c++: Move consteval folding to cp_fold_r

In the review of P2564:
<https://gcc.gnu.org/pipermail/gcc-patches/2023-August/628747.html>
it turned out that in order to correctly handle an example in the paper,
we should stop doing immediate evaluation in build_over_call and
bot_replace, and instead do it in cp_fold_r.  This patch does that.

Another benefit is that this is a pretty significant simplification, at
least in my opinion.  Also, this fixes the c++/110997 ICE (but the test
doesn't compile yet).

The main drawback seems to be that cp_fold_r doesn't process
uninstantiated templates.  We still have to handle things like
"false ? foo () : 1".  To that end, I've added cp_fold_immediate, called
on dead branches in cxx_eval_conditional_expression.

You'll see that I've reintroduced ADDR_EXPR_DENOTES_CALL_P here.  This
is to detect

  *(&foo)) ()
  (s.*&S::foo) ()

which were deemed ill-formed.

gcc/cp/ChangeLog:

	* call.cc (build_over_call): Set ADDR_EXPR_DENOTES_CALL_P.  Don't handle
	immediate_invocation_p here.
	* constexpr.cc (cxx_eval_call_expression): Use mce_true for
	DECL_IMMEDIATE_FUNCTION_P.
	(cxx_eval_conditional_expression): Call cp_fold_immediate.
	* cp-gimplify.cc (enum fold_flags): Add ff_fold_immediate.
	(maybe_replace_decl): Make static.
	(cp_fold_r): Expand immediate invocations.
	(cp_fold_immediate_r): New.
	(cp_fold_immediate): New.
	* cp-tree.h (ADDR_EXPR_DENOTES_CALL_P): Define.
	(cp_fold_immediate): Declare.
	* tree.cc (bot_replace): Don't handle immediate invocations here.

libstdc++-v3/ChangeLog:

	* testsuite/20_util/allocator/105975.cc: Add dg-error.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp23/consteval-if2.C: Add xfail.
	* g++.dg/cpp2a/consteval-memfn1.C: Adjust.
	* g++.dg/cpp2a/consteval11.C: Remove dg-message.
	* g++.dg/cpp2a/consteval3.C: Remove dg-message and dg-error.
	* g++.dg/cpp2a/consteval9.C: Remove dg-message.
	* g++.dg/cpp2a/consteval32.C: New test.
	* g++.dg/cpp2a/consteval33.C: New test.
	* g++.dg/cpp2a/consteval34.C: New test.
	* g++.dg/cpp2a/consteval35.C: New test.

5 files changed, 168 insertions, 105 deletions

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 2bbaeee..e8dafbd 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -10434,6 +10434,10 @@ build_over_call (struct z_candidate *cand, int flags, tsubst_flags_t complain)
       fn = build_addr_func (fn, complain);
       if (fn == error_mark_node)
 	return error_mark_node;
+
+      /* We're actually invoking the function.  (Immediate functions get an
+	 & when invoking it even though the user didn't use &.)  */
+      ADDR_EXPR_DENOTES_CALL_P (fn) = true;
     }
 
   tree call = build_cxx_call (fn, nargs, argarray, complain|decltype_flag);
@@ -10451,41 +10455,7 @@ build_over_call (struct z_candidate *cand, int flags, tsubst_flags_t complain)
       if (TREE_CODE (c) == CALL_EXPR)
 	suppress_warning (c /* Suppress all warnings.  */);
     }
-  if (TREE_CODE (fn) == ADDR_EXPR)
-    {
-      tree fndecl = STRIP_TEMPLATE (TREE_OPERAND (fn, 0));
-      if (immediate_invocation_p (fndecl))
-	{
-	  tree obj_arg = NULL_TREE;
-	  /* Undo convert_from_reference called by build_cxx_call.  */
-	  if (REFERENCE_REF_P (call))
-	    call = TREE_OPERAND (call, 0);
-	  if (DECL_CONSTRUCTOR_P (fndecl))
-	    obj_arg = cand->first_arg ? cand->first_arg : (*args)[0];
-	  if (obj_arg && is_dummy_object (obj_arg))
-	    {
-	      call = build_cplus_new (DECL_CONTEXT (fndecl), call, complain);
-	      obj_arg = NULL_TREE;
-	    }
-	  /* Look through *(const T *)&obj.  */
-	  else if (obj_arg && INDIRECT_REF_P (obj_arg))
-	    {
-	      tree addr = TREE_OPERAND (obj_arg, 0);
-	      STRIP_NOPS (addr);
-	      if (TREE_CODE (addr) == ADDR_EXPR)
-		{
-		  tree typeo = TREE_TYPE (obj_arg);
-		  tree typei = TREE_TYPE (TREE_OPERAND (addr, 0));
-		  if (same_type_ignoring_top_level_qualifiers_p (typeo, typei))
-		    obj_arg = TREE_OPERAND (addr, 0);
-		}
-	    }
-	  call = cxx_constant_value (call, obj_arg, complain);
-	  if (obj_arg && !error_operand_p (call))
-	    call = cp_build_init_expr (obj_arg, call);
-	  call = convert_from_reference (call);
-	}
-    }
+
   return call;
 }
 
diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 0ca4370..a673a60 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -3135,6 +3135,11 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree t,
 	  unsigned save_heap_alloc_count = ctx->global->heap_vars.length ();
 	  unsigned save_heap_dealloc_count = ctx->global->heap_dealloc_count;
 
+	  /* Make sure we fold std::is_constant_evaluated to true in an
+	     immediate function.  */
+	  if (DECL_IMMEDIATE_FUNCTION_P (fun))
+	    call_ctx.manifestly_const_eval = mce_true;
+
 	  /* If this is a constexpr destructor, the object's const and volatile
 	     semantics are no longer in effect; see [class.dtor]p5.  */
 	  if (new_obj && DECL_DESTRUCTOR_P (fun))
@@ -3807,8 +3812,7 @@ cxx_eval_binary_expression (const constexpr_ctx *ctx, tree t,
 }
 
 /* Subroutine of cxx_eval_constant_expression.
-   Attempt to evaluate condition expressions.  Dead branches are not
-   looked into.  */
+   Attempt to evaluate condition expressions.  */
 
 static tree
 cxx_eval_conditional_expression (const constexpr_ctx *ctx, tree t,
@@ -3837,12 +3841,25 @@ cxx_eval_conditional_expression (const constexpr_ctx *ctx, tree t,
 				   boolean_type_node);
     }
   /* Don't VERIFY_CONSTANT the other operands.  */
-  if (integer_zerop (val))
+  const bool zero_p = integer_zerop (val);
+  if (zero_p)
     val = TREE_OPERAND (t, 2);
   else
     val = TREE_OPERAND (t, 1);
   if (TREE_CODE (t) == IF_STMT && !val)
     val = void_node;
+
+  /* P2564: a subexpression of a manifestly constant-evaluated expression
+     or conversion is an immediate function context.  */
+  if (ctx->manifestly_const_eval != mce_true
+      && !in_immediate_context ()
+      && cp_fold_immediate (&TREE_OPERAND (t, zero_p ? 1 : 2),
+			    ctx->manifestly_const_eval))
+    {
+      *non_constant_p = true;
+      return t;
+    }
+
   /* A TARGET_EXPR may be nested inside another TARGET_EXPR, but still
      serve as the initializer for the same object as the outer TARGET_EXPR,
      as in
diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc
index 206e791..bdf6e5f 100644
--- a/gcc/cp/cp-gimplify.cc
+++ b/gcc/cp/cp-gimplify.cc
@@ -53,6 +53,8 @@ enum fold_flags {
      definitely not in a manifestly constant-evaluated
      context.  */
   ff_mce_false = 1 << 1,
+  /* Whether we're being called from cp_fold_immediate.  */
+  ff_fold_immediate = 1 << 2,
 };
 
 using fold_flags_t = int;
@@ -1000,7 +1002,7 @@ cp_genericize_target_expr (tree *stmt_p)
    replacement when cp_folding TARGET_EXPR to preserve the invariant that
    AGGR_INIT_EXPR_SLOT agrees with the enclosing TARGET_EXPR_SLOT.  */
 
-bool
+static bool
 maybe_replace_decl (tree *tp, tree decl, tree replacement)
 {
   if (!*tp || !VOID_TYPE_P (TREE_TYPE (*tp)))
@@ -1029,44 +1031,95 @@ struct cp_genericize_data
   bool handle_invisiref_parm_p;
 };
 
