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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c
Subject: Re: GCC bug
Date: Sun, 14 Dec 2025 22:38:34 -0800
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Andrey Tarasevich <noone@noone.net> writes:

> An attempt to use designated initializers to explicitly re-initialize
> a sub-member `y` of a struct member `b.a`
>
>   #include <stdio.h>
>
>   struct A { int x, y; };
>   struct B { struct A a; };
>
>   int main(void)
>   {
>     struct A ia = { 1, 2 };
>     struct B b = { .a = ia, .a.y = 42 };
>     printf("%d %d\n", b.a.x, b.a.y);
>   }
>
> GCC outputs:
>
>   0 42
>
> I.e. it does initialize `b.a.y` with `42`, but for some reason also
> produces zero in `b.a.x`. Meanwhile, Clang, MSVC output
>
>   1 42
>
> as expected.
>
> The funny part is that this exact functionality is actually directly
> illustrated by the standard in "6.7.11 Initialization" example 12.
>
> Is it just a bug or is there some defiant reasoning (e.g. "we know
> better") for GCC's behavior?

Given the example that is shown in the C standard, it seems clear
that what gcc does is wrong, so may be called a bug.