Why is an extra qualification for inline definitions of member functions disallowed?

Question

If we look at the grammar of a member declaration it loosk something like this:

member-declaration:
       function-definition ;

function-definition:
       attribute-specifier-seqopt decl-specifier-seqopt declarator 
       virt-specifier-seqopt function-body

declarator:
       noptr-declarator parameters-and-qualifiers trailing-return-type

This disallows the syntax:

struct B {
    A B::*B::read();
};

Why doesn't it allow an optional nested-name-specifier :: before the function name?

Answer 1

The grammar does not forbid to declare entities named by nested-name-specifiers at all.
As a matter of fact, it has to - for friend declarations:

class A
{
     friend void OtherNamespace::foo();
};

This is a member-declaration with the declaration specifier friend. It's covered through (bold emphasis mine):

member-declaration:
     attribute-specifier-seq_opt decl-specifier-seq_opt member-declarator-list_opt ;

And

member-declarator:
     declarator virt-specifier-seq_opt pure-specifier_opt

Now, a declarator has the following grammar:

declarator:
    ptr-declarator

ptr-declarator:
    noptr-declarator

noptr-declarator:
    declarator-id

declarator-id:
    ..._opt id-expression

Finally, an id-expression is

id-expression:
    qualified-id