Patrick Palka]

https://gcc.gnu.org/g:0f4a2fb44dad6c9c140226fc19ed16109b85e6f4

commit r12-1566-g0f4a2fb44dad6c9c140226fc19ed16109b85e6f4
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Jun 17 09:46:04 2021 -0400

    libstdc++: Refine range adaptors' "simple extra args" mechanism [PR100940]
    
    The _S_has_simple_extra_args mechanism is used to simplify forwarding
    of range adaptor's extra arguments when perfect forwarding call wrapper
    semantics isn't required for correctness, on a per-adaptor basis.
    Both views::take and views::drop are flagged as such, but it turns out
    perfect forwarding semantics are needed for these adaptors in some
    contrived cases, e.g. when their extra argument is a move-only class
    that's implicitly convertible to an integral type.
    
    To fix this, we could just clear the flag for views::take/drop as with
    views::split, but that'd come at the cost of acceptable diagnostics
    for ill-formed uses of these adaptors (see PR100577).
    
    This patch instead allows adaptors to parameterize their
    _S_has_simple_extra_args flag according the types of the captured extra
    arguments, so that we could conditionally disable perfect forwarding
    semantics only when the types of the extra arguments permit it.  We
    then use this finer-grained mechanism to safely disable perfect
    forwarding semantics for views::take/drop when the extra argument is
    integer-like, rather than incorrectly always disabling it.  Similarly,
    for views::split, rather than always enabling perfect forwarding
    semantics we now safely disable it when the extra argument is a scalar
    or a view, and recover good diagnostics for these common cases.
    
            PR libstdc++/100940
    
    libstdc++-v3/ChangeLog:
    
            * include/std/ranges (__adaptor::_RangeAdaptor): Document the
            template form of _S_has_simple_extra_args.
            (__adaptor::__adaptor_has_simple_extra_args): Add _Args template
            parameter pack.  Try to treat _S_has_simple_extra_args as a
            variable template parameterized by _Args.
            (__adaptor::_Partial): Pass _Arg/_Args to the constraint
            __adaptor_has_simple_extra_args.
            (views::_Take::_S_has_simple_extra_args): Templatize according
            to the type of the extra argument.
            (views::_Drop::_S_has_simple_extra_args): Likewise.
            (views::_Split::_S_has_simple_extra_args): Define.
            * testsuite/std/ranges/adaptors/100577.cc (test01, test02):
            Adjust after changes to _S_has_simple_extra_args mechanism.
            (test03): Define.

Diff:
---
 libstdc++-v3/include/std/ranges                    | 37 ++++++++++-----
 .../testsuite/std/ranges/adaptors/100577.cc        | 55 ++++++++++++++--------
 2 files changed, 61 insertions(+), 31 deletions(-)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index b2943490e31..a4289009bc4 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -796,7 +796,9 @@ namespace views::__adaptor
   //
   // The optional static data member _Derived::_S_has_simple_extra_args should
   // be defined to true if the behavior of this adaptor is independent of the
-  // constness/value category of the extra arguments.
+  // constness/value category of the extra arguments.  This data member could
+  // also be defined as a variable template parameterized by the types of the
+  // extra arguments.
   template<typename _Derived>
     struct _RangeAdaptor
     {
@@ -818,9 +820,10 @@ namespace views::__adaptor
     concept __closure_has_simple_call_op = _Adaptor::_S_has_simple_call_op;
 
   // True if the behavior of the range adaptor non-closure _Adaptor is
-  // independent of the value category of its extra arguments.
-  template<typename _Adaptor>
-    concept __adaptor_has_simple_extra_args = _Adaptor::_S_has_simple_extra_args;
+  // independent of the value category of its extra arguments _Args.
+  template<typename _Adaptor, typename... _Args>
+    concept __adaptor_has_simple_extra_args = _Adaptor::_S_has_simple_extra_args
+      || _Adaptor::template _S_has_simple_extra_args<_Args...>;
 
