Avoiding implicit conversion in constructor. The ‘explicit’ keyword doesn’t help here

You can delete A::A(<anything not an int>);:

struct A
{
    explicit A(int a)
    : num(a)
    {}

    template<class T>
    A(T) = delete;

    int num;
};

int main()
{
    //A a1=A(10.0); // error: use of deleted function 'A::A(T) [with T = double]'
    A a2 = A(10); // OK
    (void) a2;
}

Demo: https://coliru.stacked-crooked.com/a/425afc19003697c9