Re: [PATCH] c++, v3: Implement C++23 P2647R1 - Permitting static constexpr variables in constexpr functions

[Jason Merrill <jason@redhat.com>]

On 11/17/22 04:13, Jakub Jelinek wrote:
> On Wed, Nov 16, 2022 at 03:26:32PM -0500, Jason Merrill wrote:
>> On 11/16/22 09:46, Jakub Jelinek wrote:
>>> On Wed, Nov 16, 2022 at 09:33:27AM -0500, Jason Merrill wrote:
>>>>> and at that point I fear decl_maybe_constant_var_p will not work
>>>>> properly.  Shall this hunk be moved somewhere else (cp_finish_decl?)
>>>>> where we can already call it, or do the above in start_decl for
>>>>> cxx_dialect < cxx20 and add a cxx_dialect == cxx20 hunk in cp_finish_decl?
>>>>
>>>> Hmm, I'd expect decl_maybe_constant_var_p to work fine at this point.
>>>
>>> For static constexpr vars sure, but what about
>>> static const where start_decl doesn't know the initializer?
>>> Sure, decl_maybe_constant_var_p will not crash in that case, but
>>> it will return true even if the static const var doesn't have
>>> a constant initializer.  Sure, we'd catch that later on when actually
>>> trying to constexpr evaluate the function and hitting there the
>>> spots added for C++23 in potential_constant_expression*/cxx_eval_*,
>>> but it would mean that we don't reject it when nothing calls the functions.
>>>
>>> I meant something like:
>>> constexpr int bar (int x) { if (x) throw 1; return 0; }
>>> constexpr int foo () { static const int a = bar (1); return 0; }
>>> with -std=c++20 IMHO shouldn't be accepted, while in C++23 it should.
>>
>> I'd expect us to reject that in C++20 in potential_constant_expression, but
>> it's a fair point; it is awkward that P2242 wasn't also accepted as a DR.

I pointed out that inconsistency on the reflectors, and it seems that 
EWG didn't actually intend P2647 to be a DR, either; it was a CWG error 
to propose it as such.  But let's go ahead and build on the work you've 
done.

>> Moving the check from start_decl to cp_finish_decl makes sense to me.
> 
> So like this?  I had to outline the check from start_decl to a function
> because it is needed in cp_finish_decl in two different places (the
> processing_template_decl path and at the end and it can't be done before the
> processing_template_decl spot, because the initializer isn't finalized at
> that point for !procesing_template_decl).  Also, decl_maybe_constant_var_p
> doesn't do what is needed when !processing_template_decl, but I think
> we want decl_maybe_constant_var_p in templates so that we don't instantiate
> anything.
> 
> Lightly tested so far, ok for trunk if it passes full bootstrap/regtest?
> 
> 2022-11-17  Jakub Jelinek  <jakub@redhat.com>
> 
> gcc/c-family/
> 	* c-cppbuiltin.cc (c_cpp_builtins): Bump __cpp_constexpr
> 	value from 202207L to 202211L.
> gcc/cp/
> 	* constexpr.cc (cxx_eval_constant_expression): Implement C++23
> 	P2647R1 - Permitting static constexpr variables in constexpr functions.
> 	Allow decl_constant_var_p static or thread_local vars for
> 	C++20 and later.
> 	(potential_constant_expression_1): For C++20 or later, allow
> 	static or thread_local decl_maybe_constant_var_p vars, for
> 	!processing_template_decl only decl_constant_var_p vars.
> 	* decl.cc (diagnose_static_in_constexpr): New function.
> 	(start_decl): Use it for C++17 or earlier.
> 	(cp_finish_decl): Call it for C++20.
> gcc/testsuite/
> 	* g++.dg/cpp23/constexpr-nonlit17.C: New test.
> 	* g++.dg/cpp23/constexpr-nonlit18.C: New test.
> 	* g++.dg/cpp23/feat-cxx2b.C: Adjust expected __cpp_constexpr
> 	value.
> 	* g++.dg/ext/stmtexpr19.C: Don't expect an error for C++20 or later.
> 
> --- gcc/c-family/c-cppbuiltin.cc.jj	2022-11-17 09:00:42.106249011 +0100
> +++ gcc/c-family/c-cppbuiltin.cc	2022-11-17 09:01:49.286320527 +0100
> @@ -1074,7 +1074,7 @@ c_cpp_builtins (cpp_reader *pfile)
>   	  /* Set feature test macros for C++23.  */
>   	  cpp_define (pfile, "__cpp_size_t_suffix=202011L");
>   	  cpp_define (pfile, "__cpp_if_consteval=202106L");
> -	  cpp_define (pfile, "__cpp_constexpr=202207L");
> +	  cpp_define (pfile, "__cpp_constexpr=202211L");
>   	  cpp_define (pfile, "__cpp_multidimensional_subscript=202211L");
>   	  cpp_define (pfile, "__cpp_named_character_escapes=202207L");
>   	  cpp_define (pfile, "__cpp_static_call_operator=202207L");
> --- gcc/cp/constexpr.cc.jj	2022-11-17 08:48:30.530357181 +0100
> +++ gcc/cp/constexpr.cc	2022-11-17 09:56:50.479522863 +0100
> @@ -7098,7 +7098,8 @@ cxx_eval_constant_expression (const cons
>   	    && (TREE_STATIC (r)
>   		|| (CP_DECL_THREAD_LOCAL_P (r) && !DECL_REALLY_EXTERN (r)))
>   	    /* Allow __FUNCTION__ etc.  */
> -	    && !DECL_ARTIFICIAL (r))
> +	    && !DECL_ARTIFICIAL (r)
> +	    && (cxx_dialect < cxx20 || !decl_constant_var_p (r)))