   // A range adaptor closure that represents partial application of
   // the range adaptor _Adaptor with arguments _Args.
@@ -897,7 +900,7 @@ namespace views::__adaptor
   // This lets us get away with a single operator() overload, which makes
   // overload resolution failure diagnostics more concise.
   template<typename _Adaptor, typename... _Args>
-    requires __adaptor_has_simple_extra_args<_Adaptor>
+    requires __adaptor_has_simple_extra_args<_Adaptor, _Args...>
     struct _Partial<_Adaptor, _Args...> : _RangeAdaptorClosure
     {
       tuple<_Args...> _M_args;
@@ -926,7 +929,7 @@ namespace views::__adaptor
   // A lightweight specialization of the above template for the common case
   // where _Adaptor accepts a single extra argument.
   template<typename _Adaptor, typename _Arg>
-    requires __adaptor_has_simple_extra_args<_Adaptor>
+    requires __adaptor_has_simple_extra_args<_Adaptor, _Arg>
     struct _Partial<_Adaptor, _Arg> : _RangeAdaptorClosure
     {
       _Arg _M_arg;
@@ -2112,7 +2115,12 @@ namespace views::__adaptor
 
       using _RangeAdaptor<_Take>::operator();
       static constexpr int _S_arity = 2;
-      static constexpr bool _S_has_simple_extra_args = true;
+      // The count argument of views::take is not always simple -- it can be
+      // e.g. a move-only class that's implicitly convertible to the difference
+      // type.  But an integer-like count argument is surely simple.
+      template<typename _Tp>
+	static constexpr bool _S_has_simple_extra_args
+	  = ranges::__detail::__is_integer_like<_Tp>;
     };
 
     inline constexpr _Take take;
@@ -2352,7 +2360,9 @@ namespace views::__adaptor
 
       using _RangeAdaptor<_Drop>::operator();
       static constexpr int _S_arity = 2;
-      static constexpr bool _S_has_simple_extra_args = true;
+      template<typename _Tp>
+	static constexpr bool _S_has_simple_extra_args
+	  = _Take::_S_has_simple_extra_args<_Tp>;
     };
 
     inline constexpr _Drop drop;
@@ -3230,9 +3240,14 @@ namespace views::__adaptor
 
       using _RangeAdaptor<_Split>::operator();
       static constexpr int _S_arity = 2;
-      // The second argument of views::split is _not_ simple -- it can be a
-      // non-view range, the value category of which affects whether the call is
-      // well-formed.  So we must not define _S_has_simple_extra_args to true.
+      // The pattern argument of views::split is not always simple -- it can be
+      // a non-view range, the value category of which affects whether the call
+      // is well-formed.  But a scalar or a view pattern argument is surely
+      // simple.
+      template<typename _Pattern>
+	static constexpr bool _S_has_simple_extra_args
+	  = is_scalar_v<_Pattern> || (view<_Pattern>
+				      && copy_constructible<_Pattern>);
     };
 
     inline constexpr _Split split;
diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/100577.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/100577.cc
index 29176c8b7b0..8ef084621f9 100644
--- a/libstdc++-v3/testsuite/std/ranges/adaptors/100577.cc
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/100577.cc
@@ -28,16 +28,18 @@ namespace views = std::ranges::views;
 void
 test01()
 {
-  // Verify all multi-argument adaptors except for views::split are denoted
-  // to have simple extra arguments.
+  // Verify adaptors are deemed to have simple extra arguments when appropriate.
   using views::__adaptor::__adaptor_has_simple_extra_args;
-  static_assert(__adaptor_has_simple_extra_args<decltype(views::transform)>);
-  static_assert(__adaptor_has_simple_extra_args<decltype(views::filter)>);
-  static_assert(__adaptor_has_simple_extra_args<decltype(views::drop)>);
-  static_assert(__adaptor_has_simple_extra_args<decltype(views::take)>);
-  static_assert(__adaptor_has_simple_extra_args<decltype(views::take_while)>);
-  static_assert(__adaptor_has_simple_extra_args<decltype(views::drop_while)>);
-  static_assert(!__adaptor_has_simple_extra_args<decltype(views::split)>);
+  using std::identity;
+  static_assert(__adaptor_has_simple_extra_args<decltype(views::transform), identity>);
+  static_assert(__adaptor_has_simple_extra_args<decltype(views::filter), identity>);
+  static_assert(__adaptor_has_simple_extra_args<decltype(views::drop), int>);
+  static_assert(__adaptor_has_simple_extra_args<decltype(views::take), int>);
+  static_assert(__adaptor_has_simple_extra_args<decltype(views::take_while), identity>);
+  static_assert(__adaptor_has_simple_extra_args<decltype(views::drop_while), identity>);
+  static_assert(__adaptor_has_simple_extra_args<decltype(views::split), std::string_view>);
+  static_assert(__adaptor_has_simple_extra_args<decltype(views::split), char>);
+  static_assert(!__adaptor_has_simple_extra_args<decltype(views::split), std::string>);
 
