[libc++][test] Only `views::split(pattern)` can be piped #74961

StephanTLavavej · 2023-12-10T03:02:26Z

Found while running libc++'s tests with MSVC's STL.

N4964 [range.split.overview]/2:

The name views::split denotes a range adaptor object ([range.adaptor.object]). Given subexpressions E and F, the expression views::split(E, F) is expression-equivalent to split_view(E, F).
[range.lazy.split.overview]/2:

The name views::lazy_split denotes a range adaptor object ([range.adaptor.object]). Given subexpressions E and F, the expression views::lazy_split(E, F) is expression-equivalent to lazy_split_view(E, F).
[range.adaptor.object]/1:

A range adaptor closure object is a unary function object that accepts a range argument. For a range adaptor
closure object C and an expression R such that decltype((R)) models range, the following expressions are
equivalent:
C(R)
R | C
[range.adaptor.object]/6-8:

A range adaptor object is a customization point object ([customization.point.object]) that accepts a viewable_range as its first argument and returns a view.
If a range adaptor object accepts only one argument, then it is a range adaptor closure object.
If a range adaptor object adaptor accepts more than one argument, then let range be an expression such that decltype((range)) models viewable_range, let args... be arguments such that adaptor(range, args...) is a well-formed expression as specified in the rest of subclause [range.adaptors], and let BoundArgs be a pack that denotes decay_t<decltype((args))>.... The expression adaptor(args...) produces a range adaptor closure object f that is a perfect forwarding call wrapper ([func.require]) with the following properties: [...]

To summarize: views::split and views::lazy_split aren't unary, aren't range adaptor closure objects, and can't be piped. However, [range.adaptor.object]/8 says that views::split(pattern) and views::lazy_split(pattern) produce unary, pipeable, range adaptor closure objects.

This PR adjusts the test coverage accordingly, allowing it to portably pass for libc++ and MSVC's STL.

Note: The fact that the tests were previously passing indicates the existence of a bug. @cpplearner figured out what was going on, so I filed #75002 to track followup changes for libc++'s product code.

Click to expand the Clang compiler error that discovered this:

D:\GitHub\STL\llvm-project\libcxx\test\std\ranges\range.adaptors\range.split\adaptor.pass.cpp(42,1): error: static assertion failed
static_assert( CanBePiped<SomeView&,    decltype(std::views::split)>);
^              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
D:\GitHub\STL\llvm-project\libcxx\test\std\ranges\range.adaptors\range.split\adaptor.pass.cpp(42,16): note: because 'CanBePiped<SomeView &, decltype(std::views::split)>' evaluated to false
static_assert( CanBePiped<SomeView&,    decltype(std::views::split)>);
                ^
D:\GitHub\STL\llvm-project\libcxx\test\std\ranges\range.adaptors\range.split\adaptor.pass.cpp(25,30): note: because 'std::forward<View>(view) | std::forward<T>(t)' would be invalid: invalid operands to binary expression ('SomeView' and 'const std::ranges::views::_Split_fn')
  { std::forward<View>(view) | std::forward<T>(t) };
                              ^
D:\GitHub\STL\llvm-project\libcxx\test\std\ranges\range.adaptors\range.split\adaptor.pass.cpp(43,1): error: static assertion failed
static_assert( CanBePiped<char(&)[10],  decltype(std::views::split)>);
^              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
D:\GitHub\STL\llvm-project\libcxx\test\std\ranges\range.adaptors\range.split\adaptor.pass.cpp(43,16): note: because 'CanBePiped<char (&)[10], decltype(std::views::split)>' evaluated to false
static_assert( CanBePiped<char(&)[10],  decltype(std::views::split)>);
                ^
D:\GitHub\STL\llvm-project\libcxx\test\std\ranges\range.adaptors\range.split\adaptor.pass.cpp(25,30): note: because 'std::forward<View>(view) | std::forward<T>(t)' would be invalid: invalid operands to binary expression ('char[10]' and 'const std::ranges::views::_Split_fn')
  { std::forward<View>(view) | std::forward<T>(t) };
                              ^

