Category 

Type Trait Functions 

Syntax 

bool __is_scalar( typename T ) 

Returns true if and only if T is a scalar type. 

Scalar type is defined in Section 3.9 p10 of the Working Draft, essentially: 

__is_arithmetic( T ) || __is_enumeration(T) || 

__is_pointer(T) || __is_member_pointer(T)  

Scalar types have a built in meaning for operator < as well as operator ==. Therefore, __closure is not a scalar type.