Re: [PATCH] c++: fix ICE with constexpr ARRAY_REF [PR110382]

[Jason Merrill <jason@redhat.com>]

On 7/21/23 18:38, Marek Polacek wrote:
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/13?
> 
> -- >8 --
> 
> This code in cxx_eval_array_reference has been hard to get right.
> In r12-2304 I added some code; in r13-5693 I removed some of it.
> 
> Here the problematic line is "S s = arr[0];" which causes a crash
> on the assert in verify_ctor_sanity:
> 
>    gcc_assert (!ctx->object || !DECL_P (ctx->object)
>                || ctx->global->get_value (ctx->object) == ctx->ctor);
> 
> ctx->object is the VAR_DECL 's', which is correct here.  The second
> line points to the problem: we replaced ctx->ctor in
> cxx_eval_array_reference:
> 
>    new_ctx.ctor = build_constructor (elem_type, NULL); // #1

...and this code doesn't also clear(/set) new_ctx.object like everywhere 
else in constexpr.cc that sets new_ctx.ctor.  Fixing that should make 
the testcase work.

> which I think we shouldn't have; the CONSTRUCTOR we created in
> cxx_eval_constant_expression/DECL_EXPR
> 
>    new_ctx.ctor = build_constructor (TREE_TYPE (r), NULL);
> 
> had the right type.

Indeed, and using it rather than building a new one seems like a valid 
optimization for trunk.

I also notice that the DECL_EXPR code calls unshare_constructor, which 
should be unnecessary if init == ctx->ctor?

> We still need #1 though.  E.g., in constexpr-96241.C, we never
> set ctx.ctor/object before calling cxx_eval_array_reference, so
> we have to build a CONSTRUCTOR there.  And in constexpr-101371-2.C
> we have a ctx.ctor, but it has the wrong type, so we need a new one.
> 
> 	PR c++/110382
> 
> gcc/cp/ChangeLog:
> 
> 	* constexpr.cc (cxx_eval_array_reference): Create a new constructor
> 	only when we don't already have a matching one.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp1y/constexpr-110382.C: New test.
> ---
>   gcc/cp/constexpr.cc                           |  5 ++++-
>   gcc/testsuite/g++.dg/cpp1y/constexpr-110382.C | 17 +++++++++++++++++
>   2 files changed, 21 insertions(+), 1 deletion(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-110382.C
> 
> diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> index fb94f3cefcb..518b7c7a2d5 100644
> --- a/gcc/cp/constexpr.cc
> +++ b/gcc/cp/constexpr.cc
> @@ -4291,7 +4291,10 @@ cxx_eval_array_reference (const constexpr_ctx *ctx, tree t,
>     else
>       val = build_value_init (elem_type, tf_warning_or_error);
>   
> -  if (!SCALAR_TYPE_P (elem_type))
> +  if (!SCALAR_TYPE_P (elem_type)
> +      /* Create a new constructor only if we don't already have one that
> +	 is suitable.  */
> +      && !(ctx->ctor && same_type_p (elem_type, TREE_TYPE (ctx->ctor))))

We generally use same_type_ignoring_top_level_qualifiers_p in the 
constexpr code.

Jason