c++: Implement P1009: Array size deduction in new-expressions.

This patch implements C++20 P1009, allowing code like

  new double[]{1,2,3}; // array bound will be deduced

Since this proposal makes the initialization rules more consistent, it is
applied to all previous versions of C++ (thus, effectively, all the way back
to C++11).

My patch is based on Jason's patch that handled the basic case.  I've
extended it to work with ()-init and also the string literal case.
Further testing revealed that to handle stuff like

  new int[]{t...};

in a template, we have to consider such a NEW_EXPR type-dependent.
Obviously, we first have to expand the pack to be able to deduce the
number of elements in the array.

Curiously, while implementing this proposal, I noticed that we fail
to accept

  new char[4]{"abc"};

so I've assigned 77841 to self.  I think the fix will depend on the
build_new_1 hunk in this patch.

The new tree.c function build_constructor_from_vec helps us morph
a vector into a CONSTRUCTOR more efficiently.

gcc/cp/ChangeLog:

	PR c++/93529
	* call.c (build_new_method_call_1): Use build_constructor_from_vec
	instead of build_tree_list_vec + build_constructor_from_list.
	* init.c (build_new_1): Handle new char[]{"foo"}.  Use
	build_constructor_from_vec instead of build_tree_list_vec +
	build_constructor_from_list.
	(build_new): Deduce the array size in new-expression if not
	present.  Handle ()-init.  Handle initializing an array from
	a string literal.
	* parser.c (cp_parser_new_type_id): Leave [] alone.
	(cp_parser_direct_new_declarator): Allow [].
	* pt.c (type_dependent_expression_p): In a NEW_EXPR, consider
	array types whose dimension has to be deduced type-dependent.

gcc/ChangeLog:

	PR c++/93529
	* tree.c (build_constructor_from_vec): New.
	* tree.h (build_constructor_from_vec): Declare.

gcc/testsuite/ChangeLog:

	PR c++/93529
	* g++.dg/cpp0x/sfinae4.C: Adjust expected result after P1009.
	* g++.dg/cpp2a/new-array1.C: New test.
	* g++.dg/cpp2a/new-array2.C: New test.
	* g++.dg/cpp2a/new-array3.C: New test.
	* g++.dg/cpp2a/new-array4.C: New test.

Co-authored-by: Jason Merrill <jason@redhat.com>

4 files changed, 67 insertions, 9 deletions

diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index 4726e57..61bbb38 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -10297,8 +10297,8 @@ build_new_method_call_1 (tree instance, tree fns, vec<tree, va_gc> **args,
 	  && !vec_safe_is_empty (user_args))
 	{
 	  /* Create a CONSTRUCTOR from ARGS, e.g. {1, 2} from <1, 2>.  */
-	  tree list = build_tree_list_vec (user_args);
-	  tree ctor = build_constructor_from_list (init_list_type_node, list);
+	  tree ctor = build_constructor_from_vec (init_list_type_node,
+						  user_args);
 	  CONSTRUCTOR_IS_DIRECT_INIT (ctor) = true;
 	  CONSTRUCTOR_IS_PAREN_INIT (ctor) = true;
 	  if (is_dummy_object (instance))
diff --git a/gcc/cp/init.c b/gcc/cp/init.c
index 872c234..360ab8c 100644
--- a/gcc/cp/init.c
+++ b/gcc/cp/init.c
@@ -3559,8 +3559,8 @@ build_new_1 (vec<tree, va_gc> **placement, tree type, tree nelts,
       else if (array_p)
 	{
 	  tree vecinit = NULL_TREE;
-	  if (vec_safe_length (*init) == 1
-	      && DIRECT_LIST_INIT_P ((**init)[0]))
+	  const size_t len = vec_safe_length (*init);
+	  if (len == 1 && DIRECT_LIST_INIT_P ((**init)[0]))
 	    {
 	      vecinit = (**init)[0];
 	      if (CONSTRUCTOR_NELTS (vecinit) == 0)
@@ -3578,6 +3578,15 @@ build_new_1 (vec<tree, va_gc> **placement, tree type, tree nelts,
 		  vecinit = digest_init (arraytype, vecinit, complain);
 		}
 	    }
+	  /* This handles code like new char[]{"foo"}.  */
+	  else if (len == 1
+		   && char_type_p (TYPE_MAIN_VARIANT (type))
+		   && TREE_CODE (tree_strip_any_location_wrapper ((**init)[0]))
+		      == STRING_CST)
+	    {
+	      vecinit = (**init)[0];
+	      STRIP_ANY_LOCATION_WRAPPER (vecinit);
+	    }
 	  else if (*init)
             {
               if (complain & tf_error)
@@ -3634,8 +3643,7 @@ build_new_1 (vec<tree, va_gc> **placement, tree type, tree nelts,
 		  && AGGREGATE_TYPE_P (type)
 		  && (*init)->length () > 1)
 		{
-		  ie = build_tree_list_vec (*init);
-		  ie = build_constructor_from_list (init_list_type_node, ie);
+		  ie = build_constructor_from_vec (init_list_type_node, *init);
 		  CONSTRUCTOR_IS_DIRECT_INIT (ie) = true;
 		  CONSTRUCTOR_IS_PAREN_INIT (ie) = true;
 		  ie = digest_init (type, ie, complain);
@@ -3917,6 +3925,45 @@ build_new (location_t loc, vec<tree, va_gc> **placement, tree type,
       return error_mark_node;
     }
 
