[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 via Gcc-bugs]

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>();
                 ^~~
                 ;