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

[Jakub Jelinek <jakub@redhat.com>]

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.
> 
> 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)))
 	  {
 	    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))))
 	    {
 	      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))))
+    {
+      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);
 
   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