+  /* P1009: Array size deduction in new-expressions.  */
+  if (TREE_CODE (type) == ARRAY_TYPE
+      && !TYPE_DOMAIN (type)
+      && *init)
+    {
+      /* This means we have 'new T[]()'.  */
+      if ((*init)->is_empty ())
+	{
+	  tree ctor = build_constructor (init_list_type_node, NULL);
+	  CONSTRUCTOR_IS_DIRECT_INIT (ctor) = true;
+	  vec_safe_push (*init, ctor);
+	}
+      tree &elt = (**init)[0];
+      /* The C++20 'new T[](e_0, ..., e_k)' case allowed by P0960.  */
+      if (!DIRECT_LIST_INIT_P (elt) && cxx_dialect >= cxx20)
+	{
+	  /* Handle new char[]("foo").  */
+	  if (vec_safe_length (*init) == 1
+	      && char_type_p (TYPE_MAIN_VARIANT (TREE_TYPE (type)))
+	      && TREE_CODE (tree_strip_any_location_wrapper (elt))
+		 == STRING_CST)
+	    /* Leave it alone: the string should not be wrapped in {}.  */;
+	  else
+	    {
+	      tree ctor = build_constructor_from_vec (init_list_type_node, *init);
+	      CONSTRUCTOR_IS_DIRECT_INIT (ctor) = true;
+	      CONSTRUCTOR_IS_PAREN_INIT (ctor) = true;
+	      elt = ctor;
+	      /* We've squashed all the vector elements into the first one;
+		 truncate the rest.  */
+	      (*init)->truncate (1);
+	    }
+	}
+      /* Otherwise we should have 'new T[]{e_0, ..., e_k}'.  */
+      if (BRACE_ENCLOSED_INITIALIZER_P (elt))
+	elt = reshape_init (type, elt, complain);
+      cp_complete_array_type (&type, elt, /*do_default*/false);
+    }
+
   /* The type allocated must be complete.  If the new-type-id was
      "T[N]" then we are just checking that "T" is complete here, but
      that is equivalent, since the value of "N" doesn't matter.  */
diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index 147d537..9849e59 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -9011,7 +9011,9 @@ cp_parser_new_type_id (cp_parser* parser, tree *nelts)
       if (*nelts == error_mark_node)
 	*nelts = integer_one_node;
 
-      if (outer_declarator)
+      if (*nelts == NULL_TREE)
+	/* Leave [] in the declarator.  */;
+      else if (outer_declarator)
 	outer_declarator->declarator = declarator->declarator;
       else
 	new_declarator = NULL;
@@ -9072,6 +9074,7 @@ static cp_declarator *
 cp_parser_direct_new_declarator (cp_parser* parser)
 {
   cp_declarator *declarator = NULL;
+  bool first_p = true;
 
   while (true)
     {
@@ -9082,14 +9085,17 @@ cp_parser_direct_new_declarator (cp_parser* parser)
       cp_parser_require (parser, CPP_OPEN_SQUARE, RT_OPEN_SQUARE);
 
       token = cp_lexer_peek_token (parser->lexer);
-      expression = cp_parser_expression (parser);
+      if (token->type == CPP_CLOSE_SQUARE && first_p)
+	expression = NULL_TREE;
+      else
+	expression = cp_parser_expression (parser);
       /* The standard requires that the expression have integral
 	 type.  DR 74 adds enumeration types.  We believe that the
 	 real intent is that these expressions be handled like the
 	 expression in a `switch' condition, which also allows
 	 classes with a single conversion to integral or
 	 enumeration type.  */
-      if (!processing_template_decl)
+      if (expression && !processing_template_decl)
 	{
 	  expression
 	    = build_expr_type_conversion (WANT_INT | WANT_ENUM,
@@ -9114,6 +9120,7 @@ cp_parser_direct_new_declarator (cp_parser* parser)
 	 bounds.  */
       if (cp_lexer_next_token_is_not (parser->lexer, CPP_OPEN_SQUARE))
 	break;
+      first_p = false;
     }
 
   return declarator;
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index ad8c988..a7b7a12 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -26872,6 +26872,10 @@ type_dependent_expression_p (tree expression)
 	return dependent_type_p (TREE_VALUE (TREE_PURPOSE (type)))
 	       || value_dependent_expression_p
 		    (TREE_OPERAND (TREE_VALUE (type), 1));
+      /* Array type whose dimension has to be deduced.  */
+      else if (TREE_CODE (type) == ARRAY_TYPE
+	       && TREE_OPERAND (expression, 2) == NULL_TREE)
+	return true;
       else
 	return dependent_type_p (type);
     }