[Bug c++/107437] New: nested generic lambdas fail requiring unneded captures

[Bug c++/107437] New: nested generic lambdas fail requiring unneded captures

[development at jordi dot vilar.cat]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107437

            Bug ID: 107437
           Summary: nested generic lambdas fail requiring unneded captures
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: development at jordi dot vilar.cat
  Target Milestone: ---

This simple valid code fails to compile with all recent versions of gcc, but
compiles nicely with clang (and even msvc!)

(live demo on complier explorer: https://godbolt.org/z/eM1b9e6s9)

// compile with just -std=c++20
#include <array>
#include <functional>
#include <type_traits>

void test()
{
    std::invoke([]<typename T>(auto N, std::type_identity<T>)
    {
        using vector_type = std::array<T, N>;
        static_assert(N == std::tuple_size_v<vector_type>);
        std::invoke([]<std::size_t... i>(std::index_sequence<i...>)
        {
           
static_assert(std::conjunction_v<std::is_same<std::tuple_element_t<i,
vector_type>, T>...>);
        }, std::make_index_sequence<N>{});
    }, std::integral_constant<std::size_t, 3>{}, std::type_identity<int>{});
}

It issues the diagnostics output:
<source>: In instantiation of 'test()::<lambda(auto:3, std::type_identity<T>)>
[with T = int; auto:3 = std::integral_constant<long unsigned int, 3>]':
/opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/type_traits:2565:26:  
required by substitution of 'template<class _Fn, class ... _Args> static
std::__result_of_success<decltype (declval<_Fn>()((declval<_Args>)()...)),
std::__invoke_other> std::__result_of_other_impl::_S_test(int) [with _Fn =
test()::<lambda(auto:3, std::type_identity<T>)>; _Args =
{std::integral_constant<long unsigned int, 3>, std::type_identity<int>}]'
/opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/type_traits:2576:55:  
required from 'struct std::__result_of_impl<false, false,
test()::<lambda(auto:3, std::type_identity<T>)>, std::integral_constant<long
unsigned int, 3>, std::type_identity<int> >'
/opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/type_traits:2581:12:  
required from 'struct std::__invoke_result<test()::<lambda(auto:3,
std::type_identity<T>)>, std::integral_constant<long unsigned int, 3>,
std::type_identity<int> >'
/opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/type_traits:3022:12:  
required from 'struct std::invoke_result<test()::<lambda(auto:3,
std::type_identity<T>)>, std::integral_constant<long unsigned int, 3>,
std::type_identity<int> >'
/opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/type_traits:3034:11:  
required by substitution of 'template<class _Fn, class ... _Args> using
invoke_result_t = typename std::invoke_result::type [with _Fn =
test()::<lambda(auto:3, std::type_identity<T>)>; _Args =
{std::integral_constant<long unsigned int, 3>, std::type_identity<int>}]'
/opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/functional:107:5:  
required by substitution of 'template<class _Callable, class ... _Args>
constexpr std::invoke_result_t<_Fn, _Args ...> std::invoke(_Callable&&, _Args&&
...) [with _Callable = test()::<lambda(auto:3, std::type_identity<T>)>; _Args =
{std::integral_constant<long unsigned int, 3>, std::type_identity<int>}]'
<source>:7:16:   required from here
<source>:13:32: error: 'N' is not captured
   13 |            
static_assert(std::conjunction_v<std::is_same<std::tuple_element_t<i,
vector_type>, T>...>);
      |                          
~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:11:21: note: the lambda has no capture-default
   11 |         std::invoke([]<std::size_t... i>(std::index_sequence<i...>)
      |                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   12 |         {
      |         ~            
   13 |            
static_assert(std::conjunction_v<std::is_same<std::tuple_element_t<i,
vector_type>, T>...>);
      |            
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   14 |         }, std::make_index_sequence<N>{});
      |         ~            
<source>:7:37: note: 'std::integral_constant<long unsigned int, 3> N' declared
here
    7 |     std::invoke([]<typename T>(auto N, std::type_identity<T>)
      |                                ~~~~~^

[Bug c++/107437] nested generic lambdas fail requiring unneded captures

[development at jordi dot vilar.cat]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107437

Jordi Vilar <development at jordi dot vilar.cat> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |development at jordi dot vilar.cat

--- Comment #1 from Jordi Vilar <development at jordi dot vilar.cat> ---
BTW, capturing (the unneded!) N in the nested lambda, raises a beautiful error:

error: integral expression '(std::integral_constant<long unsigned int,
3>::value_type)N' is not constant.

Should I fill a new issue for this?

[Bug c++/107437] nested generic lambdas fail requiring unneded captures

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107437

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I suspect this is the same as PR 105518.

[Bug c++/107437] nested generic lambdas fail requiring unneded captures

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107437

--- Comment #3 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:982629bea416df976686467f235e09cb1a5531cc

commit r13-4761-g982629bea416df976686467f235e09cb1a5531cc
Author: Patrick Palka <ppalka@redhat.com>
Date:   Sat Dec 17 11:24:44 2022 -0500

    c++: constantness of non-dependent NTTP argument [PR107437]

    Here we're rejecting the use of the lambda capture of 't' (of empty
    type) as a template argument ultimately because convert_nontype_argument
    checks constantness using is_constant_expression, which returns false
    for lambda captures since want_rval=false.  But in this case I believe
    an lvalue-to-rvalue conversion of the argument is implied, so we should
    be using is_rvalue_constant_expression instead (which would return true
    here).

