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

  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. 



	Jakub
  

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
  

On 11/17/22 15:42, 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.

We could, but I wouldn't bother to enforce this specially for 
statement-expressions, which are already an extension.

OTOH, we should test that static constexpr is handled properly for 
lambdas, i.e. this should still fail:

constexpr int q = [](int i)
{ static constexpr int x = 42; return x+i; }(24);

> 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;
> +	      }
> +	    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
> +   usable in constant expressions.  */
> +
> +static void
> +diagnose_static_in_constexpr (tree decl)
> +{
> +  if (cxx_dialect >= cxx23)
> +    return;
> +  if (current_function_decl
> +      && VAR_P (decl)
> +      && DECL_DECLARED_CONSTEXPR_P (current_function_decl))
> +    {
> +      bool ok = false;
> +      if (processing_template_decl
> +	  ? decl_maybe_constant_var_p (decl)
> +	  : decl_constant_var_p (decl))
> +	{
> +	  if (CP_DECL_THREAD_LOCAL_P (decl) && !DECL_REALLY_EXTERN (decl))
> +	    pedwarn (DECL_SOURCE_LOCATION (decl), OPT_Wpedantic,
> +		     "%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))
> +	    pedwarn (DECL_SOURCE_LOCATION (decl), OPT_Wpedantic,
> +		     "%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");
> +	  ok = true;
> +	}
> +      else 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
> @@ -5860,29 +5911,6 @@ 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 (!processing_template_decl && VAR_P (decl))
>       start_decl_1 (decl, initialized);
>   
> @@ -8424,6 +8452,9 @@ cp_finish_decl (tree decl, tree init, bo
>   	  set_user_assembler_name (decl, asmspec);
>   	  DECL_HARD_REGISTER (decl) = 1;
>   	}
> +
> +      diagnose_static_in_constexpr (decl);
> +
>         return;
>       }
>   
> @@ -8749,6 +8780,8 @@ cp_finish_decl (tree decl, tree init, bo
>         && !DECL_HARD_REGISTER (decl))
>       targetm.lower_local_decl_alignment (decl);
>   
> +  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 20:55:33.550498209 +0100
> @@ -0,0 +1,12 @@
> +// P2647R1 - Permitting static constexpr variables in constexpr functions
> +// { dg-do compile { target c++23 } }
> +
> +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 21:02:20.852865509 +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++20_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++20_down } }
> +  return 0;
> +}
> +
> +constexpr int
> +f6 ()
> +{
> +  static const int a = f1 (0);		// { dg-error "'a' defined 'static' in 'constexpr' function only available with" "" { target c++20_down } }
> +  return 0;
> +}
> +
> +constexpr int a6 = f6 ();
> --- gcc/testsuite/g++.dg/cpp23/constexpr-nonlit19.C.jj	2022-11-17 20:56:07.887023431 +0100
> +++ gcc/testsuite/g++.dg/cpp23/constexpr-nonlit19.C	2022-11-17 21:04:12.618319027 +0100
> @@ -0,0 +1,50 @@
> +// P2647R1 - Permitting static constexpr variables in constexpr functions
> +// { dg-do compile { target c++14 } }
> +// { dg-options "-pedantic" }
> +
> +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-warning "'a' defined 'static' in 'constexpr' function only available with" "" { target c++20_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-warning "'a' defined 'static' in 'constexpr' function only available with" "" { target c++20_down } }
> +  return 0;
> +}
> +
> +constexpr int
> +f6 ()
> +{
> +  static const int a = f1 (0);		// { dg-warning "'a' defined 'static' in 'constexpr' function only available with" "" { target c++20_down } }
> +  return 0;
> +}
> +
> +constexpr int a6 = f6 ();
> --- gcc/testsuite/g++.dg/cpp23/constexpr-nonlit20.C.jj	2022-11-17 20:56:28.527738024 +0100
> +++ gcc/testsuite/g++.dg/cpp23/constexpr-nonlit20.C	2022-11-17 21:31:25.209729268 +0100
> @@ -0,0 +1,50 @@
> +// P2647R1 - Permitting static constexpr variables in constexpr functions
> +// { dg-do compile { target c++14 } }
> +// { dg-options "-Wno-pedantic" }
> +
> +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;
> +  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);
> +  return 0;
> +}
> +
> +constexpr int
> +f6 ()
> +{
> +  static const int a = f1 (0);
> +  return 0;
> +}
> +
> +constexpr int a6 = f6 ();
> --- 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 20:57:06.936206927 +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++20_down } }
>       };
>   
>     return &atest;
> 
> 
> 	Jakub
>
  

On Thu, Nov 17, 2022 at 07:15:05PM -0500, Marek Polacek wrote:
> > --- 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 "?

