[conv.qual]

A qualification-decomposition of a type T is a sequence of

c v {i and {P i such that T is “ c v {0 {P 0 c v {1 {P 1 \dots}_{1 \dots}}_{1 {P 1 \dots}_{1 \dots}}}_{0 c v {1 {P 1 \dots}_{1 \dots}}_{1 {P 1 \dots}_{1 \dots}}}}_{0 {P 0 c v {1 {P 1 \dots}_{1 \dots}}_{1 {P 1 \dots}_{1 \dots}}}_{0 c v {1 {P 1 \dots}_{1 \dots}}_{1 {P 1 \dots}_{1 \dots}}}}}_{i such that T is “ c v {0 {P 0 c v {1 {P 1 \dots}_{1 \dots}}_{1 {P 1 \dots}_{1 \dots}}}_{0 c v {1 {P 1 \dots}_{1 \dots}}_{1 {P 1 \dots}_{1 \dots}}}}_{0 {P 0 c v {1 {P 1 \dots}_{1 \dots}}_{1 {P 1 \dots}_{1 \dots}}}_{0 c v {1 {P 1 \dots}_{1 \dots}}_{1 {P 1 \dots}_{1 \dots c v}}}}}}_{i and {P i such that T is “ c v {0 {P 0 c v {1 {P 1 \dots}_{1 \dots}}_{1 {P 1 \dots}_{1 \dots}}}_{0 c v {1 {P 1 \dots}_{1 \dots}}_{1 {P 1 \dots}_{1 \dots}}}}_{0 {P 0 c v {1 {P 1 \dots}_{1 \dots}}_{1 {P 1 \dots}_{1 \dots}}}_{0 c v {1 {P 1 \dots}_{1 \dots}}_{1 {P 1 \dots}_{1 \dots c v}}}}}_{i such that T is “ c v {0 {P 0 c v {1 {P 1 \dots}_{1 \dots}}_{1 {P 1 \dots}_{1 \dots}}}_{0 c v {1 {P 1 \dots}_{1 \dots}}_{1 {P 1 \dots}_{1 \dots c v}}}}_{0 {P 0 c v {1 {P 1 \dots}_{1 \dots}}_{1 {P 1 \dots}_{1 \dots c v}}}_{0 c v {1 {P 1 \dots}_{1 \dots c v}}_{1 {P 1 \dots c v}_{1 \dots c v}}}}}}

{P i designates an array, the cv-qualifiers c v {i + 1 on the element type are also taken as the cv-qualifiers c v {i of the array .}_{i of the array .}}_{i + 1 on the element type are also taken as the cv-qualifiers c v {i of the array .}_{i of the array .}}}_{i designates an array, the cv-qualifiers c v {i + 1 on the element type are also taken as the cv-qualifiers c v {i of the array .}_{i of the array .}}_{i + 1 on the element type are also taken as the cv-qualifiers c v {i of the array .}_{i of the array .}}}

[Example 1:

The type denoted by the type-id const int ** has three qualification-decompositions, taking U as “int”, as “pointer to const int”, and as “pointer to pointer to const int”.

— end example]

Two types T1 and T2 are similar if they have qualification-decompositions with the same n such that corresponding

{P i components are either the same or one is “array of {N i ” and the other is “array of unknown bound of”, and the types denoted by U are the same .}_{i ” and the other is “array of unknown bound of”, and the types denoted by U are the same .}}_{i components are either the same or one is “array of {N i ” and the other is “array of unknown bound of”, and the types denoted by U are the same .}_{i ” and the other is “array of unknown bound of”, and the types denoted by U are the same .}}

The qualification-combined type of two types T1 and T2 is the type T3 similar to T1 whose qualification-decomposition is such that:

(3.1)
for every $i > 0, c v {3 i is the union of c v {1 i and c v {2 i,}_{i,}^{2 i,}}_{i and c v {2 i,}_{i,}^{2 i,}}^{1 i and c v {2 i,}_{i,}^{2 i,}}}_{i is the union of c v {1 i and c v {2 i,}_{i,}^{2 i,}}_{i and c v {2 i,}_{i,}^{2 i,}}^{1 i and c v {2 i,}_{i,}^{2 i,}}}^{3 i is the union of c v {1 i and c v {2 i,}_{i,}^{2 i,}}_{i and c v {2 i,}_{i,}^{2 i,}}^{1 i and c v {2 i,}_{i,}^{2 i,}}}$
(3.2)
if either
(3.3)
if the resulting

where

c v {j i and {P j i are the components of the qualification-decomposition of T j .}_{i are the components of the qualification-decomposition of T j .}^{j i are the components of the qualification-decomposition of T j .}}_{i and {P j i are the components of the qualification-decomposition of T j .}_{i are the components of the qualification-decomposition of T j .}^{j i are the components of the qualification-decomposition of T j .}}^{j i and {P j i are the components of the qualification-decomposition of T j .}_{i are the components of the qualification-decomposition of T j .}^{j i are the components of the qualification-decomposition of T j .}}

A prvalue of type T1 can be converted to type T2 if the qualification-combined type of T1 and T2 is T2.

[Note 1:

If a program could assign a pointer of type T** to a pointer of type const T** (that is, if line #1 below were allowed), a program could inadvertently modify a const object (as it is done on line #2).

For example, int main() { const char c = 'c'; char* pc; const char* pcc = &pc; / #1: not allowed *pcc = &c; *pc = 'C'; / #2: modifies a const object }

— end note]

[Note 2:

Given similar types T1 and T2, this construction ensures that both can be converted to the qualification-combined type of T1 and T2.

— end note]

[Note 3:

A prvalue of type “pointer to cv1 T” can be converted to a prvalue of type “pointer to cv2 T” if “cv2 T” is more cv-qualified than “cv1 T”.

A prvalue of type “pointer to member of X of type cv1 T” can be converted to a prvalue of type “pointer to member of X of type cv2 T” if “cv2 T” is more cv-qualified than “cv1 T”.

— end note]

[Note 4:

Function types (including those used in pointer-to-member-function types) are never cv-qualified ([dcl.fct]).

— end note]