In C draft ISO(N4901/2021) 6.5.4 (Argument-dependent name lookup) we have:

When the postfix-expression in a function call (7.6.1.3) is an unqualified-id, and unqualified lookup (6.5.3) for the name in the unqualified-id does not find any (1.1) — declaration of a class member, or (1.2) — function declaration inhabiting a block scope, or (1.3) — declaration not of a function or function template

I can't figure out a example of an unqualified-id that contains a unqualified name(with the two being different). Draft gives the following example, but i couldn't say what is what in the (f)(s):

namespace N {
struct S { };
void f(S);
}
void g() {
N::S s;
f(s); // OK: calls N::f
(f)(s); // error: N::f not considered; parentheses prevent argument-dependent lookup
}

CodePudding user response：

unqualified-id that contains a unqualified name

The two are synonyms. The former is the correct term.

couldn't say what is what in the (f)(s)

By the postfix-expression in this context they mean the thing being called. Consider:

[expr.call]/1 A function call is a postfix expression followed by parentheses containing ...

So, in (f)(s) the postfix-expression is (f). It's clearly not an unqualified-id, so ADL doesn't apply.

CodePudding user response：

I can't figure out a example of an unqualified-id that contains a unqualified name(with the two being different).

The wording doesn't say «unqualified name». And seem to miss the word «component» before «name». Here is an example:

namespace N
{
    struct S { };
    template<typename T>
    T f(S);
}

void g()
{
    N::S s;
    f<int>(s); // OK: calls N::f
}

unqualified-id here is f<int> (which is template-id) and the lookup is performed for its component name f.

[expr.prim.id.unqual]/2:

A component name of an unqualified-id U is
— U if it is a name or
— the component name of the template-id or type-name of U, if any.

[temp.names]/2

The component name of a simple-template-id, template-id, or template-name is the first name in it.

The paragraphs use the term «name» which is defined in [basic.pre]/4:

A name is an identifier ([lex.name]), operator-function-id ([over.oper]), literal-operator-id ([over.literal]), or conversion-function-id ([class.conv.fct]).