[class.mem]/12:

The decl-specifier-seq may be omitted in constructor, destructor, and conversion function declarations only; when declaring another kind of member the decl-specifier-seq shall contain a type-specifier that is not a cv-qualifier. The member-declarator-list can be omitted only after a class-specifier or an enum-specifier or in a friend declaration. A pure-specifier shall be used only in the declaration of a virtual function that is not a friend declaration.

The code below is accepted by C++ compilers since time immemorial:

struct A{
    const int i;
    A():i(1) {}
};