c++: Fix usage of CONSTRUCTOR_PLACEHOLDER_BOUNDARY inside array initializers [PR90996]

This PR reports that ever since the introduction of the
CONSTRUCTOR_PLACEHOLDER_BOUNDARY flag, we are sometimes failing to resolve
PLACEHOLDER_EXPRs inside array initializers that refer to some inner
constructor.  In the testcase in the PR, we have as the initializer for "S c[];"
the following

  {{.a=(int &) &_ZGR1c_, .b={*(&<PLACEHOLDER_EXPR struct S>)->a}}}

where CONSTRUCTOR_PLACEHOLDER_BOUNDARY is set on the middle constructor.  When
calling replace_placeholders from store_init_value, we pass the entire
initializer to it, and as a result we fail to resolve the PLACEHOLDER_EXPR
within due to the CONSTRUCTOR_PLACEHOLDER_BOUNDARY flag on the middle
constructor blocking replace_placeholders_r from reaching it.

To fix this, we could perhaps either call replace_placeholders in more places,
or we could change where we set CONSTRUCTOR_PLACEHOLDER_BOUNDARY.  This patch
takes this latter approach -- when building up an array initializer, we now
bubble any CONSTRUCTOR_PLACEHOLDER_BOUNDARY flag from the element initializers
up to the array initializer so that the boundary doesn't later impede us when we
call replace_placeholders from store_init_value.

Besides fixing the kind of code like in the testcase, this shouldn't cause any
other differences in PLACEHOLDER_EXPR resolution because we don't create or use
PLACEHOLDER_EXPRs of array type in the frontend, as far as I can tell.

gcc/cp/ChangeLog:

	PR c++/90996
	* tree.c (replace_placeholders): Look through all handled components,
	not just COMPONENT_REFs.
	* typeck2.c (process_init_constructor_array): Propagate
	CONSTRUCTOR_PLACEHOLDER_BOUNDARY up from each element initializer to
	the array initializer.

gcc/testsuite/ChangeLog:

	PR c++/90996
	* g++.dg/cpp1y/pr90996.C: New test.

gcc/cp/ChangeLog

+2020-04-07  Patrick Palka  <ppalka@redhat.com>
+
+	PR c++/90996
+	* tree.c (replace_placeholders): Look through all handled components,
+	not just COMPONENT_REFs.
+	* typeck2.c (process_init_constructor_array): Propagate
+	CONSTRUCTOR_PLACEHOLDER_BOUNDARY up from each element initializer to
+	the array initializer.
+
 2020-04-07  Jakub Jelinek  <jakub@redhat.com>
 
 	PR c++/94512

gcc/cp/tree.c

@@ -3247,7 +3247,7 @@ replace_placeholders (tree exp, tree obj, bool *seen_p /*= NULL*/)
 
   /* If the object isn't a (member of a) class, do nothing.  */
   tree op0 = obj;
-  while (TREE_CODE (op0) == COMPONENT_REF)
+  while (handled_component_p (op0))
     op0 = TREE_OPERAND (op0, 0);
   if (!CLASS_TYPE_P (strip_array_types (TREE_TYPE (op0))))
     return exp;

gcc/cp/typeck2.c

@@ -1488,6 +1488,17 @@ process_init_constructor_array (tree type, tree init, int nested, int flags,
 	= massage_init_elt (TREE_TYPE (type), ce->value, nested, flags,
 			    complain);
 
+      if (TREE_CODE (ce->value) == CONSTRUCTOR
+	  && CONSTRUCTOR_PLACEHOLDER_BOUNDARY (ce->value))
+	{
+	  /* Shift CONSTRUCTOR_PLACEHOLDER_BOUNDARY from the element initializer
+	     up to the array initializer, so that the call to
+	     replace_placeholders from store_init_value can resolve any
+	     PLACEHOLDER_EXPRs inside this element initializer.  */
+	  CONSTRUCTOR_PLACEHOLDER_BOUNDARY (ce->value) = 0;
+	  CONSTRUCTOR_PLACEHOLDER_BOUNDARY (init) = 1;
+	}
+
       gcc_checking_assert
 	(ce->value == error_mark_node
 	 || (same_type_ignoring_top_level_qualifiers_p
@@ -1516,6 +1527,13 @@ process_init_constructor_array (tree type, tree init, int nested, int flags,
 	      /* The default zero-initialization is fine for us; don't
 		 add anything to the CONSTRUCTOR.  */
 	      next = NULL_TREE;
+	    else if (TREE_CODE (next) == CONSTRUCTOR
+		     && CONSTRUCTOR_PLACEHOLDER_BOUNDARY (next))
+	      {
+		/* As above.  */
+		CONSTRUCTOR_PLACEHOLDER_BOUNDARY (next) = 0;
+		CONSTRUCTOR_PLACEHOLDER_BOUNDARY (init) = 1;
+	      }
 	  }
 	else if (!zero_init_p (TREE_TYPE (type)))
 	  next = build_zero_init (TREE_TYPE (type),

gcc/testsuite/ChangeLog

+2020-04-07  Patrick Palka  <ppalka@redhat.com>
+
+	PR c++/90996
+	* g++.dg/cpp1y/pr90996.C: New test.
+
 2020-04-07  Jakub Jelinek  <jakub@redhat.com>
 
 	PR target/94509

gcc/testsuite/g++.dg/cpp1y/pr90996.C

+// PR c++/90996
+// { dg-do compile { target c++14 } }
+
+struct S
+{
+  int &&a = 2;
+  int b[1] {a};
+};
+
+S c[2][2] {{{5}}};
+
+struct T
+{
+  S c[2][2] {{{7}}};
+};
+
+T d {};