From Dan Saks's Top-Level cv-Qualifiers in Function Parameters:
In C++, a cv-qualifier that applies to the first level of a type is called a toplevel cv-qualifier. For example, in:
T *const p;
the top-level cv-qualifier is const, and in:
T const *volatile q;
the top-level cv-qualifier is volatile. On the other hand:
T const volatile *q;
has no top-level cv-qualifiers. In this case, the cv-qualifiers const and volatile appear at the second level.
The signature of a function includes all cv-qualifiers appearing in that function’s parameter types, except for those qualifiers appearing at the top-level of a parameter type.
For example, in:
int f(char const *p);
the const qualifier is not at the top level in the parameter declaration, so it is part of the function’s signature.
On the other hand, in:
int f(char *const p);
the const qualifier is at the top level, so it is not part of the function’s signature. This function has the same signature as:
int f(char *p);
I couldn't find a definition in the standard either but what I posted above is explicitly stated in N3337 §8.3.5-5
After producing the list of parameter types, any top-level
cv-qualifiers modifying a parameter type are deleted when forming the
function type.
Edit:
At the time of writing the above post a definition in the standard could not be found but now there's one as pointed out by Shafik:
n4296 excerpt:
In this International Standard, the notation cv (or cv1 , cv2 , etc.),
used in the description of types, represents an arbitrary set of
cv-qualifiers, i.e., one of {const}, {volatile}, {const, volatile}, or
the empty set. For a type cv T, the top-level cv-qualifiers of that
type are those denoted by cv. [Example: The type corresponding to the
type-id const int& has no top-level cv-qualifiers. The type
corresponding to the typeid volatile int * const has the top-level
cv-qualifier const. For a class type C, the type corresponding to the
type-id void (C::* volatile)(int) const has the top-level cv-qualifier
volatile. — end example ]
