Template argument deduction/substitution failed with lambda as function pointer

Question

I'm wondering why in the following code the compiler is unable to use lambda as the argument for function foo() (template argument deduction/substitution failed), while a simple function works:

template<class ...Args>
void foo(int (*)(Args...))
{
}

int bar(int)
{
    return 0;
}

int main() {
    //foo([](int) { return 0; }); // error
    foo(bar);
    return 0;
}

The intel compiler (version 18.0.3 )

template.cxx(12): error: no instance of function template "foo" matches the argument list
            argument types are: (lambda [](int)->int)
      foo([](int) { return 0; }); // error
      ^
template.cxx(2): note: this candidate was rejected because at least one template argument could not be deduced
  void foo(int (*)(Args...))

Any ideas?

Answer 1

Template argument deduction doesn't consider implicit conversion.

Type deduction does not consider implicit conversions (other than type adjustments listed above): that's the job for overload resolution, which happens later.

You can convert the lambda to function pointer explicitly, e.g. you can use static_cast,

foo(static_cast<int(*)(int)>([](int) { return 0; }));

or operator+,

foo(+[](int) { return 0; });