Re: [PATCH] c++: Fix up maybe_warn_for_constant_evaluated calls [PR114580]

[Jason Merrill <jason@redhat.com>]

On 4/5/24 14:47, Marek Polacek wrote:
> On Fri, Apr 05, 2024 at 09:40:48AM +0200, Jakub Jelinek wrote:
>> Hi!
>>
>> When looking at maybe_warn_for_constant_evaluated for the trivial
>> infinite loops patch, I've noticed that it can emit weird diagnostics
>> for if constexpr in templates, first warn that std::is_constant_evaluted()
>> always evaluates to false (because the function template is not constexpr)
>> and then during instantiation warn that std::is_constant_evaluted()
>> always evaluates to true (because it is used in if constexpr condition).
>> Now, only the latter is actually true, even when the if constexpr
>> is in a non-constexpr function, it will still always evaluate to true.
>>
>> So, the following patch fixes it to call maybe_warn_for_constant_evaluated
>> always with IF_STMT_CONSTEXPR_P (if_stmt) as the second argument rather than
>> true if it is if constexpr with non-dependent condition etc.
>>
>> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
>>
>> 2024-04-05  Jakub Jelinek  <jakub@redhat.com>
>>
>> 	PR c++/114580
>> 	* semantics.cc (finish_if_stmt_cond): Call
>> 	maybe_warn_for_constant_evaluated with IF_STMT_CONSTEXPR_P (if_stmt)
>> 	as the second argument, rather than true/false depending on if
>> 	it is if constexpr with non-dependent constant expression with
>> 	bool type.
>>
>> 	* g++.dg/cpp2a/is-constant-evaluated15.C: New test.
>>
>> --- gcc/cp/semantics.cc.jj	2024-04-03 09:58:33.407772541 +0200
>> +++ gcc/cp/semantics.cc	2024-04-04 12:11:36.203886572 +0200
>> @@ -1126,6 +1126,9 @@ tree
>>   finish_if_stmt_cond (tree orig_cond, tree if_stmt)
>>   {
>>     tree cond = maybe_convert_cond (orig_cond);
>> +  maybe_warn_for_constant_evaluated (cond,
>> +				     /*constexpr_if=*/
>> +				     IF_STMT_CONSTEXPR_P (if_stmt));
> 
> I don't think we need the comment anymore since it's clear what the
> argument does, and then the whole call can fit on a single line.
> 
> But either way, the patch looks good, thanks.

Agreed, OK with that change.

Jason