    However, it doesn't seem necessary to consider constantness at all
    when deciding whether to instantiate a non-dependent argument in
    convert_nontype_argument.  So this patch gets rid of the problematic
    constantness test altogether, which incidentally also fixes the similar
    dg-ice'd testcase from PR87765.  This is in line with a similar
    change we made to finish_decltype_type in r12-7564-gec0f53a3a542e7.

            PR c++/107437
            PR c++/87765

    gcc/cp/ChangeLog:

            * pt.cc (convert_nontype_argument): Relax is_nondep_const_expr
            test to !inst_dep_expr_p.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp1y/lambda-generic-107437.C: New test.
            * g++.dg/cpp1z/constexpr-lambda26.C: Remove dg-ice.

[Bug c++/107437] nested generic lambdas fail requiring unneded captures

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107437

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
                 CC|                            |ppalka at gcc dot gnu.org
           Assignee|unassigned at gcc dot gnu.org      |ppalka at gcc dot gnu.org
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2022-12-17

--- Comment #4 from Patrick Palka <ppalka at gcc dot gnu.org> ---
The capturing version of the testcase is now accepted on trunk, unfortunately
we still reject the original non-capturing version.

Another workaround that's accepted by GCC 9+ is to avoid using N in an
evaluated context in the definition of vector_type:

  using vector_type = std::array<T, decltype(N){}>;

[Bug c++/107437] nested generic lambdas fail requiring unneded captures

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107437

--- Comment #5 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Reduced version of the original testcase:

struct integral_constant {
  constexpr operator int() const { return 42; }
};

template<int N>
struct A {
  static constexpr int value = N;
};

template<class T>
struct B {
  static constexpr int value = T::value;
};

template<class T>
void f(T t) {
  using alias = A<t>;
  [](auto) {
    B<alias> a; // { dg-bogus "'t' is not captured" }
    return a.value;
  }(0);
}

template void f(integral_constant);

[Bug c++/107437] nested generic lambdas fail requiring unneded captures

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107437

--- Comment #6 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:4cf64d9cc2faf4001f037a50a350abd0f95f3e29

commit r14-2170-g4cf64d9cc2faf4001f037a50a350abd0f95f3e29
Author: Patrick Palka <ppalka@redhat.com>
Date:   Wed Jun 28 15:43:33 2023 -0400

    c++: ahead of time variable template-id coercion [PR89442]

    This patch makes us coerce the arguments of a variable template-id ahead
    of time, as we do for class template-ids, which causes us to immediately
    diagnose template parm/arg kind mismatches and arity mismatches.

    Unfortunately this causes a regression in cpp1z/constexpr-if20.C: coercing
    the variable template-id m<ar, as> ahead of time means we strip it of
    typedefs, yielding m<typename C<i>::q, typename C<j>::q>, but in this
    stripped form we're directly using 'i' and so we expect to have captured
    it.  This is a variable template version of PR107437.

            PR c++/89442
            PR c++/107437

    gcc/cp/ChangeLog:

            * cp-tree.h (lookup_template_variable): Add complain parameter.
            * parser.cc (cp_parser_template_id): Pass tf_warning_or_error
            to lookup_template_variable.
            * pt.cc (lookup_template_variable): Add complain parameter.
            Coerce template arguments here ...
            (finish_template_variable): ... instead of here.
            (lookup_and_finish_template_variable): Check for error_mark_node
            result from lookup_template_variable.
            (tsubst_copy) <case TEMPLATE_ID_EXPR>: Pass complain to
            lookup_template_variable.
            (instantiate_template): Use build2 instead of
            lookup_template_variable to build a TEMPLATE_ID_EXPR
            for most_specialized_partial_spec.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp/pr64127.C: Expect "expected unqualified-id at end
            of input" error.
            * g++.dg/cpp0x/alias-decl-ttp1.C: Fix template parameter/argument
            kind mismatch for variable template has_P_match_V.
            * g++.dg/cpp1y/pr72759.C: Expect "template argument 1 is invalid"
            error.
            * g++.dg/cpp1z/constexpr-if20.C: XFAIL test due to bogus "'i' is
            not captured" error.
            * g++.dg/cpp1z/noexcept-type21.C: Fix arity of variable template d.
            * g++.dg/diagnostic/not-a-function-template-1.C: Add default
            template argument to variable template A so that A<> is valid.
            * g++.dg/parse/error56.C: Don't expect "ISO C++ forbids
            declaration with no type" error.
            * g++.dg/parse/template30.C: Don't expect "parse error in
            template argument list" error.
            * g++.dg/cpp1y/var-templ82.C: New test.

[Bug c++/107437] nested generic lambdas fail requiring unneded captures

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107437

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at gcc dot gnu.org

--- Comment #7 from Jason Merrill <jason at gcc dot gnu.org> ---
Created attachment 55466
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55466&action=edit
failed experiment

I wondered if keeping function-local typedefs would be a simple fix, but a
number of modules tests regress with this patch.  I guess that's because of
problems with entries in the specialization hash tables referring to
function-local typedefs that we can't then reconnect when loading the module. 
Really such entries (if we do add them) shouldn't persist after we're done
compiling the function.