   // Verify all adaptor closures except for views::split(pattern) have a simple
   // operator().
@@ -53,15 +55,17 @@ test01()
   __closure_has_simple_call_op auto a08 = views::common;
   __closure_has_simple_call_op auto a09 = views::reverse;
   __closure_has_simple_call_op auto a10 = views::keys;
+  __closure_has_simple_call_op auto a11 = views::split(' ');
   // Verify composition of simple closures is simple.
   __closure_has_simple_call_op auto b
-    = (a00 | a01) | (a02 | a03) | (a04 | a05 | a06) | (a07 | a08 | a09 | a10);
+    = (a00 | a01) | (a02 | a03) | (a04 | a05 | a06) | (a07 | a08 | a09 | a10) | a11;
 
-  // Verify views::split is the exception.
-  auto a11 = views::split(' ');
-  static_assert(!__closure_has_simple_call_op<decltype(a11)>);
-  static_assert(!__closure_has_simple_call_op<decltype(a11 | a00)>);
-  static_assert(!__closure_has_simple_call_op<decltype(a00 | a11)>);
+  // Verify views::split(non_view_range) is an exception.
+  extern std::string s;
+  auto a12 = views::split(s);
+  static_assert(!__closure_has_simple_call_op<decltype(a12)>);
+  static_assert(!__closure_has_simple_call_op<decltype(a12 | a00)>);
+  static_assert(!__closure_has_simple_call_op<decltype(a00 | a12)>);
 }
 
 void
@@ -71,18 +75,14 @@ test02()
   // fallback deleted overload, so when a call is ill-formed overload resolution
   // fails.
   extern int x[10];
-  auto badarg = nullptr;
+  struct { } badarg;
   views::transform(badarg)(x); // { dg-error "no match" }
   views::filter(badarg)(x); // { dg-error "no match" }
-  views::take(badarg)(x); // { dg-error "no match" }
-  views::drop(badarg)(x); // { dg-error "no match" }
   views::take_while(badarg)(x); // { dg-error "no match" }
   views::drop_while(badarg)(x); // { dg-error "no match" }
 
   (views::transform(badarg) | views::all)(x); // { dg-error "no match" }
   (views::filter(badarg) | views::all)(x); // { dg-error "no match" }
-  (views::take(badarg) | views::all)(x); // { dg-error "no match" }
-  (views::drop(badarg) | views::all)(x); // { dg-error "no match" }
   (views::take_while(badarg) | views::all)(x); // { dg-error "no match" }
   (views::drop_while(badarg) | views::all)(x); // { dg-error "no match" }
 
@@ -96,6 +96,21 @@ test02()
   a0(x); // { dg-error "no match" };
   auto a1 = a0 | views::all;
   a1(x); // { dg-error "no match" }
+
+  views::take(badarg)(x); // { dg-error "deleted" }
+  views::drop(badarg)(x); // { dg-error "deleted" }
+  (views::take(badarg) | views::all)(x); // { dg-error "deleted" }
+  (views::drop(badarg) | views::all)(x); // { dg-error "deleted" }
+}
+
+void
+test03()
+{
+  // PR libstdc++/100940
+  extern int x[10];
+  struct S { operator int() && { return 5; }; };
+  x | std::views::take(S{});
+  x | std::views::drop(S{});
 }
 
 // { dg-prune-output "in requirements" }