c++: Implement C++26 P2573R2 - = delete("should have a reason"); [PR114458]

The following patch implements the C++26 P2573R2
= delete("should have a reason"); paper.
I've tried to avoid increasing compile time memory for it when it isn't
used (e.g. by adding a new lang_decl tree member), so the reason is
represented as STRING_CST in DECL_INITIAL (which normally is for
DECL_DELETED_FN error_mark_node) and to differentiate this delete("reason")
initializer from some bogus attempt to initialize a function with "reason"
using the RID_DELETE identifier as TREE_TYPE of the STRING_CST, as nothing
needs to care about the type of the reason.  If preferred it could
be say TREE_LIST with the reason STRING_CST and RID_DELETE identifier or
something similar instead, but that would need more compile time memory when
it is used.

2024-05-02  Jakub Jelinek  <jakub@redhat.com>

	PR c++/114458
gcc/c-family/
	* c-cppbuiltin.cc (c_cpp_builtins): Predefine
	__cpp_deleted_function=202403L for C++26.
gcc/cp/ChangeLog
	* parser.cc (cp_parser_pure_specifier): Implement C++26 P2573R2
	- = delete("should have a reason");.  Parse deleted-function-body.
	* decl.cc (duplicate_decls): Copy DECL_INITIAL from DECL_DELETED_FN
	olddecl to newdecl if it is a STRING_CST.
	(cp_finish_decl): Handle deleted init with a reason.
	* decl2.cc: Include "escaped_string.h".
	(grokfield): Handle deleted init with a reason.
	(mark_used): Emit DECL_DELETED_FN reason in the message if any.
	* cp-tree.h (DECL_DELETED_FN): Document representation of
	= delete("reason") on a DECL.
gcc/testsuite/
	* g++.dg/cpp26/feat-cxx26.C (__cpp_deleted_function): Add test.
	* g++.dg/cpp26/delete-reason1.C: New test.
	* g++.dg/cpp26/delete-reason2.C: New test.
	* g++.dg/parse/error65.C (f1): Adjust expected diagnostics.

4 files changed, 55 insertions, 7 deletions

diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 5d1bd6b..933504b 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -4477,7 +4477,10 @@ get_vec_init_expr (tree t)
        && DECL_DECLARED_CONSTEXPR_P (NODE)			\
        && DECL_CLASS_SCOPE_P (NODE)))
 
-/* Nonzero if DECL was declared with '= delete'.  */
+/* Nonzero if DECL was declared with '= delete'.
+   = delete("reason") is represented in addition to this flag by DECL_INITIAL
+   being STRING_CST with the reason and TREE_TYPE of the STRING_CST the
+   RID_DELETE IDENTIFIER_NODE.  */
 #define DECL_DELETED_FN(DECL) \
   (LANG_DECL_FN_CHECK (DECL)->min.base.threadprivate_or_deleted_p)
 
diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index de0c02a..378311c 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -2420,6 +2420,10 @@ duplicate_decls (tree newdecl, tree olddecl, bool hiding, bool was_hidden)
 			"previous declaration of %qD", olddecl);
 	    }
 	  DECL_DELETED_FN (newdecl) |= DECL_DELETED_FN (olddecl);
+	  if (DECL_DELETED_FN (olddecl)
+	      && DECL_INITIAL (olddecl)
+	      && TREE_CODE (DECL_INITIAL (olddecl)) == STRING_CST)
+	    DECL_INITIAL (newdecl) = DECL_INITIAL (olddecl);
 	}
     }
 
