Can 'auto' be used as a subtype of lambda argument in C++?

Question

C++ code with auto as a part of lambda argument type is accepted by GCC, e.g.:

#include <vector>
#include <iostream>

int main()
{
    auto make_vector = []( std::initializer_list<auto> v ) 
        { return std::vector<typename decltype(v)::value_type>{v}; };
    auto v = make_vector( {1,2} );
    std::cout << v[0] << ' ' << v[1] << '\n';
}

https://gcc.godbolt.org/z/z43bEjMc9

Here the argument type is std::initializer_list<auto> and it is really convenient sometimes. But other compilers complain:

A template-argument cannot be a type that contains 'auto'

Is it a GCC-only extension of the language or a new C++ feature that will appear in other compilers as well? If the first is the case then how can it be turned off to maximize language conformance?

Answer 1

It looks like it's a bug in GCC. Altough it says it's solved in the version of GCC you're using...

You can read here more on why you can't use auto in this context.

It was correctly pointed out to me that this case is for a function parameter. It's still not allowed in C++20, see this answer.

Answer 2

This is not the valid way to declare generic lambdas. [dcl.spec.auto.general]/2:

A placeholder-type-specifier of the form type-constraint_opt auto can be used as a decl-specifier of the decl-specifier-seq of a parameter-declaration of a function declaration or lambda-expression and, if it is not the auto type-specifier introducing a trailing-return-type (see below), is a generic parameter type placeholder of the function declaration or lambda-expression.

Note that std::initializer_list<auto> is not in the form of type-constraint_opt auto.

Since C++20 you can delcare template parameter list for generic lambdas. E.g.

auto make_vector = []<typename T>( std::initializer_list<T> v ) 
    { return std::vector<typename decltype(v)::value_type>{v}; };