The limitation for a function that can have an explicit object parameter is defined as

An explicit-object-parameter-declaration is a parameter-declaration with a this specifier. An explicit-object-parameter-declaration shall appear only as the first parameter-declaration of a parameter-declaration-list of either:

• a member-declarator that declares a member function ([class.mem]), or
• a lambda-declarator ([expr.prim.lambda]).

A member-declarator with an explicit-object-parameter-declaration shall not include a ref-qualifier or a cv-qualifier-seq and shall not be declared static or virtual.

Consider a constructor of a class, which satisfies all bullets regulated above. Namely, the constructor can have an explicit object parameter, is this reasonable? Since in the call of a constructor, there never exists an implicit object argument to match the object parameter. From this perspective, a constructor with an explicit object parameter is similar to a non-member function with that object parameter, the latter does make no sense and is forbidden by the above rule.