I don't think we need to check cxx_dialect here since 
diagnose_static_in_constexpr will have already complained.

>   	  {
>   	    if (!ctx->quiet)
>   	      {
> @@ -9588,7 +9589,12 @@ potential_constant_expression_1 (tree t,
>         tmp = DECL_EXPR_DECL (t);
>         if (VAR_P (tmp) && !DECL_ARTIFICIAL (tmp))
>   	{
> -	  if (CP_DECL_THREAD_LOCAL_P (tmp) && !DECL_REALLY_EXTERN (tmp))
> +	  if (CP_DECL_THREAD_LOCAL_P (tmp)
> +	      && !DECL_REALLY_EXTERN (tmp)
> +	      && (cxx_dialect < cxx20
> +		  || (processing_template_decl
> +		      ? !decl_maybe_constant_var_p (tmp)
> +		      : !decl_constant_var_p (tmp))))

Or here.

>   	    {
>   	      if (flags & tf_error)
>   		constexpr_error (DECL_SOURCE_LOCATION (tmp), fundef_p,
> @@ -9596,7 +9602,11 @@ potential_constant_expression_1 (tree t,
>   				 "%<constexpr%> context", tmp);
>   	      return false;
>   	    }
> -	  else if (TREE_STATIC (tmp))
> +	  else if (TREE_STATIC (tmp)
> +		   && (cxx_dialect < cxx20
> +		       || (processing_template_decl
> +			   ? !decl_maybe_constant_var_p (tmp)
> +			   : !decl_constant_var_p (tmp))))
>   	    {
>   	      if (flags & tf_error)
>   		constexpr_error (DECL_SOURCE_LOCATION (tmp), fundef_p,
> --- gcc/cp/decl.cc.jj	2022-11-16 14:44:43.692339668 +0100
> +++ gcc/cp/decl.cc	2022-11-17 09:54:30.556482833 +0100
> @@ -5600,6 +5600,41 @@ groktypename (cp_decl_specifier_seq *typ
>     return type;
>   }
>   
> +/* For C++17 and older diagnose static or thread_local decls in constexpr
> +   or consteval functions.  For C++20 similarly, except if they are
> +   usable in constant expressions.  */
> +
> +static void
> +diagnose_static_in_constexpr (tree decl)
> +{
> +  if (current_function_decl && VAR_P (decl)
> +      && DECL_DECLARED_CONSTEXPR_P (current_function_decl)
> +      && cxx_dialect < cxx23
> +      && (cxx_dialect < cxx20
> +	  || (processing_template_decl
> +	      ? !decl_maybe_constant_var_p (decl)
> +	      : !decl_constant_var_p (decl))))

For (maybe) constant variables let's make this error a pedwarn in C++20 
and below.

