Implement C++20 P0388R4, DR 1307, and DR 330.

This patch implements P0388R4, Permit conversions to arrays of unknown bound,
<http://wg21.link/p0388r4>.  CWG 393 allowed references to arrays of unknown
bound and this C++20 feature allows conversions like

  void f(int(&)[]);
  int arr[1];

  void g() { f(arr); }
  int(&r)[] = arr;

The proposal seemed fairly straightforward but it turned out to be quite
shifty.  I found out that I needed to implement DR 2352 (done), and also
DR 1307 (done in this patch).  The latter DR added wording for
list-initialization ranking of references to arrays which this proposal
extends.  DR 330 was also implemented in this patch.

	PR c++/91364 - P0388R4: Permit conversions to arrays of	unknown bound.
	PR c++/69531 - DR 1307: Differently bounded array parameters.
	PR c++/88128 - DR 330: Qual convs and pointers to arrays of pointers.
	* call.c (build_array_conv): Build ck_identity at the beginning
	of the conversion.
	(standard_conversion): Pass bounds_none to comp_ptr_ttypes_const.
	(maybe_warn_array_conv): New.
	(convert_like_real): Call it.  Add an error message about converting
	from arrays of unknown bounds.
	(conv_get_original_expr): New.
	(nelts_initialized_by_list_init): New.
	(conv_binds_to_array_of_unknown_bound): New.
	(compare_ics): Implement list-initialization ranking based on
	array sizes, as specified in DR 1307 and P0388R.
	* cp-tree.h (comp_ptr_ttypes_const): Adjust declaration.
	(compare_bounds_t): New enum.
	* typeck.c (comp_array_types): New bool and compare_bounds_t
	parameters.  Use them.
	(structural_comptypes): Adjust the call to comp_array_types.
	(similar_type_p): Handle ARRAY_TYPE.
	(build_const_cast_1): Pass bounds_none to comp_ptr_ttypes_const.
	(comp_ptr_ttypes_real): Don't check cv-quals of ARRAY_TYPEs.  Use
	comp_array_types to compare array types.  Look through arrays as per
	DR 330.
	(comp_ptr_ttypes_const): Use comp_array_types to compare array types.
	Look through arrays as per DR 330.

	* g++.dg/conversion/qual1.C: New test.
	* g++.dg/conversion/qual2.C: New test.
	* g++.dg/conversion/qual3.C: New test.
	* g++.dg/conversion/ref2.C: New test.
	* g++.dg/conversion/ref3.C: New test.
	* g++.dg/cpp0x/initlist-array3.C: Remove dg-error.
	* g++.dg/cpp0x/initlist-array7.C: New test.
	* g++.dg/cpp0x/initlist-array8.C: New test.
	* g++.dg/cpp2a/array-conv1.C: New test.
	* g++.dg/cpp2a/array-conv10.C: New test.
	* g++.dg/cpp2a/array-conv11.C: New test.
	* g++.dg/cpp2a/array-conv12.C: New test.
	* g++.dg/cpp2a/array-conv13.C: New test.
	* g++.dg/cpp2a/array-conv14.C: New test.
	* g++.dg/cpp2a/array-conv15.C: New test.
	* g++.dg/cpp2a/array-conv16.C: New test.
	* g++.dg/cpp2a/array-conv17.C: New test.
	* g++.dg/cpp2a/array-conv2.C: New test.
	* g++.dg/cpp2a/array-conv3.C: New test.
	* g++.dg/cpp2a/array-conv4.C: New test.
	* g++.dg/cpp2a/array-conv5.C: New test.
	* g++.dg/cpp2a/array-conv6.C: New test.
	* g++.dg/cpp2a/array-conv7.C: New test.
	* g++.dg/cpp2a/array-conv8.C: New test.
	* g++.dg/cpp2a/array-conv9.C: New test.
	* g++.old-deja/g++.bugs/900321_01.C: Adjust dg-error.

	* testsuite/23_containers/span/lwg3255.cc: Adjust test to match the
	post-P0388R4 behavior.

