[conv.qual] Improvement and supply the omitted result

[conv.qual] p3 omitted to specify the result of the conversion, it might be

The pointer value ([basic.compound]) is unchanged by this conversion.

it implies that the result has the same pointer value as the original.

admit only two similar types can undergo qualification conversion

The current wording just says

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

According to the definition of qualification-combined type, it just concerns whether the corresponding Pi and CVi satisfy some conditions. For example: int ** and char**.

https://github.com/cplusplus/draft/pull/4860 has excluded an identity conversion from qualification conversion. But two types that have a difference only in their top-level cv-qualifications are also considered to be different types. For example:
int const vs. int, char* const vs. char*, they only differ in their top-level cv-qualifiers.

So, based on that, the wording might be changed to

A prvalue of type T1 can be converted to another type T2(ignoring cv-qualification) if T1 and T2 are similar and the qualification-combined type of T1 and T2 is T2.

duplicately add const

[conv.qual] p3.3 says

if the resulting cv³_i is different from cv¹_i or cv²_i or the resulting P³_i is different from P¹_i or P²_i, then const is added to every cv³_k for 0< k < i

Consider a const has added in cv³_k due to [conv.qual] p3.1. [conv.qual] p3.3 can result in a duplicate addition. For example:
T1 = int*** and T2 = int *const* const*, after applying [conv.qual] p3.1, T3 will be pointer to const pointer to const pointer to int where cv³₂ is different from cv¹₂, however, cv³₁ does have a const. {const const } is not a valid set defined in [basic.type.qualifier] p6.

The change might be

if the resulting cv³_i is different from cv¹_i or cv²_i or the resulting P³_i is different from P¹_i or P²_i, then const is added to every cv³_k for 0< k < i if cv³_k haven't yet comprised a const

[conv.qual] p3 should apply to a pointer, a pointer to member types

Because the bullets in p3 refer to the component Pi that is defined as “pointer to”, “pointer to member of class Ci of type”, “array of Ni”, “array of unknown bound of”. So, the precondition for this definition should be n ≥ 1

The initial sentence might be changed to

The qualification-combined type of two types T1 and T2 is the type T3 similar to T1 whose qualification-decomposition(where n ≥ 1 ) is such that:

As a similar reason to the above, the definition of similar might be changed to

Two types T1 and T2 are similar if they have qualification-decompositions with the same n such that either there is no Pi component, or the corresponding Pi components are either the same or one is “array of Ni” and the other is “array of unknown bound of”, and the types denoted by U are the same.

Start by reading [conv.qual], especially paragraph 3, together with [basic.compound] and [basic.type.qualifier], then check the definitions of qualification-combined and similar types cited in the issue. Done means reaching and documenting an agreed wording for the omitted result, similar-type restriction, duplicate const handling, and pointer or pointer-to-member precondition.