[Bug c/78079] New: implicit initialization overrides explicit designated initializer

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78079

            Bug ID: 78079
           Summary: implicit initialization overrides explicit designated
                    initializer
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

In C11 DR 413
(http://www.open-std.org/jtc1/sc22/wg14/www/docs/summary.htm#dr_413) the
committee's resolution clarifies by adding an example the intent already
present in the standard that implicit initialization of struct members doesn't
override their explicit initialization.

When compiled using GCC the added example (below) aborts at runtime when it is
expected to run successfully to completion.  (Clang does the right thing and
its output might help illuminate why.  See below.)

$ cat b.c && /build/gcc-trunk-svn/gcc/xgcc -B /build/gcc-trunk-svn/gcc -Wall
-Wextra -Wpedantic b.c && ./a.out
    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) __builtin_abort ();
    }
Aborted (core dumped)



$ /build/llvm-trunk/bin/clang -Wall -Wextra -Wpedantic b.c && ./a.out
b.c:16:30: warning: subobject initialization overrides initialization of other
      fields within its enclosing subobject [-Winitializer-overrides]
        S l = { 1, .t = x, .t.l = 41, .t.a[1] = 17};
                             ^~~~~~~
b.c:16:25: note: previous initialization is here
        S l = { 1, .t = x, .t.l = 41, .t.a[1] = 17};
                        ^
b.c:16:41: warning: subobject initialization overrides initialization of other
      fields within its enclosing subobject [-Winitializer-overrides]
        S l = { 1, .t = x, .t.l = 41, .t.a[1] = 17};
                                        ^~~~~~~~~~
b.c:16:25: note: previous initialization is here
        S l = { 1, .t = x, .t.l = 41, .t.a[1] = 17};
                        ^
2 warnings generated.