Re: [PATCH] c++: fix array initialization wrong code [PR126335]

On Fri, 24 Jul 2026, Marek Polacek wrote:

> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> 
> -- >8 --
> This is a wrong-code problem starting with the recent check_initializer
> simplification (r17-1661).  I thought the fix would be to bring some of
> those dropped conditions back, but now I think the change just uncovered
> a latent bug.

FWIW it seems the aforementioned latent bug causes PR126472 and your
patch fixes it on trunk!  I wonder if backporting this patch to the 16
branch would make sense without r17-1661?

> 
> Since r17-1661, when initializing 'm' of type 'M[2]' we no longer call
> build_aggr_init_full_exprs in check_initializer; instead, we go on to
> store_init_value -> split_nonconstant_init.  There we arrive with:
> 
> {{.a=TARGET_EXPR <D.3136, <<< Unknown tree: aggr_init_expr
>   3
>   operator""_s
>   D.3136 >>>>, .b=TARGET_EXPR <D.3139, <<< Unknown tree: aggr_init_expr
>   3
>   operator""_s
>   D.3139 >>>>}, {.a={.p=&empty.str}, .b={.p=&empty.str}}}
> 
> which so far seems OK.  The type is an array so split_nonconstant_init_1
> delegates to build_vec_init and returns true which, as the comment says,
> should mean that "the whole of the value was initialized by the generated
> statements".  This is inaccurate: since try_const and do_static_init are
> both true in build_vec_init, we have split out the constant initializer
> (the {.a={.p=&empty.str}, .b={.p=&empty.str}} part) into DECL_INITIAL:
> 
> 5374       else if (do_static_init && !vec_safe_is_empty (const_vec))
> 5375         DECL_INITIAL (obase) = build_constructor (atype, const_vec);
> 
> so we have both dynamic and static initializers.  But since
> split_nonconstant_init_1 returns bool, it's not ready to signal this case
> to split_nonconstant_init, which then does:
> 
>  943       if (split_nonconstant_init_1 (dest, init, true, &flags))
>  944         init = NULL_TREE;
> 
> and then overwrites DECL_INITIAL (dest).  So we've lost a half of the
> initializer and got wrong-code as the result.
> 
> This patch fixes it by not throwing away the DECL_INITIAL that
> build_vec_init set for us.  I suppose another approach would be
> to somehow change split_nonconstant_init_1/ARRAY_TYPE to follow
> the element pruning/add_stmt like the rest of the function, but that
> seems more complicated.
> 
> 	PR c++/126335
> 
> gcc/cp/ChangeLog:
> 
> 	* typeck2.cc (split_nonconstant_init): Don't clear DECL_INITIAL
> 	if build_vec_init set it.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/init/array68.C: New test.
> ---
>  gcc/cp/typeck2.cc                   |  9 ++++++++-
>  gcc/testsuite/g++.dg/init/array68.C | 19 +++++++++++++++++++
>  2 files changed, 27 insertions(+), 1 deletion(-)
>  create mode 100644 gcc/testsuite/g++.dg/init/array68.C
> 
> diff --git a/gcc/cp/typeck2.cc b/gcc/cp/typeck2.cc
> index 2d7c65c3a51..720528e2505 100644
> --- a/gcc/cp/typeck2.cc
> +++ b/gcc/cp/typeck2.cc
> @@ -950,7 +950,14 @@ split_nonconstant_init (tree dest, tree init)
>        code = pop_stmt_list (code);
>        if (VAR_P (dest) && !is_local_temp (dest))
>  	{
> -	  DECL_INITIAL (dest) = init;
> +	  /* If we are initializing an array, split_nonconstant_init_1
> +	     might've delegated to build_vec_init in which case it always
> +	     returns true so we clear INIT.  But if we're initializing
> +	     a static array, build_vec_init can put constant initializers
> +	     into DECL_INITIAL.  Clearing it would mean losing some of the
> +	     initializers as in c++/126335.  */
> +	  if (init)
> +	    DECL_INITIAL (dest) = init;
>  	  TREE_READONLY (dest) = 0;
>  	}
>        else if (init)
> diff --git a/gcc/testsuite/g++.dg/init/array68.C b/gcc/testsuite/g++.dg/init/array68.C
> new file mode 100644
> index 00000000000..f9afda7eb53
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/init/array68.C
> @@ -0,0 +1,19 @@
> +// PR c++/126335
> +// { dg-do run { target c++20 } }
> +
> +struct D {};
> +struct Lit {
> +  constexpr Lit(char const *) {}
> +  D str;
> +};
> +auto empty = Lit("");
> +struct S {
> +  D * p;
> +  constexpr S() { p = &empty.str; }
> +  S(Lit) {}
> +  ~S() {}
> +};
> +template<Lit L> S operator ""_s() { return L; }
> +struct M { S a, b; };
> +static M m[2]{{""_s, ""_s}, {}};
> +int main() { return m[1].a.p == nullptr; }
> 
> base-commit: b5e6f5d2d435aba79ed8568f19da25378c716714
> -- 
> 2.55.0
> 
>