c++: empty union member activation during constexpr [PR102163]

  Here, the union's constructor is defined to activate its empty data
member _M_rest, but during constexpr evaluation of this constructor the
subobject constructor call to O::O(&_M_rest, 42) produces no side
effects that actually activates the member, so the union still appears
uninitialized after the fact.  This patch fixes this by faking up a
dummy MODIFY_EXPR in this situation, whose evaluation ensures the member
gets activated.

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK
for trunk?

	PR c++/102163

gcc/cp/ChangeLog:

	* constexpr.c (cxx_eval_call_expression): After evaluating a
	constructor call for an empty union member, produce a side
	effect that makes sure the member is activated.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-empty17.C: New test.
---
 gcc/cp/constexpr.c                            | 34 +++++++++++++++----
 .../g++.dg/cpp0x/constexpr-empty17.C          | 21 ++++++++++++
 2 files changed, 49 insertions(+), 6 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-empty17.C
  

On 9/13/21 3:54 PM, Patrick Palka wrote:
> Here, the union's constructor is defined to activate its empty data
> member _M_rest, but during constexpr evaluation of this constructor the
> subobject constructor call to O::O(&_M_rest, 42) produces no side
> effects that actually activates the member, so the union still appears
> uninitialized after the fact.  This patch fixes this by faking up a
> dummy MODIFY_EXPR in this situation, whose evaluation ensures the member
> gets activated.
> 
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK
> for trunk?

OK.

> 	PR c++/102163
> 
> gcc/cp/ChangeLog:
> 
> 	* constexpr.c (cxx_eval_call_expression): After evaluating a
> 	constructor call for an empty union member, produce a side
> 	effect that makes sure the member is activated.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp0x/constexpr-empty17.C: New test.
> ---
>   gcc/cp/constexpr.c                            | 34 +++++++++++++++----
>   .../g++.dg/cpp0x/constexpr-empty17.C          | 21 ++++++++++++
>   2 files changed, 49 insertions(+), 6 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-empty17.C
> 
> diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
> index 7772fe62d95..40b0b80b438 100644
> --- a/gcc/cp/constexpr.c
> +++ b/gcc/cp/constexpr.c
> @@ -2787,12 +2787,34 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree t,
>   					&jump_target);
>   
>   	  if (DECL_CONSTRUCTOR_P (fun))
> -	    /* This can be null for a subobject constructor call, in
> -	       which case what we care about is the initialization
> -	       side-effects rather than the value.  We could get at the
> -	       value by evaluating *this, but we don't bother; there's
> -	       no need to put such a call in the hash table.  */
> -	    result = lval ? ctx->object : ctx->ctor;
> +	    {
> +	      /* This can be null for a subobject constructor call, in
> +		 which case what we care about is the initialization
> +		 side-effects rather than the value.  We could get at the
> +		 value by evaluating *this, but we don't bother; there's
> +		 no need to put such a call in the hash table.  */
> +	      result = lval ? ctx->object : ctx->ctor;
> +
> +	      if (!result && new_obj
> +		  && TREE_CODE (new_obj) == COMPONENT_REF
> +		  && TREE_CODE (TREE_TYPE
> +				(TREE_OPERAND (new_obj, 0))) == UNION_TYPE
> +		  && is_really_empty_class (TREE_TYPE (new_obj),
> +					    /*ignore_vptr*/false))
> +		{
> +		  /* This constructor call for an empty union member might not
> +		     have produced a side effect that actually activated the
> +		     member.  So produce such a side effect now to ensure the
> +		     union appears initialized.  */
> +		  tree activate = build2 (MODIFY_EXPR, TREE_TYPE (new_obj),
> +					  new_obj,
> +					  build_constructor (TREE_TYPE (new_obj),
> +							     NULL));
> +		  cxx_eval_constant_expression (ctx, activate, lval,
> +						non_constant_p, overflow_p);
> +		  ggc_free (activate);
> +		}
> +	    }
>   	  else if (VOID_TYPE_P (TREE_TYPE (res)))
>   	    result = void_node;
>   	  else
> diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-empty17.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-empty17.C
> new file mode 100644
> index 00000000000..9d753a3bb69
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-empty17.C
> @@ -0,0 +1,21 @@
> +// PR c++/102163
> +// { dg-do compile { target c++11 } }
> +
> +struct O {
> +  constexpr O(int) { }
> +};
> +
> +union _Variadic_union {
> +  constexpr _Variadic_union(int __arg) : _M_rest(__arg) { }
> +
> +  int _M_first;
> +  O _M_rest;
> +};
> +
> +
> +struct _Variant_storage {
> +  constexpr _Variant_storage() : _M_u(42) {}
> +  _Variadic_union _M_u;
> +};
> +
> +constexpr _Variant_storage w;
>