Why can I prevent implicit conversions for primitives but not user-defined types?

Question

The High Integrity C++ Standards suggest that rvalue arguments to functions can be deleted thus preventing implicit conversions.

http://www.codingstandard.com/rule/8-3-4

Answer 1

In your code, there is no difference in treatment between user-defined types and primitive types. The difference between the behaviour of these two lines:

foo(a);
foo(3.3);

is that a is an lvalue, and 3.3 is an rvalue. The rvalue argument matches your overload 1 (which only accepts rvalues), the lvalue argument does not.

If you try to invoke foo with an rvalue argument it will also match 1 and fail, e.g. foo(A{});.