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

[Jason Merrill <jason@redhat.com>]

On 9/19/23 20:55, Nathaniel Shead wrote:
> On Tue, Sep 19, 2023 at 05:25:20PM -0400, Jason Merrill wrote:
>> 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.
> 
> Thanks for the review, I've updated these messages and will send through
> an updated patch once bootstrap/regtest is complete.
> 
>> 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);
> 
> This appears to segfault even without the patch since GCC 13.1.
> https://godbolt.org/z/45sPh8WaK
> 
> I haven't done a bisect yet to work out what commit exactly caused this.
> Should I aim to fix this first before coming back with this patch?

Ah, I was just assuming it was related, never mind.  I'll fix it.

Jason