Why the default assignment operator is not synthesized by the compiler if the class has a reference data element

In C ++, if a class has a reference data element, the default assignment operator is not synthesized by the compiler. Why?

What copy assignment should do is defined in:

C ++ 03 Standard 12.8 / 13:

Each subobject is assigned according to its type:

• if the subobject has a class type, the instance assignment operator for the class is used (as if it were explicitly qualified, i.e. ignored any possible functions of virtual redefinition in more derived classes);

• , ;

• , .

,
,
?
:

• , , ^{[Ref 1]}.
• #1 , , .

, . , ++ , , , , .

:
++ 03 12.8/12:

, , . , , , :
 .......
 - ,
 .......

^{[Ref 1]}
++ 03 8.5.3/2:

. , , . (5.2.2) (6.6.3) .