为什么这个const const const声明有效

Question

A friend asked me to explain why

const const const const const int const i = 0;

is valid syntax. I declined to have any thoughts on the subject. Though I'm curious if it is just a grammar thing?

Edit. Tag is C++, my friend was referencing gcc, so I should probably add the C tag.

Answer 1

From n1256:

6.7.3/4

If the same qualifier appears more than once in the same specifier-qualifier-list,either directly or via one or more typedefs, the behavior is the same as if it appeared only once.

In C++, this should give an error.

From C++03 standard:

7.1.5/1 [dcl.type]

As a general rule, at most one type-specifier is allowed in the complete decl-specifier-seq of a declaration. The only exceptions to this rule are the following:

— const or volatile can be combined with any other type-specifier. However, redundant cv- qualifiers are prohibited except when introduced through the use of typedefs (7.1.3) or template type arguments (14.3), in which case the redundant cv-qualifiers are ignored.

From N3797 (C++11):

7.1.6/2 [dcl.type]

As a general rule, at most one type-specifier is allowed in the complete decl-specifier-seq of a declaration or in a type-specifier-seq or trailing-type-specifier-seq. The only exceptions to this rule are the following:

 — const can be combined with any type specifier except itself.