[llvm-bugs] [Bug 50715] New: Undefined uses of std::is_invocable in <concepts> and <ranges> tests

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=50715

            Bug ID: 50715
           Summary: Undefined uses of std::is_invocable in <concepts> and
                    <ranges> tests
           Product: libc++
           Version: 12.0
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: All Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: zilla at kayari.org
                CC: llvm-bugs at lists.llvm.org, mclow.lists at gmail.com

The tests for the std::invocable concept use std::is_invocable with an argument
of incomplete type, which is undefined (it violates the precondition for
std::is_invocable):

In file included from /home/jwakely/gcc/12/include/c++/12.0.0/ratio:39,
                 from /home/jwakely/gcc/12/include/c++/12.0.0/chrono:39,
                 from
std/concepts/concepts.callable/concept.invocable/invocable.compile.pass.cpp:15:
/home/jwakely/gcc/12/include/c++/12.0.0/type_traits: In instantiation of
'struct std::is_invocable<int S::*, S>':
/home/jwakely/gcc/12/include/c++/12.0.0/type_traits:3001:73:   required from
'constexpr const bool std::is_invocable_v<int S::*, S>'
/home/jwakely/gcc/12/include/c++/12.0.0/concepts:338:25:   required from
'constexpr bool pointer_to_member_functions::check_member_is_invocable() [with
Member = int S::*; T = S; Args = {}]'
std/concepts/concepts.callable/concept.invocable/invocable.compile.pass.cpp:259:53:
  required from here
/home/jwakely/gcc/12/include/c++/12.0.0/type_traits:2942:7: error: static
assertion failed: each argument type must be a complete class or an unbounded
array

Similarly for std::regular_invocable:

In file included from /home/jwakely/gcc/12/include/c++/12.0.0/ratio:39,
                 from /home/jwakely/gcc/12/include/c++/12.0.0/chrono:39,
                 from
std/concepts/concepts.callable/concept.regularinvocable/regular_invocable.compile.pass.cpp:15:
/home/jwakely/gcc/12/include/c++/12.0.0/type_traits: In instantiation of
'struct std::is_invocable<int S::*, S>':
/home/jwakely/gcc/12/include/c++/12.0.0/type_traits:3001:73:   required from
'constexpr const bool std::is_invocable_v<int S::*, S>'
/home/jwakely/gcc/12/include/c++/12.0.0/concepts:338:25:   required from
'constexpr bool pointer_to_member_functions::check_member_is_invocable() [with
Member = int S::*; T = S; Args = {}]'
std/concepts/concepts.callable/concept.regularinvocable/regular_invocable.compile.pass.cpp:285:53:
  required from here
/home/jwakely/gcc/12/include/c++/12.0.0/type_traits:2942:7: error: static
assertion failed: each argument type must be a complete class or an unbounded
array



The tests for std::ranges::data use std::is_invocable_v to check that an array
of incomplete type can't be used, but that's undefined as stated above:

In file included from /home/jwakely/gcc/12/include/c++/12.0.0/concepts:44,
                 from /home/jwakely/gcc/12/include/c++/12.0.0/ranges:37,
                 from std/ranges/range.access/range.prim/data.pass.cpp:15:
/home/jwakely/gcc/12/include/c++/12.0.0/type_traits: In instantiation of
'struct std::is_invocable<const std::ranges::__cust_access::_Data, Incomplete
[2]>':
/home/jwakely/gcc/12/include/c++/12.0.0/type_traits:3001:73:   required from
'constexpr const bool std::is_invocable_v<const
std::ranges::__cust_access::_Data, Incomplete [2]>'
std/ranges/range.access/range.prim/data.pass.cpp:28:21:   required from here
/home/jwakely/gcc/12/include/c++/12.0.0/type_traits:2942:7: error: static
assertion failed: each argument type must be a complete class or an unbounded
array

Shouldn't those first three static assertions in
std/ranges/range.access/range.prim/data.pass.cpp be using a reference to an
array anyway? An array rvalue is not a borrowed_range, so it's ill-formed. If
they are supposed to be testing the array of incomplete type condition, they
aren't doing that, they're testing the "lvalue or borrowed range" condition.

Also, that test is not portable (should it be guarded by a preprocessor check?)
because the standard doesn't require a diagnostic for trying to use
ranges::data on an array of incomplete type. With libstdc++ you do get an
error, but not in the immediate context (so it's not SFINAE-friendly and you
can't check for it with is_invocable).

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