13 Templates [temp]

13.4 Template arguments [temp.arg]

13.4.2 Template non-type arguments [temp.arg.nontype]

1
#
If the type T of a template-parameter contains a placeholder type ([dcl.spec.auto]) or a placeholder for a deduced class type ([dcl.type.class.deduct]), the type of the parameter is the type deduced for the variable x in the invented declaration 
T x = template-argument ;
If a deduced parameter type is not permitted for a template-parameter declaration ([temp.param]), the program is ill-formed.
2
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A template-argument for a non-type template-parameter shall be a converted constant expression ([expr.const]) of the type of the template-parameter.
[ Note
:
If the template-argument is an overload set (or the address of such, including forming a pointer-to-member), the matching function is selected from the set ([over.over]).
— end note
 ]
3
#
For a non-type template-parameter of reference or pointer type, or for each non-static data member of reference or pointer type in a non-type template-parameter of class type or subobject thereof, the reference or pointer value shall not refer to or be the address of (respectively):
4
#
[ Example
: 
template<const int* pci> struct X { /* ... */ };
int ai[10];
X<ai> xi;                       // array to pointer and qualification conversions

struct Y { /* ... */ };
template<const Y& b> struct Z { /* ... */ };
Y y;
Z<y> z;                         // no conversion, but note extra cv-qualification

template<int (&pa)[5]> struct W { /* ... */ };
int b[5];
W<b> w;                         // no conversion

void f(char);
void f(int);

template<void (*pf)(int)> struct A { /* ... */ };

A<&f> a;                        // selects f(int)

template<auto n> struct B { /* ... */ };
B<5> b1;                        // OK, template parameter type is int
B<'a'> b2;                      // OK, template parameter type is char
B<2.5> b3;                      // OK, template parameter type is double
B<void(0)> b4;                  // error: template parameter type cannot be void
— end example
 ]
5
#
[ Note
:
A string-literal is not an acceptable template-argument for a template-parameter of non-class type.
[ Example
: 
template<class T, T p> class X {
  /* ... */
};

X<const char*, "Studebaker"> x; // error: string literal object as template-argument
X<const char*, "Knope" + 1> x2; // error: subobject of string literal object as template-argument

const char p[] = "Vivisectionist";
X<const char*, p> y;            // OK

struct A {
  constexpr A(const char*) {}
};

X<A, "Pyrophoricity"> z;        // OK, string-literal is a constructor argument to A
— end example
 ]
— end note
 ]
6
#
[ Note
:
A temporary object is not an acceptable template-argument when the corresponding template-parameter has reference type.
[ Example
: 
template<const int& CRI> struct B { /* ... */ };

B<1> b1;                        // error: temporary would be required for template argument

int c = 1;
B<c> b2;                        // OK

struct X { int n; };
struct Y { const int &r; };
template<Y y> struct C { /* ... */ };
C<Y{X{1}.n}> c;                 // error: subobject of temporary object used to initialize
                                // reference member of template parameter
— end example
 ]
— end note
 ]