[Bug c++/102590] New: Templated operations on variables in structured binding don't work when templated functions/lambdas require type deduction

[Bug c++/102590] New: Templated operations on variables in structured binding don't work when templated functions/lambdas require type deduction

[1997.rajatjain at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102590

            Bug ID: 102590
           Summary: Templated operations on variables in structured
                    binding don't work when templated functions/lambdas
                    require type deduction
           Product: gcc
           Version: 8.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: 1997.rajatjain at gmail dot com
  Target Milestone: ---

Foreach loops written with a structured-binding declaration error out if any of
the captured types contains a template member-function which is used inside the
loop.

This only happens if such a loop is placed inside a templated lambda/function.

A minimal example of the above is:

struct Foo
{
    template<typename T>
    T get() const
    {
        return static_cast<T>(0);
    }
};

template<typename T>
void error_func(T elem)
{
    std::array<std::pair<int, Foo>, 1> arr;
    for(const auto& [_, val] : arr)
    {
        val.get<int>(); // Error here
    }
}

The error disappears if T is replaced with any concrete type. It also
disappears if instead of structured bindings, a tuple is used with auto type
specifier.

The above code also works if one writes:

const auto& [_, val] = arr[0];
val.get<int>();

Compilation flags: -std=c++17

Reported error:

test.cpp:19:17: error: expected primary-expression before 'int'
         val.get<int>();
                 ^~~
test.cpp:19:17: error: expected ';' before 'int'
         val.get<int>();
                 ^~~
                 ;

[Bug c++/102590] Templated operations on variables in structured binding don't work when templated functions/lambdas require type deduction

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102590

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
My bet is gcc mistaken it for being a dependent type which meaning you can use
the template keyword to workaround the issue. Also gcc 8.x series is over 3
years old so this might be fixed already in a newer version of gcc.

[Bug c++/102590] Templated operations on variables in structured binding don't work when templated functions/lambdas require type deduction

[1997.rajatjain at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102590

--- Comment #2 from Rajat Jain <1997.rajatjain at gmail dot com> ---
Yeah .template works as well. This is in the latest release too. I wasn't sure
whether I should write the oldest version that this is in (since other 8.x
releases would need a patch as well), or the latest. Let me change that.

[Bug c++/102590] structured binding inside for all loop thinks it is type depedent when it is not.

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102590

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
           Keywords|                            |rejects-valid
            Summary|Templated operations on     |structured binding inside
                   |variables in structured     |for all loop thinks it is
                   |binding don't work when     |type depedent when it is
                   |templated functions/lambdas |not.
                   |require type deduction      |
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2021-10-05

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Reduced testcase (without any includes):
struct Foo
{
    template<class T> T get() const { return 0;  }
};

struct f
{
  int t;
  Foo t1;
};

template<typename T>
void error_func(T elem)
{
    f arr[1];
    for(const auto& [_, val] : arr)
    {
        val.get<int>(); // Error here
    }
}

[Bug c++/102590] structured binding inside for all loop thinks it is type depedent when it is not (inside a template)

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102590

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |DUPLICATE
             Status|NEW                         |RESOLVED

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Dup of bug 84469.

*** This bug has been marked as a duplicate of bug 84469 ***