I just forgot to update the function comment.

Anyway, I think:

> 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.

this is actually a real bug and I'm not sure how to resolve that.

We have there:

int main()
{
  [](auto i) { if (i) { int j; static int k; return i + j; } return i; }(0);
}

and for C++17/20 I presume something (haven't figured out yet what) marks
the lambda operator() when still a template as constexpr and then
cp_finish_decl -> diagnose_static_in_constexpr pedwarns on it.
For the above perhaps we could figure out there is a static int k; in the
operator() and don't turn it into constexpr, but what if there is
something that would e.g. satisfy decl_maybe_constant_var_p but not
decl_constant_var_p when actually instantiated?
Without my patch, the diagnostics is in start_decl which isn't called again
during instantiation, so I presume we mark it as constexpr and then we'd
diagnose it during constant evaluation.

	Jakub
  

On Fri, Nov 18, 2022 at 08:48:32AM +0100, Jakub Jelinek wrote:
> On Thu, Nov 17, 2022 at 07:15:05PM -0500, Marek Polacek wrote:
> > > --- 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 "?
> 
> I just forgot to update the function comment.
> 
> Anyway, I think:
> 
> > 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.
> 
> this is actually a real bug and I'm not sure how to resolve that.
> 
> We have there:
> 
> int main()
> {
>   [](auto i) { if (i) { int j; static int k; return i + j; } return i; }(0);
> }
> 
> and for C++17/20 I presume something (haven't figured out yet what) marks

Right, that's the C++17 implicit constexpr for lambdas, finish_function:

  /* Lambda closure members are implicitly constexpr if possible.  */
  if (cxx_dialect >= cxx17
      && LAMBDA_TYPE_P (CP_DECL_CONTEXT (fndecl)))
    DECL_DECLARED_CONSTEXPR_P (fndecl)
      = ((processing_template_decl
          || is_valid_constexpr_fn (fndecl, /*complain*/false))
         && potential_constant_expression (DECL_SAVED_TREE (fndecl)));

> the lambda operator() when still a template as constexpr and then
> cp_finish_decl -> diagnose_static_in_constexpr pedwarns on it.
> For the above perhaps we could figure out there is a static int k; in the
> operator() and don't turn it into constexpr, but what if there is
> something that would e.g. satisfy decl_maybe_constant_var_p but not
> decl_constant_var_p when actually instantiated?
> Without my patch, the diagnostics is in start_decl which isn't called again
> during instantiation, so I presume we mark it as constexpr and then we'd
> diagnose it during constant evaluation.

Um, can we give up on trying to handle C++17/C++20 then?

Marek
  

On Fri, Nov 18, 2022 at 10:03:18AM -0500, Marek Polacek wrote:
> > the lambda operator() when still a template as constexpr and then
> > cp_finish_decl -> diagnose_static_in_constexpr pedwarns on it.
> > For the above perhaps we could figure out there is a static int k; in the
> > operator() and don't turn it into constexpr, but what if there is
> > something that would e.g. satisfy decl_maybe_constant_var_p but not
> > decl_constant_var_p when actually instantiated?
> > Without my patch, the diagnostics is in start_decl which isn't called again
> > during instantiation, so I presume we mark it as constexpr and then we'd
> > diagnose it during constant evaluation.
> 
> Um, can we give up on trying to handle C++17/C++20 then?

That was why I've posted the other two variant patches (with the 3rd one
being a strict C++23 only change).  Even if it is just a temporary state,
make C++23 work first and then iterate if it is possible to make C++17/20
working with the pedwarns right.

	Jakub
  

On 11/18/22 10:03, Marek Polacek wrote:
> On Fri, Nov 18, 2022 at 08:48:32AM +0100, Jakub Jelinek wrote:
>> On Thu, Nov 17, 2022 at 07:15:05PM -0500, Marek Polacek wrote:
>>>> --- 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 "?
>>
>> I just forgot to update the function comment.
>>
>> Anyway, I think:
>>
>>> 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.
>>
>> this is actually a real bug and I'm not sure how to resolve that.
>>
>> We have there:
>>
>> int main()
>> {
>>    [](auto i) { if (i) { int j; static int k; return i + j; } return i; }(0);
>> }
>>
>> and for C++17/20 I presume something (haven't figured out yet what) marks
> 
> Right, that's the C++17 implicit constexpr for lambdas, finish_function:
> 
>    /* Lambda closure members are implicitly constexpr if possible.  */
>    if (cxx_dialect >= cxx17
>        && LAMBDA_TYPE_P (CP_DECL_CONTEXT (fndecl)))
>      DECL_DECLARED_CONSTEXPR_P (fndecl)
>        = ((processing_template_decl
>            || is_valid_constexpr_fn (fndecl, /*complain*/false))
>           && potential_constant_expression (DECL_SAVED_TREE (fndecl)));

Yeah, I guess potential_constant_expression needs to be stricter in a 
lambda. Or perhaps any function that isn't already 
DECL_DECLARED_CONSTEXPR_P?