From-SVN: r276771

33 files changed, 902 insertions, 31 deletions

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index 4eb18294..e47e8ca 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,5 +1,34 @@
 2019-10-09  Marek Polacek  <polacek@redhat.com>
 
+	PR c++/91364 - P0388R4: Permit conversions to arrays of	unknown bound.
+	PR c++/69531 - DR 1307: Differently bounded array parameters.
+	PR c++/88128 - DR 330: Qual convs and pointers to arrays of pointers.
+	* call.c (build_array_conv): Build ck_identity at the beginning
+	of the conversion.
+	(standard_conversion): Pass bounds_none to comp_ptr_ttypes_const.
+	(maybe_warn_array_conv): New.
+	(convert_like_real): Call it.  Add an error message about converting
+	from arrays of unknown bounds.
+	(conv_get_original_expr): New.
+	(nelts_initialized_by_list_init): New.
+	(conv_binds_to_array_of_unknown_bound): New.
+	(compare_ics): Implement list-initialization ranking based on
+	array sizes, as specified in DR 1307 and P0388R.
+	* cp-tree.h (comp_ptr_ttypes_const): Adjust declaration.
+	(compare_bounds_t): New enum.
+	* typeck.c (comp_array_types): New bool and compare_bounds_t
+	parameters.  Use them.
+	(structural_comptypes): Adjust the call to comp_array_types.
+	(similar_type_p): Handle ARRAY_TYPE.
+	(build_const_cast_1): Pass bounds_none to comp_ptr_ttypes_const.
+	(comp_ptr_ttypes_real): Don't check cv-quals of ARRAY_TYPEs.  Use
+	comp_array_types to compare array types.  Look through arrays as per
+	DR 330.
+	(comp_ptr_ttypes_const): Use comp_array_types to compare array types.
+	Look through arrays as per DR 330.
+
+2019-10-09  Marek Polacek  <polacek@redhat.com>
+
 	PR c++/92032 - DR 1601: Promotion of enum with fixed underlying	type.
 	* call.c (standard_conversion): When converting an enumeration with
 	a fixed underlying type to the underlying type, give it the cr_promotion
diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index 33ec6a2..55d2aba 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -122,7 +122,8 @@ struct conversion {
        of using this field directly.  */
     conversion *next;
     /* The expression at the beginning of the conversion chain.  This
-       variant is used only if KIND is ck_identity or ck_ambig.  */
+       variant is used only if KIND is ck_identity or ck_ambig.  You can
+       use conv_get_original_expr to get this expression.  */
     tree expr;
     /* The array of conversions for an initializer_list, so this
        variant is used only when KIN D is ck_list.  */
@@ -223,6 +224,8 @@ static void add_candidates (tree, tree, const vec<tree, va_gc> *, tree, tree,
 			    tsubst_flags_t);
 static conversion *merge_conversion_sequences (conversion *, conversion *);
 static tree build_temp (tree, tree, int, diagnostic_t *, tsubst_flags_t);
+static conversion *build_identity_conv (tree, tree);
+static inline bool conv_binds_to_array_of_unknown_bound (conversion *);
 
 /* Returns nonzero iff the destructor name specified in NAME matches BASETYPE.
    NAME can take many forms...  */
@@ -1066,7 +1069,7 @@ build_array_conv (tree type, tree ctor, int flags, tsubst_flags_t complain)
   c->rank = rank;
   c->user_conv_p = user;
   c->bad_p = bad;
-  c->u.next = NULL;
+  c->u.next = build_identity_conv (TREE_TYPE (ctor), ctor);
   return c;
 }
 
@@ -1366,7 +1369,7 @@ standard_conversion (tree to, tree from, tree expr, bool c_cast_p,
 