This was failing with: D:\GitHub\STL\llvm-project\libcxx\test\std\ranges\range.adaptors\range.split\adaptor.pass.cpp(42,1): error: static assertion failed static_assert( CanBePiped<SomeView&, decltype(std::views::split)>); ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ D:\GitHub\STL\llvm-project\libcxx\test\std\ranges\range.adaptors\range.split\adaptor.pass.cpp(42,16): note: because 'CanBePiped<SomeView &, decltype(std::views::split)>' evaluated to false static_assert( CanBePiped<SomeView&, decltype(std::views::split)>); ^ D:\GitHub\STL\llvm-project\libcxx\test\std\ranges\range.adaptors\range.split\adaptor.pass.cpp(25,30): note: because 'std::forward<View>(view) | std::forward<T>(t)' would be invalid: invalid operands to binary expression ('SomeView' and 'const std::ranges::views::_Split_fn') { std::forward<View>(view) | std::forward<T>(t) }; ^ D:\GitHub\STL\llvm-project\libcxx\test\std\ranges\range.adaptors\range.split\adaptor.pass.cpp(43,1): error: static assertion failed static_assert( CanBePiped<char(&)[10], decltype(std::views::split)>); ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ D:\GitHub\STL\llvm-project\libcxx\test\std\ranges\range.adaptors\range.split\adaptor.pass.cpp(43,16): note: because 'CanBePiped<char (&)[10], decltype(std::views::split)>' evaluated to false static_assert( CanBePiped<char(&)[10], decltype(std::views::split)>); ^ D:\GitHub\STL\llvm-project\libcxx\test\std\ranges\range.adaptors\range.split\adaptor.pass.cpp(25,30): note: because 'std::forward<View>(view) | std::forward<T>(t)' would be invalid: invalid operands to binary expression ('char[10]' and 'const std::ranges::views::_Split_fn') { std::forward<View>(view) | std::forward<T>(t) }; ^ Similarly for range.lazy.split/adaptor.pass.cpp.

llvmbot · 2023-12-10T03:02:54Z

@llvm/pr-subscribers-libcxx

Author: Stephan T. Lavavej (StephanTLavavej)

Changes

Found while running libc++'s tests with MSVC's STL.

N4964 [range.split.overview]/2:
> The name views::split denotes a range adaptor object ([range.adaptor.object]). Given subexpressions E and F, the expression views::split(E, F) is expression-equivalent to split_view(E, F).
[range.lazy.split.overview]/2:
> The name views::lazy_split denotes a range adaptor object ([range.adaptor.object]). Given subexpressions E and F, the expression views::lazy_split(E, F) is expression-equivalent to lazy_split_view(E, F).
[range.adaptor.object]/1:
> A range adaptor closure object is a unary function object that accepts a range argument. For a range adaptor
closure object C and an expression R such that decltype((R)) models range, the following expressions are
equivalent:
> C(R)
> R | C
[range.adaptor.object]/6-8:
> A range adaptor object is a customization point object ([customization.point.object]) that accepts a viewable_range as its first argument and returns a view.
> If a range adaptor object accepts only one argument, then it is a range adaptor closure object.
> If a range adaptor object adaptor accepts more than one argument, then let range be an expression such that decltype((range)) models viewable_range, let args... be arguments such that adaptor(range, args...) is a well-formed expression as specified in the rest of subclause [range.adaptors], and let BoundArgs be a pack that denotes decay_t<decltype((args))>.... The expression adaptor(args...) produces a range adaptor closure object f that is a perfect forwarding call wrapper ([func.require]) with the following properties: [...]

To summarize: views::split and views::lazy_split aren't unary, aren't range adaptor closure objects, and can't be piped. However, [range.adaptor.object]/8 says that views::split(pattern) and views::lazy_split(pattern) produce unary, pipeable, range adaptor closure objects.

This PR adjusts the test coverage accordingly.

Note: The fact that the tests were previously passing may indicate the existence of a bug (or an extension?) in libc++'s product code. I looked at the machinery briefly but it was far beyond me (I barely understand microsoft/STL's ranges machinery). As this PR changes the tests to be portable, I would like to request that if changes to libc++'s product code are desired, that should be done in a followup PR (and I can file a followup issue if a reminder is desired).

