Does a namespace-scope constructor definition require a class-qualified identifier?

Question

Here's something we all learned on Day 1 of C++, which we take for granted but doesn't clearly follow from the wording of the Standard.

Given a class S, we can define its constructor

struct S { S(); };
S::S() { … }

But the Standard seems to allow this just as well:

struct S { S(); };
S() { … }

Qualifying the name of a class with itself is always allowed but always redundant. For example S::S::S::S() { … } is also a valid declaration. If S::S is, why not plain S?

From C++11 §12.1/1,

Constructors do not have names. A special declarator syntax is used to declare or define the constructor. The syntax uses:

— an optional decl-specifier-seq in which each decl-specifier is either a function-specifier or constexpr,

— the constructor’s class name, and

— a parameter list

in that order.

This applies equally to class or namespace scope. There is a special rule about namespace scope, §9.3/5,

If the definition of a member function is lexically outside its class definition, the member function name shall be qualified by its class name using the :: operator.

However, constructors do not have names, so this doesn't apply, right? Moreover, there's no reason to require the qualification, because there is no syntactic ambiguity. A function declared with no return type and a class-name for an identifier is always a syntax error under currently observed rules. Right?

Not that we should start writing code with the qualification omitted, but is there a reason that no compiler accepts this, or is it just tradition?

Answer 1

Yes, it says that,

If the definition of a member function is lexically outside its class definition the member function name shall be qualified by its class name using the :: operator.

But it doesn't says that member function w/o name shall not be qualified by its class name. Does it? ;)

That seems to lead to an uncertain area depending on implementations. However, the form of A::A is defined by the Standard.

5.1 Primary Expressions

Where class-name :: class-name is used, and the two class-names refer to the same class, this notation names the constructor..

As to whether A(){..} is allowed or not, I guess there is no reason to do it conventionally(Is there ANY C++ compiler allow it?? AFAIK, nope):

1. Since constructor is a special member function, the way of A::A(){..} is more consistent with other member functions. Why borther allow it to behave specially? That's probably not worth the effort.

2. No one wants to run the risk of writing non-compliant code that's not explicitly stated in the Standard.