       if (same_type_p (from, to))
 	/* OK */;
-      else if (c_cast_p && comp_ptr_ttypes_const (to, from))
+      else if (c_cast_p && comp_ptr_ttypes_const (to, from, bounds_either))
 	/* In a C-style cast, we ignore CV-qualification because we
 	   are allowed to perform a static_cast followed by a
 	   const_cast.  */
@@ -1668,7 +1671,14 @@ reference_binding (tree rto, tree rfrom, tree expr, bool c_cast_p, int flags,
       maybe_warn_cpp0x (CPP0X_INITIALIZER_LISTS);
       /* DR 1288: Otherwise, if the initializer list has a single element
 	 of type E and ... [T's] referenced type is reference-related to E,
-	 the object or reference is initialized from that element... */
+	 the object or reference is initialized from that element...
+
+	 ??? With P0388R4, we should bind 't' directly to U{}:
+	   using U = A[2];
+	   A (&&t)[] = {U{}};
+	 because A[] and A[2] are reference-related.  But we don't do it
+	 because grok_reference_init has deduced the array size (to 1), and
+	 A[1] and A[2] aren't reference-related.  */
       if (CONSTRUCTOR_NELTS (expr) == 1)
 	{
 	  tree elt = CONSTRUCTOR_ELT (expr, 0)->value;
@@ -6958,6 +6968,27 @@ maybe_inform_about_fndecl_for_bogus_argument_init (tree fn, int argnum)
 	    "  initializing argument %P of %qD", argnum, fn);
 }
 
+/* Maybe warn about C++20 Conversions to arrays of unknown bound.  C is
+   the conversion, EXPR is the expression we're converting.  */
+
+static void
+maybe_warn_array_conv (location_t loc, conversion *c, tree expr)
+{
+  if (cxx_dialect >= cxx2a)
+    return;
+
+  tree type = TREE_TYPE (expr);
+  type = strip_pointer_operator (type);
+
+  if (TREE_CODE (type) != ARRAY_TYPE
+      || TYPE_DOMAIN (type) == NULL_TREE)
+    return;
+
+  if (conv_binds_to_array_of_unknown_bound (c))
+    pedwarn (loc, OPT_Wpedantic, "conversions to arrays of unknown bound "
+	     "are only available with %<-std=c++2a%> or %<-std=gnu++2a%>");
+}
+
 /* Perform the conversions in CONVS on the expression EXPR.  FN and
    ARGNUM are used for diagnostics.  ARGNUM is zero based, -1
    indicates the `this' argument of a method.  INNER is nonzero when
@@ -7377,8 +7408,20 @@ convert_like_real (conversion *convs, tree expr, tree fn, int argnum,
 	      error_at (loc, "cannot bind non-const lvalue reference of "
 			"type %qH to an rvalue of type %qI", totype, extype);
 	    else if (!reference_compatible_p (TREE_TYPE (totype), extype))
-	      error_at (loc, "binding reference of type %qH to %qI "
-			"discards qualifiers", totype, extype);
+	      {
+		/* If we're converting from T[] to T[N], don't talk
+		   about discarding qualifiers.  (Converting from T[N] to
+		   T[] is allowed by P0388R4.)  */
+		if (TREE_CODE (extype) == ARRAY_TYPE
+		    && TYPE_DOMAIN (extype) == NULL_TREE
+		    && TREE_CODE (TREE_TYPE (totype)) == ARRAY_TYPE
+		    && TYPE_DOMAIN (TREE_TYPE (totype)) != NULL_TREE)
+		  error_at (loc, "cannot bind reference of type %qH to %qI "
+			    "due to different array bounds", totype, extype);
+		else
+		  error_at (loc, "binding reference of type %qH to %qI "
+			    "discards qualifiers", totype, extype);
+	      }
 	    else
 	      gcc_unreachable ();
 	    maybe_print_user_conv_context (convs);
@@ -7386,6 +7429,8 @@ convert_like_real (conversion *convs, tree expr, tree fn, int argnum,
 
 	    return error_mark_node;
 	  }
+	else if (complain & tf_warning)
+	  maybe_warn_array_conv (loc, convs, expr);
 
 	/* If necessary, create a temporary. 
 
