c - getting error: cannot take the address of an rvalue of type 'int' -

i tryed compile old code new compiler , got next error:

error: cannot take address of rvalue of type 'int' 

here example 2 lines - 1 compiles , other gives error

struct mstct {     int myfield;     int myfield2[5];     int myfield3[5]; };  typedef struct mstct db_data_px;  int foo(int a, int b, int c){    //the next code compiles successfully.   unsigned val1 = ((18 == c) ? ((unsigned) & (((db_data_px *) 0)->myfield)) : ((unsigned) & (((db_data_px *) 0)->myfield3[b]))); //successes     //the next code failing   unsigned val2 = (unsigned) & ((18 == c) ? (((db_data_px *) 0)->myfield) : (((db_data_px *) 0)->myfield3[b]));   return 0; // failing } 

why first line compiles , second failing ? why need cast (unsigned) & in both of select expression , not enough cast after select expression valued ?

in code

   ((18 == c) ? (((db_data_px *) 0)->myfield) : (((db_data_px *) 0)->myfield3[b])) 

is conditional expression not produce lvalue.

the above expression gives rvalue (non-lvaue) , cannot use & operator on that.

to elaborate, quoting c11 standard, chapter §6.5.3.2, address , indirection operators

the operand of unary & operator shall either function designator, result of [] or unary * operator, or lvalue designates object not bit-field , not declared register storage-class specifier.

otoh, result type of conditional operator, chapter §6.5.15, footnote

a conditional expression not yield lvalue.

just imagine, &5, not possible.