Re: [PATCH v2] c++: Catch indirect change of active union member in constexpr [PR101631]

[Jason Merrill <jason@redhat.com>]

On 9/1/23 08:22, Nathaniel Shead wrote:
> On Wed, Aug 30, 2023 at 04:28:18PM -0400, Jason Merrill wrote:
>> On 8/29/23 09:35, Nathaniel Shead wrote:
>>> This is an attempt to improve the constexpr machinery's handling of
>>> union lifetime by catching more cases that cause UB. Is this approach
>>> OK?
>>>
>>> I'd also like some feedback on a couple of pain points with this
>>> implementation; in particular, is there a good way to detect if a type
>>> has a non-deleted trivial constructor? I've used 'is_trivially_xible' in
>>> this patch, but that also checks for a trivial destructor which by my
>>> reading of [class.union.general]p5 is possibly incorrect. Checking for a
>>> trivial default constructor doesn't seem too hard but I couldn't find a
>>> good way of checking if that constructor is deleted.
>>
>> I guess the simplest would be
>>
>> (TYPE_HAS_TRIVIAL_DFLT (t) && locate_ctor (t))
>>
>> because locate_ctor returns null for a deleted default ctor.  It would be
>> good to make this a separate predicate.
>>
>>> I'm also generally unsatisfied with the additional complexity with the
>>> third 'refs' argument in 'cxx_eval_store_expression' being pushed and
>>> popped; would it be better to replace this with a vector of some
>>> specific structure type for the data that needs to be passed on?
>>
>> Perhaps, but what you have here is fine.  Another possibility would be to
>> just have a vec of the refs and extract the index from the ref later as
>> needed.
>>
>> Jason
>>
> 
> Thanks for the feedback. I've kept the refs as-is for now. I've also
> cleaned up a couple of other typos I'd had with comments and diagnostics.
> 
> Bootstrapped and regtested on x86_64-pc-linux-gnu.
> 
> @@ -6192,10 +6197,16 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
>   
>         type = reftype;
>   
> -      if (code == UNION_TYPE && CONSTRUCTOR_NELTS (*valp)
> -	  && CONSTRUCTOR_ELT (*valp, 0)->index != index)
> +      if (code == UNION_TYPE
> +	  && TREE_CODE (t) == MODIFY_EXPR
> +	  && (CONSTRUCTOR_NELTS (*valp) == 0
> +	      || CONSTRUCTOR_ELT (*valp, 0)->index != index))
>   	{
> -	  if (cxx_dialect < cxx20)
> +	  /* We changed the active member of a union. Ensure that this is
> +	     valid.  */
> +	  bool has_active_member = CONSTRUCTOR_NELTS (*valp) != 0;
> +	  tree inner = strip_array_types (reftype);
> +	  if (has_active_member && cxx_dialect < cxx20)
>   	    {
>   	      if (!ctx->quiet)
>   		error_at (cp_expr_loc_or_input_loc (t),

While we're looking at this area, this error message should really 
mention that it's allowed in C++20.

> @@ -6205,8 +6216,36 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
>   			  index);
>   	      *non_constant_p = true;
>   	    }
> -	  else if (TREE_CODE (t) == MODIFY_EXPR
> -		   && CONSTRUCTOR_NO_CLEARING (*valp))
> +	  else if (!is_access_expr
> +		   || (CLASS_TYPE_P (inner)
> +		       && !type_has_non_deleted_trivial_default_ctor (inner)))
> +	    {
> +	      /* Diagnose changing active union member after initialisation
> +		 without a valid member access expression, as described in
> +		 [class.union.general] p5.  */
> +	      if (!ctx->quiet)
> +		{
> +		  if (has_active_member)
> +		    error_at (cp_expr_loc_or_input_loc (t),
> +			      "accessing %qD member instead of initialized "
> +			      "%qD member in constant expression",
> +			      index, CONSTRUCTOR_ELT (*valp, 0)->index);
> +		  else
> +		    error_at (cp_expr_loc_or_input_loc (t),
> +			      "accessing uninitialized member %qD",
> +			      index);
> +		  if (is_access_expr)
> +		    {
> +		      inform (DECL_SOURCE_LOCATION (index),
> +			      "%qD does not implicitly begin its lifetime "
> +			      "because %qT does not have a non-deleted "
> +			      "trivial default constructor",
> +			      index, inner);
> +		    }

The !is_access_expr case could also use an explanatory message.

Also, I notice that this testcase crashes with the patch:

union U { int i; float f; };
constexpr auto g (U u) { return (u.i = 42); }
static_assert (g({.f = 3.14}) == 42);

Jason