@@ -7469,7 +7514,10 @@ convert_like_real (conversion *convs, tree expr, tree fn, int argnum,
     case ck_qual:
       /* Warn about deprecated conversion if appropriate.  */
       if (complain & tf_warning)
-	string_conv_p (totype, expr, 1);
+	{
+	  string_conv_p (totype, expr, 1);
+	  maybe_warn_array_conv (loc, convs, expr);
+	}
       break;
 
     case ck_ptr:
@@ -10061,6 +10109,50 @@ maybe_handle_ref_bind (conversion **ics)
   return NULL;
 }
 
+/* Get the expression at the beginning of the conversion chain C.  */
+
+static tree
+conv_get_original_expr (conversion *c)
+{
+  for (; c; c = next_conversion (c))
+    if (c->kind == ck_identity || c->kind == ck_ambig)
+      return c->u.expr;
+  return NULL_TREE;
+}
+
+/* Return a tree representing the number of elements initialized by the
+   list-initialization C.  The caller must check that C converts to an
+   array type.  */
+
+static tree
+nelts_initialized_by_list_init (conversion *c)
+{
+  /* If the array we're converting to has a dimension, we'll use that.  */
+  if (TYPE_DOMAIN (c->type))
+    return array_type_nelts_top (c->type);
+  else
+    {
+      /* Otherwise, we look at how many elements the constructor we're
+	 initializing from has.  */
+      tree ctor = conv_get_original_expr (c);
+      return size_int (CONSTRUCTOR_NELTS (ctor));
+    }
+}
+
+/* True iff C is a conversion that binds a reference or a pointer to
+   an array of unknown bound.  */
+
+static inline bool
+conv_binds_to_array_of_unknown_bound (conversion *c)
+{
+  /* ck_ref_bind won't have the reference stripped.  */
+  tree type = non_reference (c->type);
+  /* ck_qual won't have the pointer stripped.  */
+  type = strip_pointer_operator (type);
+  return (TREE_CODE (type) == ARRAY_TYPE
+	  && TYPE_DOMAIN (type) == NULL_TREE);
+}
+
 /* Compare two implicit conversion sequences according to the rules set out in
    [over.ics.rank].  Return values:
 
@@ -10174,6 +10266,38 @@ compare_ics (conversion *ics1, conversion *ics2)
 	  if (f1 != f2)
 	    return 0;
 	}
+      /* List-initialization sequence L1 is a better conversion sequence than
+	 list-initialization sequence L2 if
+
+	 -- L1 and L2 convert to arrays of the same element type, and either
+	 the number of elements n1 initialized by L1 is less than the number
+	 of elements n2 initialized by L2, or n1=n2 and L2 converts to an array
+	 of unknown bound and L1 does not.  (Added in CWG 1307 and extended by
+	 P0388R4.)  */
+      else if (t1->kind == ck_aggr
+	       && TREE_CODE (t1->type) == ARRAY_TYPE
+	       && TREE_CODE (t2->type) == ARRAY_TYPE)
+	{
+	  /* The type of the array elements must be the same.  */
+	  if (!same_type_p (TREE_TYPE (t1->type), TREE_TYPE (t2->type)))
+	    return 0;
+
+	  tree n1 = nelts_initialized_by_list_init (t1);
+	  tree n2 = nelts_initialized_by_list_init (t2);
+	  if (tree_int_cst_lt (n1, n2))
+	    return 1;
+	  else if (tree_int_cst_lt (n2, n1))
+	    return -1;
+	  /* The n1 == n2 case.  */
+	  bool c1 = conv_binds_to_array_of_unknown_bound (t1);
+	  bool c2 = conv_binds_to_array_of_unknown_bound (t2);
+	  if (c1 && !c2)
+	    return -1;
+	  else if (!c1 && c2)
+	    return 1;
+	  else
+	    return 0;
+	}
       else
 	{
 	  /* For ambiguous or aggregate conversions, use the target type as
@@ -10469,6 +10593,28 @@ compare_ics (conversion *ics1, conversion *ics2)
 
       if (same_type_ignoring_top_level_qualifiers_p (to_type1, to_type2))
 	{
+	  /* Per P0388R4:
+
+	    void f (int(&)[]),     // (1)
+		 f (int(&)[1]),    // (2)
+		 f (int*);	   // (3)
+
+	    (2) is better than (1), but (3) should be equal to (1) and to
+	    (2).  For that reason we don't use ck_qual for (1) which would
+	    give it the cr_exact rank while (3) remains ck_identity.
+	    Therefore we compare (1) and (2) here.  For (1) we'll have
+
+	      ck_ref_bind <- ck_identity
+		int[] &	       int[1]
+
+	    so to handle this we must look at ref_conv.  */
+	  bool c1 = conv_binds_to_array_of_unknown_bound (ref_conv1);
+	  bool c2 = conv_binds_to_array_of_unknown_bound (ref_conv2);
+	  if (c1 && !c2)
+	    return -1;
+	  else if (!c1 && c2)
+	    return 1;
+
 	  int q1 = cp_type_quals (TREE_TYPE (ref_conv1->type));
 	  int q2 = cp_type_quals (TREE_TYPE (ref_conv2->type));
 	  if (ref_conv1->bad_p)
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 9ff617b..c1301a4 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -7372,6 +7372,10 @@ extern void cxx_print_error_function		(diagnostic_context *,
 						 struct diagnostic_info *);
 