@@ -8597,17 +8601,20 @@ cp_finish_decl (tree decl, tree init, bool init_const_expr_p,
   if (init && TREE_CODE (decl) == FUNCTION_DECL)
     {
       tree clone;
-      if (init == ridpointers[(int)RID_DELETE])
+      if (init == ridpointers[(int)RID_DELETE]
+	  || (TREE_CODE (init) == STRING_CST
+	      && TREE_TYPE (init) == ridpointers[(int)RID_DELETE]))
 	{
 	  /* FIXME check this is 1st decl.  */
 	  DECL_DELETED_FN (decl) = 1;
 	  DECL_DECLARED_INLINE_P (decl) = 1;
-	  DECL_INITIAL (decl) = error_mark_node;
+	  DECL_INITIAL (decl)
+	    = TREE_CODE (init) == STRING_CST ? init : error_mark_node;
 	  FOR_EACH_CLONE (clone, decl)
 	    {
 	      DECL_DELETED_FN (clone) = 1;
 	      DECL_DECLARED_INLINE_P (clone) = 1;
-	      DECL_INITIAL (clone) = error_mark_node;
+	      DECL_INITIAL (clone) = DECL_INITIAL (decl);
 	    }
 	  init = NULL_TREE;
 	}
diff --git a/gcc/cp/decl2.cc b/gcc/cp/decl2.cc
index 1f84878..6913efa 100644
--- a/gcc/cp/decl2.cc
+++ b/gcc/cp/decl2.cc
@@ -50,6 +50,7 @@ along with GCC; see the file COPYING3.  If not see
 #include "asan.h"
 #include "optabs-query.h"
 #include "omp-general.h"
+#include "escaped_string.h"
 
 /* Id for dumping the raw trees.  */
 int raw_dump_id;
@@ -1038,7 +1039,10 @@ grokfield (const cp_declarator *declarator,
     init = NULL_TREE;
 
   int initialized;
-  if (init == ridpointers[(int)RID_DELETE])
+  if (init == ridpointers[(int)RID_DELETE]
+      || (init
+	  && TREE_CODE (init) == STRING_CST
+	  && TREE_TYPE (init) == ridpointers[(int)RID_DELETE]))
     initialized = SD_DELETED;
   else if (init == ridpointers[(int)RID_DEFAULT])
     initialized = SD_DEFAULTED;
@@ -1123,10 +1127,14 @@ grokfield (const cp_declarator *declarator,
     {
       if (TREE_CODE (value) == FUNCTION_DECL)
 	{
-	  if (init == ridpointers[(int)RID_DELETE])
+	  if (init == ridpointers[(int)RID_DELETE]
+	      || (TREE_CODE (init) == STRING_CST
+		  && TREE_TYPE (init) == ridpointers[(int)RID_DELETE]))
 	    {
 	      DECL_DELETED_FN (value) = 1;
 	      DECL_DECLARED_INLINE_P (value) = 1;
+	      if (TREE_CODE (init) == STRING_CST)
+		DECL_INITIAL (value) = init;
 	    }
 	  else if (init == ridpointers[(int)RID_DEFAULT])
 	    {
@@ -5912,7 +5920,16 @@ mark_used (tree decl, tsubst_flags_t complain /* = tf_warning_or_error */)
 	    sorry ("converting lambda that uses %<...%> to function pointer");
 	  else if (complain & tf_error)
 	    {
-	      error ("use of deleted function %qD", decl);
+	      if (DECL_INITIAL (decl)
+		  && TREE_CODE (DECL_INITIAL (decl)) == STRING_CST)
+		{
+		  escaped_string msg;
+		  msg.escape (TREE_STRING_POINTER (DECL_INITIAL (decl)));
+		  error ("use of deleted function %qD: %s",
+			 decl, (const char *) msg);
+		}
+	      else
+		error ("use of deleted function %qD", decl);
 	      if (!maybe_explain_implicit_delete (decl))
 		inform (DECL_SOURCE_LOCATION (decl), "declared here");
 	    }
diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index a2bc6f6..7c3cfcf 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -28634,6 +28634,27 @@ cp_parser_pure_specifier (cp_parser* parser)
       || token->keyword == RID_DELETE)
     {
       maybe_warn_cpp0x (CPP0X_DEFAULTED_DELETED);
+      if (cp_lexer_next_token_is (parser->lexer, CPP_OPEN_PAREN))
+	{
+	  if (cxx_dialect >= cxx11 && cxx_dialect < cxx26)
+	    pedwarn (cp_lexer_peek_token (parser->lexer)->location,
+		     OPT_Wc__26_extensions,
+		     "%<delete%> reason only available with "
+		     "%<-std=c++2c%> or %<-std=gnu++2c%>");
+
+	  /* Consume the `('.  */
+	  matching_parens parens;
+	  parens.consume_open (parser);
+	  tree reason = cp_parser_unevaluated_string_literal (parser);
+	  /* Consume the `)'.  */
+	  parens.require_close (parser);
+	  if (TREE_CODE (reason) == STRING_CST)
+	    {
+	      TREE_TYPE (reason) = token->u.value;
+	      return reason;
+	    }
+	}
+
       return token->u.value;
     }