Re: [PATCH] c++, v4: Implement C++23 P2647R1 - Permitting static constexpr variables in constexpr functions

[Marek Polacek <polacek@redhat.com>]

On Thu, Nov 17, 2022 at 09:42:17PM +0100, Jakub Jelinek wrote:
> On Thu, Nov 17, 2022 at 07:42:40PM +0100, Jakub Jelinek via Gcc-patches wrote:
> > I thought for older C++ this is to catch
> > void
> > foo ()
> > {
> >   constexpr int a = ({ static constexpr int b = 2; b; });
> > }
> > and for C++23 the only 3 spots that diagnose those.
> > But perhaps for C++20 or older we can check if the var has a context
> > of a constexpr function (then assume cp_finish_decl errored or pedwarned
> > already) and only error or pedwarn otherwise.
> 
> So, here is an updated patch, which in constexpr.cc will accept
> DECL_EXPR of decl_*constant_var_p static/thread_local non-extern vars
> for C++23 or if they are not declared in constexpr/consteval function.
> So, the statement expression case will remain hard error for C++ <= 20 rather than
> pedwarn, because due to the ctx->quiet vs. !ctx->quiet case I don't see
> what else we could do, either something is a constant expression, or
> it is not, but whether it is or is not shouldn't depend on
> -Wpedantic/-Wno-pedantic/-Werror=pedantic.
> 
> 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++23 and later or if they are declared inside of constexpr or
> 	consteval function.
> 	(potential_constant_expression_1): Similarly, except use
> 	decl_maybe_constant_var_p instead of decl_constant_var_p if
> 	processing_template_decl.
> 	* decl.cc (diagnose_static_in_constexpr): New function.
> 	(start_decl): Remove diagnostics of static or thread_local
> 	vars in constexpr or consteval functions.
> 	(cp_finish_decl): Call diagnose_static_in_constexpr.
> gcc/testsuite/
> 	* g++.dg/cpp23/constexpr-nonlit17.C: New test.
> 	* g++.dg/cpp23/constexpr-nonlit18.C: New test.
> 	* g++.dg/cpp23/constexpr-nonlit19.C: New test.
> 	* g++.dg/cpp23/constexpr-nonlit20.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 20:53:15.432408015 +0100
> @@ -7100,17 +7100,35 @@ cxx_eval_constant_expression (const cons
>  	    /* Allow __FUNCTION__ etc.  */
>  	    && !DECL_ARTIFICIAL (r))
>  	  {
> -	    if (!ctx->quiet)
> +	    bool ok = decl_constant_var_p (r);
> +	    /* Since P2647R1 control can pass through definitions of static
> +	       or thread_local vars usable in constant expressions.
> +	       In C++20 or older, if such vars are declared inside of
> +	       constexpr or consteval function, diagnose_static_in_constexpr
> +	       should have already pedwarned on those.  Otherwise they could
> +	       be e.g. in a statement expression, reject those before
> +	       C++23.  */
> +	    if (ok && cxx_dialect < cxx23)
>  	      {
> -		if (CP_DECL_THREAD_LOCAL_P (r))
> -		  error_at (loc, "control passes through definition of %qD "
> -				 "with thread storage duration", r);
> -		else
> -		  error_at (loc, "control passes through definition of %qD "
> -				 "with static storage duration", r);
> +		tree fnctx = decl_function_context (r);
> +		if (fnctx == NULL_TREE
> +		    || !DECL_DECLARED_CONSTEXPR_P (fnctx))
> +		  ok = false;

FWIW, I couldn't find a way to trigger this code.

> +	      }
> +	    if (!ok)
> +	      {
> +		if (!ctx->quiet)
> +		  {
> +		    if (CP_DECL_THREAD_LOCAL_P (r))
> +		      error_at (loc, "control passes through definition of "
> +				     "%qD with thread storage duration", r);
> +		    else
> +		      error_at (loc, "control passes through definition of "
> +				     "%qD with static storage duration", r);
> +		  }
> +		*non_constant_p = true;
> +		break;
>  	      }
> -	    *non_constant_p = true;
> -	    break;
>  	  }
>  
>  	if (AGGREGATE_TYPE_P (TREE_TYPE (r))
> @@ -9588,21 +9606,41 @@ 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 (flags & tf_error)
> -		constexpr_error (DECL_SOURCE_LOCATION (tmp), fundef_p,
> -				 "%qD defined %<thread_local%> in "
> -				 "%<constexpr%> context", tmp);
> -	      return false;
> -	    }
> -	  else if (TREE_STATIC (tmp))
> +	  if (TREE_STATIC (tmp)
> +	      || (CP_DECL_THREAD_LOCAL_P (tmp) && !DECL_REALLY_EXTERN (tmp)))
>  	    {
> -	      if (flags & tf_error)
> -		constexpr_error (DECL_SOURCE_LOCATION (tmp), fundef_p,
> -				 "%qD defined %<static%> in %<constexpr%> "
> -				 "context", tmp);
> -	      return false;
> +	      bool ok = (processing_template_decl
> +			 ? decl_maybe_constant_var_p (tmp)
> +			 : decl_constant_var_p (tmp));
> +	      /* Since P2647R1 control can pass through definitions of static
> +		 or thread_local vars usable in constant expressions.
> +		 In C++20 or older, if such vars are declared inside of
> +		 constexpr or consteval function, diagnose_static_in_constexpr
> +		 should have already pedwarned on those.  Otherwise they could
> +		 be e.g. in a statement expression, reject those before
> +		 C++23.  */
> +	      if (ok && cxx_dialect < cxx23)
> +		{
> +		  tree fnctx = decl_function_context (tmp);
> +		  if (fnctx == NULL_TREE
> +		      || !DECL_DECLARED_CONSTEXPR_P (fnctx))
> +		    ok = false;
> +		}
> +	      if (!ok)
> +		{
> +		  if (flags & tf_error)
> +		    {
> +		      if (CP_DECL_THREAD_LOCAL_P (tmp))
> +			constexpr_error (DECL_SOURCE_LOCATION (tmp), fundef_p,
> +					 "%qD defined %<thread_local%> in "
> +					 "%<constexpr%> context", tmp);
> +		      else
> +			constexpr_error (DECL_SOURCE_LOCATION (tmp), fundef_p,
> +					 "%qD defined %<static%> in "
> +					 "%<constexpr%> context", tmp);
> +		    }
> +		  return false;
> +		}
>  	    }
>  	  else if (!check_for_uninitialized_const_var
>  		   (tmp, /*constexpr_context_p=*/true, flags))
> --- gcc/cp/decl.cc.jj	2022-11-16 14:44:43.692339668 +0100
> +++ gcc/cp/decl.cc	2022-11-17 20:53:44.102011594 +0100
> @@ -5600,6 +5600,57 @@ 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

In C++17 we don't support consteval so I guess drop the "or consteval "?

BTW, I notice that the patch breaks
g++.dg/cpp1y/lambda-generic-func1.C
g++.dg/cpp1z/constexpr-lambda16.C
Maybe they just need dg- tweaks.

Marek