-/* Perform any pre-gimplification folding of C++ front end trees to
-   GENERIC.
-   Note:  The folding of non-omp cases is something to move into
-     the middle-end.  As for now we have most foldings only on GENERIC
-     in fold-const, we need to perform this before transformation to
-     GIMPLE-form.  */
+/* A subroutine of cp_fold_r to handle immediate functions.  */
 
 static tree
-cp_fold_r (tree *stmt_p, int *walk_subtrees, void *data_)
+cp_fold_immediate_r (tree *stmt_p, int *walk_subtrees, void *data_)
 {
-  cp_fold_data *data = (cp_fold_data*)data_;
+  auto data = static_cast<cp_fold_data *>(data_);
   tree stmt = *stmt_p;
-  enum tree_code code = TREE_CODE (stmt);
+  /* The purpose of this is not to emit errors for mce_unknown.  */
+  const tsubst_flags_t complain = (data->flags & ff_mce_false
+				   ? tf_error : tf_none);
 
-  switch (code)
+  /* No need to look into types or unevaluated operands.
+     NB: This affects cp_fold_r as well.  */
+  if (TYPE_P (stmt) || unevaluated_p (TREE_CODE (stmt)))
+    {
+      *walk_subtrees = 0;
+      return NULL_TREE;
+    }
+
+  switch (TREE_CODE (stmt))
     {
+    /* Unfortunately we must handle code like
+	 false ? bar () : 42
+       where we have to check bar too.  The cp_fold call in cp_fold_r could
+       fold the ?: into a constant before we see it here.  */
+    case COND_EXPR:
+      /* If we are called from cp_fold_immediate, we don't need to worry about
+	 cp_fold folding away the COND_EXPR.  */
+      if (data->flags & ff_fold_immediate)
+	break;
+      if (TREE_OPERAND (stmt, 1)
+	  && cp_walk_tree (&TREE_OPERAND (stmt, 1), cp_fold_immediate_r, data,
+			   nullptr))
+	return error_mark_node;
+      if (TREE_OPERAND (stmt, 2)
+	  && cp_walk_tree (&TREE_OPERAND (stmt, 2), cp_fold_immediate_r, data,
+			   nullptr))
+	return error_mark_node;
+      /* We're done here.  Don't clear *walk_subtrees here though: we're called
+	 from cp_fold_r and we must let it recurse on the expression with
+	 cp_fold.  */
+      break;
     case PTRMEM_CST:
       if (TREE_CODE (PTRMEM_CST_MEMBER (stmt)) == FUNCTION_DECL
 	  && DECL_IMMEDIATE_FUNCTION_P (PTRMEM_CST_MEMBER (stmt)))
 	{
-	  if (!data->pset.add (stmt))
-	    error_at (PTRMEM_CST_LOCATION (stmt),
-		      "taking address of an immediate function %qD",
-		      PTRMEM_CST_MEMBER (stmt));
-	  stmt = *stmt_p = build_zero_cst (TREE_TYPE (stmt));
-	  break;
+	  if (!data->pset.add (stmt) && (complain & tf_error))
+	    {
+	      error_at (PTRMEM_CST_LOCATION (stmt),
+			"taking address of an immediate function %qD",
+			PTRMEM_CST_MEMBER (stmt));
+	      *stmt_p = build_zero_cst (TREE_TYPE (stmt));
+	    }
+	  return error_mark_node;
 	}
       break;
 
+    /* Expand immediate invocations.  */
+    case CALL_EXPR:
+    case AGGR_INIT_EXPR:
+      if (tree fn = cp_get_callee (stmt))
+	if (TREE_CODE (fn) != ADDR_EXPR || ADDR_EXPR_DENOTES_CALL_P (fn))
+	  if (tree fndecl = cp_get_fndecl_from_callee (fn, /*fold*/false))
+	    if (DECL_IMMEDIATE_FUNCTION_P (fndecl))
+	      {
+		stmt = cxx_constant_value (stmt, complain);
+		if (stmt == error_mark_node)
+		  {
+		    if (complain & tf_error)
+		      *stmt_p = error_mark_node;
+		    return error_mark_node;
+		  }
+		*stmt_p = stmt;
+	      }
+      break;
+
     case ADDR_EXPR:
       if (TREE_CODE (TREE_OPERAND (stmt, 0)) == FUNCTION_DECL
-	  && DECL_IMMEDIATE_FUNCTION_P (TREE_OPERAND (stmt, 0)))
+	  && DECL_IMMEDIATE_FUNCTION_P (TREE_OPERAND (stmt, 0))
+	  && !ADDR_EXPR_DENOTES_CALL_P (stmt))
 	{
-	  error_at (EXPR_LOCATION (stmt),
-		    "taking address of an immediate function %qD",
-		    TREE_OPERAND (stmt, 0));
-	  stmt = *stmt_p = build_zero_cst (TREE_TYPE (stmt));
-	  break;
+	  if (complain & tf_error)
+	    {
+	      error_at (EXPR_LOCATION (stmt),
+			"taking address of an immediate function %qD",
+			TREE_OPERAND (stmt, 0));
+	      *stmt_p = build_zero_cst (TREE_TYPE (stmt));
+	    }
+	  return error_mark_node;
 	}
       break;
 
