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

[Jason Merrill <jason@redhat.com>]

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
>