[dcl.type.general]

The type-specifiers are

{o p t type-specifier type-specifier-seq}_{o p t type-specifier type-specifier-seq}

{o p t defining-type-specifier defining-type-specifier-seq}_{o p t defining-type-specifier defining-type-specifier-seq}

The attribute-specifier-seq affects the type only for the declaration it appears in, not other declarations involving the same type.

As a general rule, at most one defining-type-specifier is allowed in the complete decl-specifier-seq of a declaration or in a defining-type-specifier-seq, and at most one type-specifier is allowed in a type-specifier-seq.

The only exceptions to this rule are the following:

(2.1)
const can be combined with any type specifier except itself.
(2.2)
volatile can be combined with any type specifier except itself.
(2.3)
signed or unsigned can be combined with char, long, short, or int.
(2.4)
short or long can be combined with int.
(2.5)
long can be combined with double.
(2.6)
long can be combined with long.

Except in a declaration of a constructor, destructor, or conversion function, at least one defining-type-specifier that is not a cv-qualifier shall appear in a complete type-specifier-seq or a complete decl-specifier-seq.

[Note 1:

The remaining type-specifiers are discussed in the rest of [dcl.type].

— end note]