How can I prevent implicit conversions in a functi

Question

How can I define a function template to prevent implicit conversions?

It seems I can prevent implicit conversions using non-template functions but not using function templates.

Defining a forwarding reference function template as = delete is too aggressive as it prevents invocation with non-const lvalue references.

Defining an function template with a const rvalue argument as =delete [1] does not prevent implicit conversions.

Defining an rvalue overload for a specific type as =delete works but I'd like to accomplish this with templates.

Minimal code example:

struct A {};

struct B {
  B() = default;

  B(const A&) {}
};

// Delete const rvalue reference.
template <class T>
void t_no_rvalue(const T&&) = delete; // 1

void t_no_rvalue(const B&) {}         // 2


// Delete forwarding reference.
template <class T>
void t_no_fwd_ref(T&&) = delete;     // 3

void t_no_fwd_ref(const B&) {}       // 4


// (non-template) Delete const rvalue reference.
void no_rvalue(const B&&) = delete;  // 5

void no_rvalue(const B&) {}          // 6


int main(int argc, char* argv[]) {
  A a;
  B b;

  // Undesired behaviour, implicit conversion allowed.
  t_no_rvalue(a);   // resolves to 2
  t_no_rvalue(b);   // resolves to 2

  // Undesired behaviour, invocation with non-const reference disallowed.
  t_no_fwd_ref(a);  // resolves to 3
  t_no_fwd_ref(b);  // resolves to 3

  // Desired behaviour.
  no_rvalue(a);     // resolves to 5
  no_rvalue(b);     // resolves to 6
}

My real-world use case is hashing of variants where implicit conversion of a variant sub-type back to the variant-like type will cause infinite recursion if the hash function is not specialized for all the variant constituents. The sample code above is clearer though.

[1] Attempted in Why can I prevent implicit conversions for primitives but not user-defined types? but with a broken code example.

Answer 1

The following overload will prevent implicit conversions:

template <class T>
void no_conversions(T) = delete; // 7

void no_conversions(const B&) {} // 8

and leads to:

// Requested behaviour.
no_conversions(a); // resolves to 7
no_conversions(b); // resolves to 8

A value-overload poisons the overload set for implicit conversions as it will be an exact match.

Edit:

template <class T>
void no_conversions(const T&) = delete; // 9

void no_conversions(const B&) {}        // 10

works just as well.