@@ -1074,6 +1127,44 @@ cp_fold_r (tree *stmt_p, int *walk_subtrees, void *data_)
       break;
     }
 
+  return NULL_TREE;
+}
+
+/* A wrapper around cp_fold_immediate_r.  Return true if we found
+   a non-constant immediate function, or taking the address of an
+   immediate function.  */
+
+bool
+cp_fold_immediate (tree *tp, mce_value manifestly_const_eval)
+{
+  if (cxx_dialect <= cxx17)
+    return false;
+
+  fold_flags_t flags = ff_fold_immediate;
+  if (manifestly_const_eval == mce_false)
+    flags |= ff_mce_false;
+
+  cp_fold_data data (flags);
+  return !!cp_walk_tree_without_duplicates (tp, cp_fold_immediate_r, &data);
+}
+
+/* Perform any pre-gimplification folding of C++ front end trees to
+   GENERIC.
+   Note:  The folding of non-omp cases is something to move into
+     the middle-end.  As for now we have most foldings only on GENERIC
+     in fold-const, we need to perform this before transformation to
+     GIMPLE-form.  */
+
+static tree
+cp_fold_r (tree *stmt_p, int *walk_subtrees, void *data_)
+{
+  cp_fold_data *data = (cp_fold_data*)data_;
+  tree stmt = *stmt_p;
+  enum tree_code code = TREE_CODE (stmt);
+
+  if (cxx_dialect > cxx17)
+    cp_fold_immediate_r (stmt_p, walk_subtrees, data);
+
   *stmt_p = stmt = cp_fold (*stmt_p, data->flags);
 
   if (data->pset.add (stmt))
@@ -1084,7 +1175,7 @@ cp_fold_r (tree *stmt_p, int *walk_subtrees, void *data_)
 	 always the same tree, which the first time cp_fold_r has been
 	 called on it had the subtrees walked.  */
       *walk_subtrees = 0;
-      return NULL;
+      return NULL_TREE;
     }
 
   code = TREE_CODE (stmt);
@@ -1136,7 +1227,7 @@ cp_fold_r (tree *stmt_p, int *walk_subtrees, void *data_)
 	}
       cp_walk_tree (&OMP_FOR_PRE_BODY (stmt), cp_fold_r, data, NULL);
       *walk_subtrees = 0;
-      return NULL;
+      return NULL_TREE;
 
     case IF_STMT:
       if (IF_STMT_CONSTEVAL_P (stmt))
@@ -1146,7 +1237,7 @@ cp_fold_r (tree *stmt_p, int *walk_subtrees, void *data_)
 	  cp_walk_tree (&ELSE_CLAUSE (stmt), cp_fold_r, data, NULL);
 	  cp_walk_tree (&IF_SCOPE (stmt), cp_fold_r, data, NULL);
 	  *walk_subtrees = 0;
-	  return NULL;
+	  return NULL_TREE;
 	}
       break;
 
@@ -1183,7 +1274,7 @@ cp_fold_r (tree *stmt_p, int *walk_subtrees, void *data_)
       break;
     }
 
-  return NULL;
+  return NULL_TREE;
 }
 
 /* Fold ALL the trees!  FIXME we should be able to remove this, but
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 3ca011c..5084932 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -4784,6 +4784,11 @@ get_vec_init_expr (tree t)
 #define PTRMEM_OK_P(NODE) \
   TREE_LANG_FLAG_0 (TREE_CHECK3 ((NODE), ADDR_EXPR, OFFSET_REF, SCOPE_REF))
 
+/* True if this ADDR_EXPR denotes a function call; that is, it's
+   fn() rather than &fn.  */
+#define ADDR_EXPR_DENOTES_CALL_P(NODE) \
+  (ADDR_EXPR_CHECK(NODE)->base.protected_flag)
+
 /* Get the POINTER_TYPE to the METHOD_TYPE associated with this
    pointer to member function.  TYPE_PTRMEMFUNC_P _must_ be true,
    before using this macro.  */
@@ -6580,6 +6585,24 @@ extern int class_dump_id;
 extern int module_dump_id;
 extern int raw_dump_id;
 
