[Bug c++/99895] New: Function parameters generated wrong in call to member of non-type template parameter in lambda

[Bug c++/99895] New: Function parameters generated wrong in call to member of non-type template parameter in lambda

[bisqwit at iki dot fi]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99895

            Bug ID: 99895
           Summary: Function parameters generated wrong in call to member
                    of non-type template parameter in lambda
           Product: gcc
           Version: 10.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bisqwit at iki dot fi
  Target Milestone: ---

GCC produces false error message:

bug1.cc: In instantiation of ‘consteval void VerifyHash() [with unsigned int
expected_hash = 5; fixed_string<...auto...> ...s = {fixed_string<6>{"khaki"},
fixed_string<6>{"plums"}}]’:
bug1.cc:24:37:   required from here
bug1.cc:19:41: error: no matching function for call to
‘fixed_string<6>::data(const fixed_string<6>*)’
   19 |       [](auto){static_assert(hash(s.data(), s.size()) ==
expected_hash);}(s)
      |                                   ~~~~~~^~
bug1.cc:11:27: note: candidate: ‘consteval const char* fixed_string<N>::data()
const [with long unsigned int N = 6]’
   11 |     consteval const char* data() const { return str; }
      |                           ^~~~
bug1.cc:11:27: note:   candidate expects 0 arguments, 1 provided

On this code:

#include <algorithm> // copy_n and size_t
static constexpr unsigned hash(const char* s, std::size_t length)
{
    s=s;
    return length;
}
template<std::size_t N>
struct fixed_string
{
    constexpr fixed_string(const char (&s)[N]) { std::copy_n(s, N, str); }
    consteval const char* data() const { return str; }
    consteval std::size_t size() const { return N-1; }
    char str[N];
};
template<unsigned expected_hash, fixed_string... s>
static consteval void VerifyHash()
{
    (
      [](auto){static_assert(hash(s.data(), s.size()) == expected_hash);}(s)
    ,...);
    // The compiler mistakenly translates s.data() into s.data(&s)
    // and then complains that the call is not valid, because
    // the function expects 0 parameters and 1 "was provided".
}
void foo()
{
    VerifyHash<5, "khaki", "plums">();
}


Compiler version:
g++-10 (Debian 10.2.1-6) 10.2.1 20210110

[Bug c++/99895] Function parameters generated wrong in call to member of non-type template parameter in lambda

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99895

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |ppalka at gcc dot gnu.org
             Status|UNCONFIRMED                 |ASSIGNED
                 CC|                            |ppalka at gcc dot gnu.org
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2021-05-27

--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Confirmed.  Reduced:

struct fixed_string { consteval auto size() const { return 42; } };

template<fixed_string s>
static void VerifyHash() {
  [](auto){ s.size(); };
}

void foo() { VerifyHash<{}>(); }


Declaring size() constexpr instead of consteval makes us accept.

[Bug c++/99895] Function parameters generated wrong in call to member of non-type template parameter in lambda

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99895

--- Comment #2 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:dec8d0e5fa00ceb2ded78b8a3eba8976d860a90e

commit r12-6897-gdec8d0e5fa00ceb2ded78b8a3eba8976d860a90e
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Jan 27 10:56:49 2022 -0500

    c++: non-dependent immediate member fn call [PR99895]

    Here we're emitting a bogus error during ahead of time evaluation of a
    non-dependent immediate member function call such as a.f(args) because
    the defacto templated form for such a call is (a.f)(args) but we're
    trying to evaluate it using the intermediate CALL_EXPR built by
    build_over_call, which has the non-member form f(a, args).  The defacto
    member form is built in build_new_method_call, so it seems we should
    handle the immediate call there instead, or perhaps make build_over_call
    build the correct form in the first place.

    Giiven that there are many spots other than build_new_method_call that
    call build_over_call for member functions, e.g. build_op_call, this
    patch takes the latter approach.

    In passing, this patch makes us avoid wrapping PARM_DECL in
    NON_DEPENDENT_EXPR for benefit of the third testcase below.

            PR c++/99895

    gcc/cp/ChangeLog:

            * call.cc (build_over_call): For a non-dependent member call,
            build up a CALL_EXPR using a COMPONENT_REF callee, as in
            build_new_method_call.
            * pt.cc (build_non_dependent_expr): Don't wrap PARM_DECL either.
            * tree.cc (build_min_non_dep_op_overload): Adjust accordingly
            after the build_over_call change.

    gcc/ChangeLog:

            * tree.cc (build_call_vec): Add const to second parameter.
            * tree.h (build_call_vec): Likewise.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp2a/consteval-memfn1.C: New test.
            * g++.dg/cpp2a/consteval-memfn2.C: New test.
            * g++.dg/cpp2a/consteval28.C: New test.

[Bug c++/99895] Function parameters generated wrong in call to member of non-type template parameter in lambda

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99895

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED
   Target Milestone|---                         |12.0

--- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Fixed for GCC 12, thanks for the bug report.