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

[Jakub Jelinek <jakub@redhat.com>]

On Thu, Nov 17, 2022 at 09:42:18AM -0500, Jason Merrill wrote:
> > --- 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.

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.

> 
> >   	  {
> >   	    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,

And these too.

> > +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.

Ok.
> 
> > +    {
> > +      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)?

Yes, will do it.

	Jakub