static return type in PHP 7 interfaces -

why not possible in php 7, declare interface static return type?

let's have following classes:

interface bignumber {     /**      * @param bignumber $that      *      * @return static      */     public function plus(bignumber $that); }  class biginteger implements bignumber { ... } class bigdecimal implements bignumber { ... } 

i want enforce return type of plus() method static, is:

• biginteger::plus() must return biginteger
• bigdecimal::plus() must return bigdecimal

i can declare interface in following way:

public function plus(bignumber $that) : bignumber; 

but doesn't enforce above. is:

public function plus(bignumber $that) : static; 

but php 7, date, not happy it:

php parse error: syntax error, unexpected 'static' (t_static)

is there specific reason this, or bug should reported?

it's not bug, doesn't make sense design-wise object-oriented programming perspective.

if biginteger , bigdecimal implement both bignumber, care contract fulfil. case, it's bignumber's interface.

so return type should using in interface bignumber since coding against interface not know else members of interface. if need know 1 returned, interface perhaps wide in first place.

note: programming languages generics can achieve effect specifying return type generic type, php not have generics , not have in near future.