> +    {
> +      bool ok = false;
> +      if (CP_DECL_THREAD_LOCAL_P (decl) && !DECL_REALLY_EXTERN (decl))
> +	error_at (DECL_SOURCE_LOCATION (decl),
> +		  "%qD defined %<thread_local%> in %qs function only "
> +		  "available with %<-std=c++2b%> or %<-std=gnu++2b%>", decl,
> +		  DECL_IMMEDIATE_FUNCTION_P (current_function_decl)
> +		  ? "consteval" : "constexpr");
> +      else if (TREE_STATIC (decl))
> +	error_at (DECL_SOURCE_LOCATION (decl),
> +		  "%qD defined %<static%> in %qs function only available "
> +		  "with %<-std=c++2b%> or %<-std=gnu++2b%>", decl,
> +		  DECL_IMMEDIATE_FUNCTION_P (current_function_decl)
> +		  ? "consteval" : "constexpr");
> +      else
> +	ok = true;
> +      if (!ok)
> +	cp_function_chain->invalid_constexpr = true;
> +    }
> +}
> +
>   /* Process a DECLARATOR for a function-scope or namespace-scope
>      variable or function declaration.
>      (Function definitions go through start_function; class member
> @@ -5860,28 +5895,8 @@ start_decl (const cp_declarator *declara
>         DECL_THIS_STATIC (decl) = 1;
>       }
>   
> -  if (current_function_decl && VAR_P (decl)
> -      && DECL_DECLARED_CONSTEXPR_P (current_function_decl)
> -      && cxx_dialect < cxx23)
> -    {
> -      bool ok = false;
> -      if (CP_DECL_THREAD_LOCAL_P (decl) && !DECL_REALLY_EXTERN (decl))
> -	error_at (DECL_SOURCE_LOCATION (decl),
> -		  "%qD defined %<thread_local%> in %qs function only "
> -		  "available with %<-std=c++2b%> or %<-std=gnu++2b%>", decl,
> -		  DECL_IMMEDIATE_FUNCTION_P (current_function_decl)
> -		  ? "consteval" : "constexpr");
> -      else if (TREE_STATIC (decl))
> -	error_at (DECL_SOURCE_LOCATION (decl),
> -		  "%qD defined %<static%> in %qs function only available "
> -		  "with %<-std=c++2b%> or %<-std=gnu++2b%>", decl,
> -		  DECL_IMMEDIATE_FUNCTION_P (current_function_decl)
> -		  ? "consteval" : "constexpr");
> -      else
> -	ok = true;
> -      if (!ok)
> -	cp_function_chain->invalid_constexpr = true;
> -    }
> +  if (cxx_dialect < cxx20)
> +    diagnose_static_in_constexpr (decl);

Can we drop this call (and make the ones in cp_finish_decl unconditional)?