<details><summary>Click to expand the Clang compiler error that discovered this:</summary>

D:\GitHub\STL\llvm-project\libcxx\test\std\ranges\range.adaptors\range.split\adaptor.pass.cpp(42,1): error: static assertion failed
static_assert( CanBePiped&lt;SomeView&amp;,    decltype(std::views::split)&gt;);
^              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
D:\GitHub\STL\llvm-project\libcxx\test\std\ranges\range.adaptors\range.split\adaptor.pass.cpp(42,16): note: because 'CanBePiped&lt;SomeView &amp;, decltype(std::views::split)&gt;' evaluated to false
static_assert( CanBePiped&lt;SomeView&amp;,    decltype(std::views::split)&gt;);
                ^
D:\GitHub\STL\llvm-project\libcxx\test\std\ranges\range.adaptors\range.split\adaptor.pass.cpp(25,30): note: because 'std::forward&lt;View&gt;(view) | std::forward&lt;T&gt;(t)' would be invalid: invalid operands to binary expression ('SomeView' and 'const std::ranges::views::_Split_fn')
  { std::forward&lt;View&gt;(view) | std::forward&lt;T&gt;(t) };
                              ^
D:\GitHub\STL\llvm-project\libcxx\test\std\ranges\range.adaptors\range.split\adaptor.pass.cpp(43,1): error: static assertion failed
static_assert( CanBePiped&lt;char(&amp;)[10],  decltype(std::views::split)&gt;);
^              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
D:\GitHub\STL\llvm-project\libcxx\test\std\ranges\range.adaptors\range.split\adaptor.pass.cpp(43,16): note: because 'CanBePiped&lt;char (&amp;)[10], decltype(std::views::split)&gt;' evaluated to false
static_assert( CanBePiped&lt;char(&amp;)[10],  decltype(std::views::split)&gt;);
                ^
D:\GitHub\STL\llvm-project\libcxx\test\std\ranges\range.adaptors\range.split\adaptor.pass.cpp(25,30): note: because 'std::forward&lt;View&gt;(view) | std::forward&lt;T&gt;(t)' would be invalid: invalid operands to binary expression ('char[10]' and 'const std::ranges::views::_Split_fn')
  { std::forward&lt;View&gt;(view) | std::forward&lt;T&gt;(t) };
                              ^

</details>

Full diff: https://github.com/llvm/llvm-project/pull/74961.diff

2 Files Affected:

(modified) libcxx/test/std/ranges/range.adaptors/range.lazy.split/adaptor.pass.cpp (+4-4)
(modified) libcxx/test/std/ranges/range.adaptors/range.split/adaptor.pass.cpp (+4-4)

diff --git a/libcxx/test/std/ranges/range.adaptors/range.lazy.split/adaptor.pass.cpp b/libcxx/test/std/ranges/range.adaptors/range.lazy.split/adaptor.pass.cpp
index da4bd9fbbe1794..58be3713263c73 100644
--- a/libcxx/test/std/ranges/range.adaptors/range.lazy.split/adaptor.pass.cpp
+++ b/libcxx/test/std/ranges/range.adaptors/range.lazy.split/adaptor.pass.cpp
@@ -40,10 +40,10 @@ static_assert(!std::is_invocable_v<decltype(std::views::lazy_split), SomeView, N
 static_assert(!std::is_invocable_v<decltype(std::views::lazy_split), NotAView, SomeView>);
 static_assert( std::is_invocable_v<decltype(std::views::lazy_split), SomeView, SomeView>);
 
-static_assert( CanBePiped<SomeView&,    decltype(std::views::lazy_split)>);
-static_assert( CanBePiped<char(&)[10],  decltype(std::views::lazy_split)>);
-static_assert(!CanBePiped<char(&&)[10], decltype(std::views::lazy_split)>);
-static_assert(!CanBePiped<NotAView,     decltype(std::views::lazy_split)>);
+static_assert( CanBePiped<SomeView&,    decltype(std::views::lazy_split('x'))>);
+static_assert( CanBePiped<char(&)[10],  decltype(std::views::lazy_split('x'))>);
+static_assert(!CanBePiped<char(&&)[10], decltype(std::views::lazy_split('x'))>);
+static_assert(!CanBePiped<NotAView,     decltype(std::views::lazy_split('x'))>);
 
 static_assert(std::same_as<decltype(std::views::lazy_split), decltype(std::ranges::views::lazy_split)>);
 