+/* Whether the current context is manifestly constant-evaluated.
+   Used by the constexpr machinery to control folding of
+   __builtin_is_constant_evaluated.  */
+
+enum class mce_value
+{
+  /* Unknown, so treat __builtin_is_constant_evaluated as non-constant.  */
+  mce_unknown = 0,
+  /* Fold it to true.  */
+  mce_true = 1,
+  /* Fold it to false.  Primarily used during cp_fold_function and
+     cp_fully_fold_init.  */
+  mce_false = -1,
+};
+constexpr mce_value mce_unknown = mce_value::mce_unknown;
+constexpr mce_value mce_true = mce_value::mce_true;
+constexpr mce_value mce_false = mce_value::mce_false;
+
 /* in call.cc */
 extern bool check_dtor_name			(tree, tree);
 int magic_varargs_p				(tree);
@@ -8354,6 +8377,7 @@ extern tree process_stmt_assume_attribute	(tree, tree, location_t);
 extern bool simple_empty_class_p		(tree, tree, tree_code);
 extern tree fold_builtin_source_location	(const_tree);
 extern tree get_source_location_impl_type	();
+extern bool cp_fold_immediate			(tree *, mce_value);
 
 /* in name-lookup.cc */
 extern tree strip_using_decl                    (tree);
@@ -8515,24 +8539,6 @@ struct GTY((for_user)) constexpr_fundef {
   tree result;
 };
 
-/* Whether the current context is manifestly constant-evaluated.
-   Used by the constexpr machinery to control folding of
-   __builtin_is_constant_evaluated.  */
-
-enum class mce_value
-{
-  /* Unknown, so treat __builtin_is_constant_evaluated as non-constant.  */
-  mce_unknown = 0,
-  /* Fold it to true.  */
-  mce_true = 1,
-  /* Fold it to false.  Primarily used during cp_fold_function and
-     cp_fully_fold_init.  */
-  mce_false = -1,
-};
-constexpr mce_value mce_unknown = mce_value::mce_unknown;
-constexpr mce_value mce_true = mce_value::mce_true;
-constexpr mce_value mce_false = mce_value::mce_false;
-
 extern void fini_constexpr			(void);
 extern bool literal_type_p                      (tree);
 extern void maybe_save_constexpr_fundef		(tree);
diff --git a/gcc/cp/tree.cc b/gcc/cp/tree.cc
index 799183d..eaf882f 100644
--- a/gcc/cp/tree.cc
+++ b/gcc/cp/tree.cc
@@ -3254,7 +3254,7 @@ bot_manip (tree* tp, int* walk_subtrees, void* data_)
    variables.  */
 
 static tree
-bot_replace (tree* t, int* walk_subtrees, void* data_)
+bot_replace (tree* t, int */*walk_subtrees*/, void* data_)
 {
   bot_data &data = *(bot_data*)data_;
   splay_tree target_remap = data.target_remap;
@@ -3284,27 +3284,6 @@ bot_replace (tree* t, int* walk_subtrees, void* data_)
 			    /*check_access=*/false, /*nonnull=*/true,
 			    tf_warning_or_error);
     }
-  else if (cxx_dialect >= cxx20
-	   && (TREE_CODE (*t) == CALL_EXPR
-	       || TREE_CODE (*t) == AGGR_INIT_EXPR)
-	   && !in_immediate_context ())
-    {
-      /* Expand immediate invocations.  */
-      if (tree fndecl = cp_get_callee_fndecl_nofold (*t))
-	if (DECL_IMMEDIATE_FUNCTION_P (fndecl))
-	  {
-	    /* Make in_immediate_context true within the args.  */
-	    in_consteval_if_p_temp_override ito;
-	    in_consteval_if_p = true;
-	    int nargs = call_expr_nargs (*t);
-	    for (int i = 0; i < nargs; ++i)
-	      cp_walk_tree (&get_nth_callarg (*t, i), bot_replace, data_, NULL);
-	    *t = cxx_constant_value (*t);
-	    if (*t == error_mark_node)
-	      return error_mark_node;
-	    *walk_subtrees = 0;
-	  }
-    }
 
   return NULL_TREE;
 }