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.

1 files changed, 20 insertions, 3 deletions

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