diff --git a/libcxx/test/std/ranges/range.adaptors/range.split/adaptor.pass.cpp b/libcxx/test/std/ranges/range.adaptors/range.split/adaptor.pass.cpp
index cd12011daeab5d..4b13d1b361198f 100644
--- a/libcxx/test/std/ranges/range.adaptors/range.split/adaptor.pass.cpp
+++ b/libcxx/test/std/ranges/range.adaptors/range.split/adaptor.pass.cpp
@@ -39,10 +39,10 @@ static_assert(!std::is_invocable_v<decltype(std::views::split), SomeView, NotAVi
 static_assert(!std::is_invocable_v<decltype(std::views::split), NotAView, SomeView>);
 static_assert( std::is_invocable_v<decltype(std::views::split), SomeView, SomeView>);
 
-static_assert( CanBePiped<SomeView&,    decltype(std::views::split)>);
-static_assert( CanBePiped<char(&)[10],  decltype(std::views::split)>);
-static_assert(!CanBePiped<char(&&)[10], decltype(std::views::split)>);
-static_assert(!CanBePiped<NotAView,     decltype(std::views::split)>);
+static_assert( CanBePiped<SomeView&,    decltype(std::views::split('x'))>);
+static_assert( CanBePiped<char(&)[10],  decltype(std::views::split('x'))>);
+static_assert(!CanBePiped<char(&&)[10], decltype(std::views::split('x'))>);
+static_assert(!CanBePiped<NotAView,     decltype(std::views::split('x'))>);
 
 static_assert(std::same_as<decltype(std::views::split), decltype(std::ranges::views::split)>);

github-actions · 2023-12-10T03:04:48Z

✅ With the latest revision this PR passed the C/C++ code formatter.

cpplearner · 2023-12-10T03:34:37Z

It seems that libc++'s decltype(std::views::split) derives from __range_adaptor_closure, which provides operator|.

llvm-project/libcxx/include/__ranges/split_view.h

Lines 195 to 197 in b85f1f9

    
           namespace views { 
        
           namespace __split_view { 
        
           struct __fn : __range_adaptor_closure<__fn> {

llvm-project/libcxx/include/__ranges/range_adaptor.h

Lines 57 to 63 in b85f1f9

    
           template <class _Tp> 
        
           struct __range_adaptor_closure { 
        
               template <ranges::viewable_range _View, _RangeAdaptorClosure _Closure> 
        
                   requires same_as<_Tp, remove_cvref_t<_Closure>> && 
        
                            invocable<_Closure, _View> 
        
               [[nodiscard]] _LIBCPP_HIDE_FROM_ABI 
        
               friend constexpr decltype(auto) operator|(_View&& __view, _Closure&& __closure)

I think decltype(std::views::split) should be like decltype(std::views::take_while), which doesn't have a base class.

llvm-project/libcxx/include/__ranges/take_while_view.h

Lines 134 to 137 in b85f1f9

    
           namespace views { 
        
           namespace __take_while { 
        
           struct __fn {

StephanTLavavej · 2023-12-13T01:22:58Z

Closing this PR as it has been superseded by #75266 which has passed stage1 checks (I'll wait for stage2 and stage3 before merging it, of course).

@cpplearner

…aptor closures (#75266) Fixes #75002. Found while running libc++'s tests with MSVC's STL. This is a superset of #74961 that also fixes the product code and adds a regression test. Thanks again, @cpplearner! To summarize: `views::split` and `views::lazy_split` aren't unary, aren't range adaptor **closure** objects, and can't be piped. However, \[range.adaptor.object\]/8 says that `views::split(pattern)` and `views::lazy_split(pattern)` produce unary, pipeable, range adaptor closure objects. This PR adjusts the test coverage accordingly, allowing it to portably pass for libc++ and MSVC's STL.

StephanTLavavej requested a review from a team as a code owner December 10, 2023 03:02

llvmbot added the libc++ libc++ C++ Standard Library. Not GNU libstdc++. Not libc++abi. label Dec 10, 2023

Apply clang-format from CI.

e9ffcc0

This was referenced Dec 10, 2023

[libc++] views::split and views::lazy_split shouldn't be range adaptor closures #75002

Closed

[libc++] views::split and views::lazy_split shouldn't be range adaptor closures #75266

Merged

StephanTLavavej closed this Dec 13, 2023

StephanTLavavej deleted the stl-24-split-can-be-piped branch December 13, 2023 01:23

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