CODEWITHSUNDEEP
c++c++11templateslambdalanguage-lawyer
IvanIvanovich
IvanIvanovich

Reputation: 171

Non-dependent name lookup and lambda

I have the following code:

template <class T>
class Outer
{
public:
    Outer();

    template <class U>
    void templateFunc()
    {
    }

private:
    class Inner
    {
    public:
        Inner(Outer& outer)
        {
            outer.templateFunc<int>();
            Outer* outer_ptr = &outer;
            [outer_ptr]() 
            {
                outer_ptr->templateFunc<int>();
            }();
        }
    };

    Inner m_inner;
};

template <class T>
Outer<T>::Outer()
    : m_inner(*this)
{
}

int main()
{
    Outer<double> outer;
}

As you can see, there is a template class that contains a nested class, which in constructor calls some template method of its enclosing class. AFAIK, even though enclosing class is a template class - for the nested class it is a non-dependent name, so calling its template method without template should not be a problem. The problem happens when I define a lambda inside nested class' constructor, capture pointer to outer class, and try to call the same template method - g++7.2 gives the following compilation error:

prog.cc: In lambda function:

prog.cc:22:41: error: expected primary-expression before 'int'
                outer_ptr->templateFunc<int>();
                                        ^~~
prog.cc:22:41: error: expected ';' before 'int'

However, g++-5.4 and g++-6.3 compile this code just fine. So it seems that g++-7.2 treats the outer_ptr's type inside the lambda as a dependent name - and I cannot understand why. Can someone explain this to me?

Upvotes: 16

Views: 417

Answers (1)

Barry
Barry

Reputation: 304122

Yes, this is a gcc regression. Filed as 82980.

Here's a reduced example:

template <class T>
struct Outer
{
    template <class U>
    void f();

    void bar(Outer outer) {
        [outer](){ outer.f<int>(); };
    }
};

int main() { }

outer.f is member access for the current instantiation, so that expression shouldn't count as type dependent, so you shouldn't need to provide the template keyword.

Upvotes: 2

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