1 files changed, 69 insertions, 0 deletions
diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
index 7215dc672b6..f5bb0c1beb4 100644
--- a/gcc/cp/semantics.c
+++ b/gcc/cp/semantics.c
@@ -7763,4 +7763,73 @@ capture_decltype (tree decl)
   return type;
 }
 
+/* Build a unary fold expression of EXPR over OP. If IS_RIGHT is true,
+   this is a right unary fold. Otherwise it is a left unary fold. */
+
+static tree
+finish_unary_fold_expr (tree expr, int op, tree_code dir)
+{
+  // Build a pack expansion (assuming expr has pack type).
+  if (!uses_parameter_packs (expr))
+    {
+      error_at (location_of (expr), "operand of fold expression has no "
+		"unexpanded parameter packs");
+      return error_mark_node;
+    }
+  tree pack = make_pack_expansion (expr);
+
+  // Build the fold expression.
+  tree code = build_int_cstu (integer_type_node, abs (op));
+  tree fold = build_min (dir, unknown_type_node, code, pack);
+  FOLD_EXPR_MODIFY_P (fold) = (op < 0);
+  return fold;
+}
+
+tree
+finish_left_unary_fold_expr (tree expr, int op)
+{
+  return finish_unary_fold_expr (expr, op, UNARY_LEFT_FOLD_EXPR);
+}
+
+tree
+finish_right_unary_fold_expr (tree expr, int op)
+{
+  return finish_unary_fold_expr (expr, op, UNARY_RIGHT_FOLD_EXPR);
+}
+
+/* Build a binary fold expression over EXPR1 and EXPR2. The
+   associativity of the fold is determined by EXPR1 and EXPR2 (whichever
+   has an unexpanded parameter pack). */
+
+tree
+finish_binary_fold_expr (tree pack, tree init, int op, tree_code dir)
+{
+  pack = make_pack_expansion (pack);
+  tree code = build_int_cstu (integer_type_node, abs (op));
+  tree fold = build_min (dir, unknown_type_node, code, pack, init);
+  FOLD_EXPR_MODIFY_P (fold) = (op < 0);
+  return fold;
+}
+
+tree
+finish_binary_fold_expr (tree expr1, tree expr2, int op)
+{
+  // Determine which expr has an unexpanded parameter pack and
+  // set the pack and initial term.
+  bool pack1 = uses_parameter_packs (expr1);
+  bool pack2 = uses_parameter_packs (expr2);
+  if (pack1 && !pack2)
+    return finish_binary_fold_expr (expr1, expr2, op, BINARY_RIGHT_FOLD_EXPR);
+  else if (pack2 && !pack1)
+    return finish_binary_fold_expr (expr2, expr1, op, BINARY_LEFT_FOLD_EXPR);
+  else
+    {
+      if (pack1)
+        error ("both arguments in binary fold have unexpanded parameter packs");
+      else
+        error ("no unexpanded parameter packs in binary fold");
+    }
+  return error_mark_node;
+}
+
 #include "gt-cp-semantics.h"