[Bug c/63944] New: [DR#413] Partial overriding of nonconstant struct/union initializers with designated initializers

["jsm28 at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63944

            Bug ID: 63944
           Summary: [DR#413] Partial overriding of nonconstant
                    struct/union initializers with designated initializers
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jsm28 at gcc dot gnu.org
                CC: jakub at gcc dot gnu.org

C11 DR#413 has now moved to closed (see N1892).  This says that if an
initializer (a) initializes a struct or union member of a larger object using
an expression of that struct or union type and (b) then uses a designated
initializer to override just part of that struct or union initialization, then
the overriding does not affect the initialization of the rest of the struct or
union.  Thus, the following should pass (based on the test in the DR):

extern void exit (int);
extern void abort (void);
typedef struct {
int k;
int l;
int a[2];
} T;

typedef struct {
int i;
T t;
} S;

T x = {.l = 43, .k = 42, .a[1] = 19, .a[0] = 18 };

int
main (void)
{
S l = { 1, .t = x, .t.l = 41, .t.a[1] = 17};
if (l.t.k == 42) exit (0); else abort ();
}

Clearly the existing approach for string constants (of splitting them
individual characters) won't work here (but is still needed for initializers of
objects with static storage duration).  This bug is specific to initializers of
objects with automatic storage duration; it can apply both to initialization
with an expression of struct type, and to initialization with one of union type
(in the union case, the override would have to be of just part of a union
member, as setting the whole of a union member sets the whole union).  I
suppose some initializations with such partial overriding need to generate more
complicated code - one possibility might be for such partial overriding to
replace the initializer for .t with (tmp = x, tmp.l = 41, tmp) for a new
temporary variable tmp.