[Bug c/70921] New: Initializer for sub-sub-object overrides whole initializer for sub-object

[ch3root at openwall dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70921

            Bug ID: 70921
           Summary: Initializer for sub-sub-object overrides whole
                    initializer for sub-object
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ch3root at openwall dot com
  Target Milestone: ---

Source code:

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#include <stdio.h>

int main()
{
  struct s { int x, y; } s1 = { 1, 2 };
  struct { struct s sub; } s2 = { s1, .sub.y = 4 };
  printf("%d %d\n", s2.sub.x, s2.sub.y);
}
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Results:

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$ gcc -std=c11 -pedantic -Wall -Wextra example.c && ./a.out
0 4
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gcc version: gcc (GCC) 7.0.0 20160502 (experimental)

It seems wrong that the initializer for the .sub.y subsubobject overrides the
previously listed initializer for the whole .sub subobject, loosing explicit
initialization of .sub.x.
The relevant rule -- C11, 6.7.9p19: "The initialization shall occur in
initializer list order, each initializer provided for a particular subobject
overriding any previously listed initializer for the same subobject". It
doesn't seem to cover this example. But, at least, the lack of warnings from
gcc is wrong.

For comparison:

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$ clang -std=c11 -Weverything example.c && ./a.out
example.c:6:43: warning: subobject initialization overrides initialization of
other fields within its enclosing subobject [-Winitializer-overrides]
  struct { struct s sub; } s2 = { s1, .sub.y = 4 };
                                          ^~~~~~
example.c:6:35: note: previous initialization is here
  struct { struct s sub; } s2 = { s1, .sub.y = 4 };
                                  ^~
1 warning generated.
1 4
----------------------------------------------------------------------