How compiler translate postfix/prefix operators?

in c/c++

a++=5;
++a=6;

From above expressions Only 2nd expression is valid . HOw compiler translate these expressions , so that it's able to find error: lvalue required as left operand of assignment .

I have tried following translation scheme for 1st expression but it's wrong .

  ...
 postfix-expression:
  primary-expression{
    $<info.place>$ = $<info.place>1;
  }
  | postfix-expression PLUSPLUS{
     $<info.place>$ = newTemp(); //Generate temporary var
     genCode($<info.place>$,$<info.place>1); //Assign temp with a 
     genCode($<info.place>1,$<info.place>$,"+","1");//Increment a by 1 
 }
 ...

When i use a++=5 then it generates

0000 : T0   =   a
0001 : a    =   T0  +   1
0002 : T1   =   5
0003 : T0   =   T1

What are the changes have to be made in action so that it works like c/c++ .