c++: constexpr empty subobject confusion [PR110197]

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk/13 (later)?

-- >8 --

Now that init_subob_ctx no longer sets new_ctx.ctor for a subobject of
empty type, it seems we need to ensure its callers cxx_eval_bare_aggregate
and cxx_eval_vec_init_1 consistently omit entries for such subobjects in
the parent ctx->ctor.  We also need to allow cxx_eval_array_reference
to synthesize an empty element object even if the array CONSTRUCTOR
has CONSTRUCTOR_NO_CLEARING set.

	PR c++/110197

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_array_reference): Return a synthesized
	empty subobject even if CONSTRUCTOR_NO_CLEARING is set.
	(cxx_eval_bare_aggregate): Set 'no_slot' to true more generally
	whenever new_ctx.ctor is empty, i.e. for any subobject of empty
	type.
	(cxx_eval_vec_init_1): Define 'no_slot' as above and use it
	accordingly.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-empty18.C: New test.
	* g++.dg/cpp0x/constexpr-empty19.C: New test.
---
 gcc/cp/constexpr.cc                           | 23 +++++++++++++------
 .../g++.dg/cpp0x/constexpr-empty18.C          |  7 ++++++
 .../g++.dg/cpp0x/constexpr-empty19.C          | 12 ++++++++++
 3 files changed, 35 insertions(+), 7 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-empty18.C
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-empty19.C

On 7/26/23 12:57, Patrick Palka wrote:
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk/13 (later)?

OK.

> -- >8 --
> 
> Now that init_subob_ctx no longer sets new_ctx.ctor for a subobject of
> empty type, it seems we need to ensure its callers cxx_eval_bare_aggregate
> and cxx_eval_vec_init_1 consistently omit entries for such subobjects in
> the parent ctx->ctor.  We also need to allow cxx_eval_array_reference
> to synthesize an empty element object even if the array CONSTRUCTOR
> has CONSTRUCTOR_NO_CLEARING set.
> 
> 	PR c++/110197
> 
> gcc/cp/ChangeLog:
> 
> 	* constexpr.cc (cxx_eval_array_reference): Return a synthesized
> 	empty subobject even if CONSTRUCTOR_NO_CLEARING is set.
> 	(cxx_eval_bare_aggregate): Set 'no_slot' to true more generally
> 	whenever new_ctx.ctor is empty, i.e. for any subobject of empty
> 	type.
> 	(cxx_eval_vec_init_1): Define 'no_slot' as above and use it
> 	accordingly.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp0x/constexpr-empty18.C: New test.
> 	* g++.dg/cpp0x/constexpr-empty19.C: New test.
> ---
>   gcc/cp/constexpr.cc                           | 23 +++++++++++++------
>   .../g++.dg/cpp0x/constexpr-empty18.C          |  7 ++++++
>   .../g++.dg/cpp0x/constexpr-empty19.C          | 12 ++++++++++
>   3 files changed, 35 insertions(+), 7 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-empty18.C
>   create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-empty19.C
> 
> diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> index f2fcb54626d..da2c3116810 100644
> --- a/gcc/cp/constexpr.cc
> +++ b/gcc/cp/constexpr.cc
> @@ -4297,6 +4297,9 @@ cxx_eval_array_reference (const constexpr_ctx *ctx, tree t,
>   
>     /* Not found.  */
>   
> +  if (is_really_empty_class (elem_type, /*ignore_vptr*/false))
> +    return build_constructor (elem_type, NULL);
> +
>     if (TREE_CODE (ary) == CONSTRUCTOR
>         && CONSTRUCTOR_NO_CLEARING (ary))
>       {
> @@ -4314,9 +4317,7 @@ cxx_eval_array_reference (const constexpr_ctx *ctx, tree t,
>        directly for non-aggregates to avoid creating a garbage CONSTRUCTOR.  */
>     tree val;
>     constexpr_ctx new_ctx;
> -  if (is_really_empty_class (elem_type, /*ignore_vptr*/false))
> -    return build_constructor (elem_type, NULL);
> -  else if (CP_AGGREGATE_TYPE_P (elem_type))
> +  if (CP_AGGREGATE_TYPE_P (elem_type))
>       {
>         tree empty_ctor = build_constructor (init_list_type_node, NULL);
>         val = digest_init (elem_type, empty_ctor, tf_warning_or_error);
> @@ -5095,9 +5096,9 @@ cxx_eval_bare_aggregate (const constexpr_ctx *ctx, tree t,
>     FOR_EACH_CONSTRUCTOR_ELT (v, i, index, value)
>       {
>         tree orig_value = value;
> -      /* Like in cxx_eval_store_expression, omit entries for empty fields.  */
> -      bool no_slot = TREE_CODE (type) == RECORD_TYPE && is_empty_field (index);
>         init_subob_ctx (ctx, new_ctx, index, value);
> +      /* Like in cxx_eval_store_expression, omit entries for empty fields.  */
> +      bool no_slot = new_ctx.ctor == NULL_TREE;
>         int pos_hint = -1;
>         if (new_ctx.ctor != ctx->ctor && !no_slot)
>   	{
> @@ -5261,7 +5262,8 @@ cxx_eval_vec_init_1 (const constexpr_ctx *ctx, tree atype, tree init,
>         bool reuse = false;
>         constexpr_ctx new_ctx;
>         init_subob_ctx (ctx, new_ctx, idx, pre_init ? init : elttype);
> -      if (new_ctx.ctor != ctx->ctor)
> +      bool no_slot = new_ctx.ctor == NULL_TREE;
> +      if (new_ctx.ctor != ctx->ctor && !no_slot)
>   	{
>   	  if (zeroed_out)
>   	    CONSTRUCTOR_NO_CLEARING (new_ctx.ctor) = false;
> @@ -5306,7 +5308,14 @@ cxx_eval_vec_init_1 (const constexpr_ctx *ctx, tree atype, tree init,
>   	}
>         if (*non_constant_p)
>   	break;
> -      if (new_ctx.ctor != ctx->ctor)
> +      if (no_slot)
> +	{
> +	  /* This is an initializer for an empty subobject; now that we've
> +	     checked that it's constant, we can ignore it.  */
> +	  gcc_checking_assert (i == 0);
> +	  break;
> +	}
> +      else if (new_ctx.ctor != ctx->ctor)
>   	{
>   	  /* We appended this element above; update the value.  */
>   	  gcc_assert ((*p)->last().index == idx);
> diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-empty18.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-empty18.C
> new file mode 100644
> index 00000000000..4bb9e3dcb64
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-empty18.C
> @@ -0,0 +1,7 @@
> +// PR c++/110197
> +// { dg-do compile { target c++11 } }
> +
> +struct A { constexpr A(int) { } };
> +struct B { A a; };
> +constexpr B f(int n) { return B{A{n}}; }
> +constexpr B b = f(1);
> diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-empty19.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-empty19.C
> new file mode 100644
> index 00000000000..5ad67682c5b
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-empty19.C
> @@ -0,0 +1,12 @@
> +// PR c++/110197
> +// { dg-do compile { target c++11 } }
> +
> +struct A {
> +  constexpr A() : A(__builtin_is_constant_evaluated()) { }
> +  constexpr A(int) { }
> +};
> +constexpr A a1[1] = {{}};
> +constexpr A a2[2] = {{}, {}};
> +constexpr A a3[3] = {{}, {}, {}};
> +constexpr A a4[4] = {};
> +constexpr A a5[5] = {};