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

The following patch implements this paper.
Per further discussions it is implemented for C++23 only, so isn't
treated as a DR, e.g. because the part of the standard the paper is
changing didn't even exist in C++20.
And we gave up on trying to implement it as a pedwarn rather than
error for C++20 and older, because of implicit constexpr lambdas or
-fimplicit-constexpr reasons.
For C++20 and older, the only change is that passing through
definitions of static or thread_local vars usable in constant expressions
is now accepted in statement expressions if they aren't inside of constexpr
or consteval functions.

2022-11-19  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_EXPRs of decl_constant_var_p static or thread_local vars.
	(potential_constant_expression_1): Similarly, except use
	decl_maybe_constant_var_p instead of decl_constant_var_p if
	processing_template_decl.
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.
	* g++.dg/ext/stmtexpr25.C: New test.

7 files changed, 88 insertions, 6 deletions

diff --git a/gcc/c-family/c-cppbuiltin.cc b/gcc/c-family/c-cppbuiltin.cc
index fb11d87..333f3e1 100644
--- a/gcc/c-family/c-cppbuiltin.cc
+++ b/gcc/c-family/c-cppbuiltin.cc
@@ -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");
diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index ad43897..0b43ae4 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -7124,7 +7124,8 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 	    && (TREE_STATIC (r)
 		|| (CP_DECL_THREAD_LOCAL_P (r) && !DECL_REALLY_EXTERN (r)))
 	    /* Allow __FUNCTION__ etc.  */
-	    && !DECL_ARTIFICIAL (r))
+	    && !DECL_ARTIFICIAL (r)
+	    && !decl_constant_var_p (r))
 	  {
 	    if (!ctx->quiet)
 	      {
@@ -9630,7 +9631,10 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
 
     case DECL_EXPR:
       tmp = DECL_EXPR_DECL (t);
-      if (VAR_P (tmp) && !DECL_ARTIFICIAL (tmp))
+      if (VAR_P (tmp) && !DECL_ARTIFICIAL (tmp)
+	  && (processing_template_decl
+	      ? !decl_maybe_constant_var_p (tmp)
+	      : !decl_constant_var_p (tmp)))
 	{
 	  if (CP_DECL_THREAD_LOCAL_P (tmp) && !DECL_REALLY_EXTERN (tmp))
 	    {
diff --git a/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit17.C b/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit17.C
new file mode 100644
index 0000000..a4c78bb
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit17.C
@@ -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 () == ' ');
diff --git a/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit18.C b/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit18.C
new file mode 100644
index 0000000..8e230ef
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit18.C
@@ -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 ();		// { dg-error "called in a constant expression" "" { target c++20_down } }
diff --git a/gcc/testsuite/g++.dg/cpp23/feat-cxx2b.C b/gcc/testsuite/g++.dg/cpp23/feat-cxx2b.C
index 3ce36e4..6f4f6bc 100644
--- a/gcc/testsuite/g++.dg/cpp23/feat-cxx2b.C
+++ b/gcc/testsuite/g++.dg/cpp23/feat-cxx2b.C
@@ -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
diff --git a/gcc/testsuite/g++.dg/ext/stmtexpr19.C b/gcc/testsuite/g++.dg/ext/stmtexpr19.C
index 0c19a21..be395b6 100644
--- a/gcc/testsuite/g++.dg/ext/stmtexpr19.C
+++ b/gcc/testsuite/g++.dg/ext/stmtexpr19.C
@@ -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; })
     };
 
   return &atest;
diff --git a/gcc/testsuite/g++.dg/ext/stmtexpr25.C b/gcc/testsuite/g++.dg/ext/stmtexpr25.C
new file mode 100644
index 0000000..79c40fc
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/stmtexpr25.C
@@ -0,0 +1,17 @@
+// PR c++/81073
+// { dg-options "" }
+// { dg-do compile { target c++11 } }
+
+struct test { const int *addr; };
+
+const test* setup()
+{
+  static constexpr test atest =
+    {
+      ({ static const int inner = (throw 1, 1); &inner; }) // { dg-error "static" "" }
+    };
+
+  return &atest;
+}
+
+int main(){}