>> the lambda operator() when still a template as constexpr and then
>> cp_finish_decl -> diagnose_static_in_constexpr pedwarns on it.
>> For the above perhaps we could figure out there is a static int k; in the
>> operator() and don't turn it into constexpr, but what if there is
>> something that would e.g. satisfy decl_maybe_constant_var_p but not
>> decl_constant_var_p when actually instantiated?

I'd expect the above change to potential_c_e to handle that case.

Jason
  

On Fri, Nov 18, 2022 at 11:24:45AM -0500, Jason Merrill wrote:
> > Right, that's the C++17 implicit constexpr for lambdas, finish_function:
> > 
> >    /* Lambda closure members are implicitly constexpr if possible.  */
> >    if (cxx_dialect >= cxx17
> >        && LAMBDA_TYPE_P (CP_DECL_CONTEXT (fndecl)))
> >      DECL_DECLARED_CONSTEXPR_P (fndecl)
> >        = ((processing_template_decl
> >            || is_valid_constexpr_fn (fndecl, /*complain*/false))
> >           && potential_constant_expression (DECL_SAVED_TREE (fndecl)));
> 
> Yeah, I guess potential_constant_expression needs to be stricter in a
> lambda. Or perhaps any function that isn't already
> DECL_DECLARED_CONSTEXPR_P?

potential_constant_expression can't be relied on that it catches up
everything if it, even a simple if statement with a condition not yet
known to be 0 or non-0 results in just a requirement that at least
one of the substatements is potential constant, etc.
Similarly switch statements etc.
If there is a way to distinguish between functions with user
specified constexpr/consteval and DECL_DECLARED_CONSTEXPR_P set
through the above if condition, sure, cp_finish_decl ->
check_static_in_constexpr could be perhaps silent about those, but then
we want to diagnose it during constexpr evaluation at least.  But in that
case having it a pedwarn rather than "this is a constant expression"
vs. "this is not a constant expression, if !ctx->quiet emit an error"
is something I don't see how to handle.  Because something needs
to be returned, it is a constant expression or it is not.

	Jakub
  

On 11/18/22 11:34, Jakub Jelinek wrote:
> On Fri, Nov 18, 2022 at 11:24:45AM -0500, Jason Merrill wrote:
>>> Right, that's the C++17 implicit constexpr for lambdas, finish_function:
>>>
>>>     /* Lambda closure members are implicitly constexpr if possible.  */
>>>     if (cxx_dialect >= cxx17
>>>         && LAMBDA_TYPE_P (CP_DECL_CONTEXT (fndecl)))
>>>       DECL_DECLARED_CONSTEXPR_P (fndecl)
>>>         = ((processing_template_decl
>>>             || is_valid_constexpr_fn (fndecl, /*complain*/false))
>>>            && potential_constant_expression (DECL_SAVED_TREE (fndecl)));
>>
>> Yeah, I guess potential_constant_expression needs to be stricter in a
>> lambda. Or perhaps any function that isn't already
>> DECL_DECLARED_CONSTEXPR_P?
> 
> potential_constant_expression can't be relied on that it catches up
> everything if it, even a simple if statement with a condition not yet
> known to be 0 or non-0 results in just a requirement that at least
> one of the substatements is potential constant, etc.
> Similarly switch statements etc.
> If there is a way to distinguish between functions with user
> specified constexpr/consteval and DECL_DECLARED_CONSTEXPR_P set
> through the above if condition, sure, cp_finish_decl ->
> check_static_in_constexpr could be perhaps silent about those, but then
> we want to diagnose it during constexpr evaluation at least.  But in that
> case having it a pedwarn rather than "this is a constant expression"
> vs. "this is not a constant expression, if !ctx->quiet emit an error"
> is something I don't see how to handle.  Because something needs
> to be returned, it is a constant expression or it is not.

True.  Let's go with your option 2, then, thanks.

Jason