c++: more mce_false folding from cp_fully_fold_init [PR108243]

We should also fold the overall initializer passed to cp_fully_fold_init
with mce_false, which allows folding of the copy-initialization of
'a1' in the below testcase (the initializer here is an AGGR_INIT_EXPR).

Unfortunately this doesn't help with direct- or default-initialization
because we don't call cp_fully_fold_init in that case, and even if we
did the initializer in that case is expressed as a bare CALL_EXPR
instead of an AGGR_INIT_EXPR, which cp_fully_fold_init can't really
fold.

	PR c++/108243
	PR c++/97553

gcc/cp/ChangeLog:

	* cp-gimplify.cc (cp_fully_fold): Add an internal overload that
	additionally takes and propagate an mce_value parameter, and
	define the existing public overload in terms of it.
	(cp_fully_fold_init): Pass mce_false to cp_fully_fold.

gcc/testsuite/ChangeLog:

	* g++.dg/opt/is_constant_evaluated3.C: New test.

gcc/cp/cp-gimplify.cc

@@ -2449,16 +2449,16 @@ cp_fold_rvalue (tree x)
 
 /* Perform folding on expression X.  */
 
-tree
-cp_fully_fold (tree x)
+static tree
+cp_fully_fold (tree x, mce_value manifestly_const_eval)
 {
   if (processing_template_decl)
     return x;
   /* FIXME cp_fold ought to be a superset of maybe_constant_value so we don't
      have to call both.  */
   if (cxx_dialect >= cxx11)
     {
-      x = maybe_constant_value (x);
+      x = maybe_constant_value (x, /*decl=*/NULL_TREE, manifestly_const_eval);
       /* Sometimes we are given a CONSTRUCTOR but the call above wraps it into
 	 a TARGET_EXPR; undo that here.  */
       if (TREE_CODE (x) == TARGET_EXPR)
@@ -2468,7 +2468,16 @@ cp_fully_fold (tree x)
 	       && TREE_TYPE (TREE_OPERAND (x, 0)) == TREE_TYPE (x))
 	x = TREE_OPERAND (x, 0);
     }
-  return cp_fold_rvalue (x);
+  fold_flags_t flags = ff_none;
+  if (manifestly_const_eval == mce_false)
+    flags |= ff_mce_false;
+  return cp_fold_rvalue (x, flags);
+}
+
+tree
+cp_fully_fold (tree x)
+{
+  return cp_fully_fold (x, mce_unknown);
 }
 
 /* Likewise, but also fold recursively, which cp_fully_fold doesn't perform
@@ -2479,7 +2488,7 @@ cp_fully_fold_init (tree x)
 {
   if (processing_template_decl)
     return x;
-  x = cp_fully_fold (x);
+  x = cp_fully_fold (x, mce_false);
   cp_fold_data data (ff_mce_false);
   cp_walk_tree (&x, cp_fold_r, &data, NULL);
   return x;

gcc/testsuite/g++.dg/opt/is_constant_evaluated3.C

@@ -0,0 +1,23 @@
+// PR c++/108243
+// { dg-do compile { target c++11 } }
+// { dg-additional-options "-O -fdump-tree-original" }
+
+struct A {
+  constexpr A(int n) : n(n), m(__builtin_is_constant_evaluated()) { }
+  constexpr A() : A(42) { }
+  int n, m;
+};
+
+int main() {
+  A a1 = {42};
+  A a2{42};
+  A a3(42);
+  A a4;
+  A a5{};
+}
+
+// { dg-final { scan-tree-dump "a1 = {\\.n=42, \\.m=0}" "original" } }
+// { dg-final { scan-tree-dump "a2 = {\\.n=42, \\.m=0}" "original" { xfail *-*-* } } }
+// { dg-final { scan-tree-dump "a3 = {\\.n=42, \\.m=0}" "original" { xfail *-*-* } } }
+// { dg-final { scan-tree-dump "a4 = {\\.n=42, \\.m=0}" "original" { xfail *-*-* } } }
+// { dg-final { scan-tree-dump "a5 = {\\.n=42, \\.m=0}" "original" { xfail *-*-* } } }