>   
>     if (!processing_template_decl && VAR_P (decl))
>       start_decl_1 (decl, initialized);
> @@ -8424,6 +8439,10 @@ cp_finish_decl (tree decl, tree init, bo
>   	  set_user_assembler_name (decl, asmspec);
>   	  DECL_HARD_REGISTER (decl) = 1;
>   	}
> +
> +      if (cxx_dialect == cxx20)
> +	diagnose_static_in_constexpr (decl);
> +
>         return;
>       }
>   
> @@ -8749,6 +8768,9 @@ cp_finish_decl (tree decl, tree init, bo
>         && !DECL_HARD_REGISTER (decl))
>       targetm.lower_local_decl_alignment (decl);
>   
> +  if (cxx_dialect == cxx20)
> +    diagnose_static_in_constexpr (decl);
> +
>     invoke_plugin_callbacks (PLUGIN_FINISH_DECL, decl);
>   }
>   
> --- gcc/testsuite/g++.dg/cpp23/constexpr-nonlit17.C.jj	2022-11-17 09:00:42.108248984 +0100
> +++ gcc/testsuite/g++.dg/cpp23/constexpr-nonlit17.C	2022-11-17 09:00:42.108248984 +0100
> @@ -0,0 +1,12 @@
> +// P2647R1 - Permitting static constexpr variables in constexpr functions
> +// { dg-do compile { target c++20 } }
> +
> +constexpr char
> +test ()
> +{
> +  static const int x = 5;
> +  static constexpr char c[] = "Hello World";
> +  return *(c + x);
> +}
> +
> +static_assert (test () == ' ');
> --- gcc/testsuite/g++.dg/cpp23/constexpr-nonlit18.C.jj	2022-11-17 09:29:45.776136195 +0100
> +++ gcc/testsuite/g++.dg/cpp23/constexpr-nonlit18.C	2022-11-17 10:04:32.894045579 +0100
> @@ -0,0 +1,49 @@
> +// P2647R1 - Permitting static constexpr variables in constexpr functions
> +// { dg-do compile { target c++14 } }
> +
> +constexpr int
> +f1 (int x)
> +{
> +  if (x)
> +    throw 1;
> +  return 0;
> +}
> +
> +constexpr int
> +f2 ()
> +{
> +  static const int a = f1 (1);		// { dg-error "'a' defined 'static' in 'constexpr' function only available with" "" { target c++20_down } }
> +  return 0;
> +}
> +
> +constexpr int
> +f3 ()
> +{
> +  static const int a = 5;		// { dg-error "'a' defined 'static' in 'constexpr' function only available with" "" { target c++17_down } }
> +  return 0;
> +}
> +
> +constexpr int
> +f4 ()					// { dg-message "declared here" "" { target c++20_down } }
> +{					// { dg-message "is not usable as a 'constexpr' function because:" "" { target c++23 } .-1 }
> +  static const int a = f1 (1);		// { dg-error "'a' defined 'static' in 'constexpr' function only available with" "" { target c++20_down } }
> +  return 0;				// { dg-error "'a' defined 'static' in 'constexpr' context" "" { target c++23 } .-1 }
> +}
> +
> +constexpr int a4 = f4 ();		// { dg-error "called in a constant expression" }
> +
> +constexpr int
> +f5 ()
> +{
> +  static const int a = f1 (0);		// { dg-error "'a' defined 'static' in 'constexpr' function only available with" "" { target c++17_down } }
> +  return 0;
> +}
> +
> +constexpr int
> +f6 ()					// { dg-message "declared here" "" { target c++17_down } }
> +{
> +  static const int a = f1 (0);		// { dg-error "'a' defined 'static' in 'constexpr' function only available with" "" { target c++17_down } }
> +  return 0;
> +}
> +
> +constexpr int a6 = f6 ();		// { dg-error "called in a constant expression" "" { target c++17_down } }
> --- gcc/testsuite/g++.dg/cpp23/feat-cxx2b.C.jj	2022-11-17 08:48:30.561356753 +0100
> +++ gcc/testsuite/g++.dg/cpp23/feat-cxx2b.C	2022-11-17 09:00:42.108248984 +0100
> @@ -134,8 +134,8 @@
>   
>   #ifndef __cpp_constexpr
>   #  error "__cpp_constexpr"
> -#elif __cpp_constexpr != 202207
> -#  error "__cpp_constexpr != 202207"
> +#elif __cpp_constexpr != 202211
> +#  error "__cpp_constexpr != 202211"
>   #endif
>   
>   #ifndef __cpp_decltype_auto
> --- gcc/testsuite/g++.dg/ext/stmtexpr19.C.jj	2022-11-17 08:48:02.730741221 +0100
> +++ gcc/testsuite/g++.dg/ext/stmtexpr19.C	2022-11-17 09:00:42.109248970 +0100
> @@ -8,7 +8,7 @@ const test* setup()
>   {
>     static constexpr test atest =
>       {
> -      ({ static const int inner = 123; &inner; }) // { dg-error "static" }
> +      ({ static const int inner = 123; &inner; }) // { dg-error "static" "" { target c++17_down } }
>       };
>   
>     return &atest;
> 
> 
> 	Jakub
>