local variable as a non-typename argument

Question

Why is it illegal to use a local variable as a non-type argument?

For example, in the next code local_var cannot be argument to X.

Answer 1

Because template arguments must be evaluated at compile time, and the compiler won't know the address of a local variable until run-time (in order to bind a reference to an object, the compiler needs to know the address of that object).

Notice, that the C++11 Standard tells exactly what non-type template arguments can be provided in paragraph 14.3.2/1:

A template-argument for a non-type, non-template template-parameter shall be one of:

— for a non-type template-parameter of integral or enumeration type, a converted constant expression (5.19) of the type of the template-parameter; or

— the name of a non-type template-parameter; or

— a constant expression (5.19) that designates the address of an object with static storage duration and external or internal linkage or a function with external or internal linkage, including function templates and function template-ids but excluding non-static class members, expressed (ignoring parentheses) as & id-expression, except that the & may be omitted if the name refers to a function or array and shall be omitted if the corresponding template-parameter is a reference; or

— a constant expression that evaluates to a null pointer value (4.10); or

— a constant expression that evaluates to a null member pointer value (4.11); or

— a pointer to member expressed as described in 5.3.1; or

— an address constant expression of type std::nullptr_t.

As you can see, local variables are not in this list.