 /* in typeck.c */
+/* Says how we should behave when comparing two arrays one of which
+   has unknown bounds.  */
+enum compare_bounds_t { bounds_none, bounds_either, bounds_first };
+
 extern bool cxx_mark_addressable		(tree, bool = false);
 extern int string_conv_p			(const_tree, const_tree, int);
 extern tree cp_truthvalue_conversion		(tree);
@@ -7462,7 +7466,7 @@ extern tree convert_for_initialization		(tree, tree, tree, int,
 						 impl_conv_rhs, tree, int,
                                                  tsubst_flags_t);
 extern int comp_ptr_ttypes			(tree, tree);
-extern bool comp_ptr_ttypes_const		(tree, tree);
+extern bool comp_ptr_ttypes_const		(tree, tree, compare_bounds_t);
 extern bool error_type_p			(const_tree);
 extern bool ptr_reasonably_similar		(const_tree, const_tree);
 extern tree build_ptrmemfunc			(tree, tree, int, bool,
diff --git a/gcc/cp/typeck.c b/gcc/cp/typeck.c
index ba334e7..a67dd4b 100644
--- a/gcc/cp/typeck.c
+++ b/gcc/cp/typeck.c
@@ -54,7 +54,7 @@ static tree rationalize_conditional_expr (enum tree_code, tree,
 					  tsubst_flags_t);
 static int comp_ptr_ttypes_real (tree, tree, int);
 static bool comp_except_types (tree, tree, bool);
-static bool comp_array_types (const_tree, const_tree, bool);
+static bool comp_array_types (const_tree, const_tree, compare_bounds_t, bool);
 static tree pointer_diff (location_t, tree, tree, tree, tsubst_flags_t, tree *);
 static tree get_delta_difference (tree, tree, bool, bool, tsubst_flags_t);
 static void casts_away_constness_r (tree *, tree *, tsubst_flags_t);
@@ -1084,11 +1084,15 @@ comp_except_specs (const_tree t1, const_tree t2, int exact)
   return exact == ce_derived || base == NULL_TREE || length == list_length (t1);
 }
 
-/* Compare the array types T1 and T2.  ALLOW_REDECLARATION is true if
-   [] can match [size].  */
+/* Compare the array types T1 and T2.  CB says how we should behave when
+   comparing array bounds: bounds_none doesn't allow dimensionless arrays,
+   bounds_either says than any array can be [], bounds_first means that
+   onlt T1 can be an array with unknown bounds.  STRICT is true if
+   qualifiers must match when comparing the types of the array elements.  */
 
 static bool
-comp_array_types (const_tree t1, const_tree t2, bool allow_redeclaration)
+comp_array_types (const_tree t1, const_tree t2, compare_bounds_t cb,
+		  bool strict)
 {
   tree d1;
   tree d2;
@@ -1098,7 +1102,9 @@ comp_array_types (const_tree t1, const_tree t2, bool allow_redeclaration)
     return true;
 
   /* The type of the array elements must be the same.  */
-  if (!same_type_p (TREE_TYPE (t1), TREE_TYPE (t2)))
+  if (strict
+      ? !same_type_p (TREE_TYPE (t1), TREE_TYPE (t2))
+      : !similar_type_p (TREE_TYPE (t1), TREE_TYPE (t2)))
     return false;
 
   d1 = TYPE_DOMAIN (t1);
@@ -1119,8 +1125,10 @@ comp_array_types (const_tree t1, const_tree t2, bool allow_redeclaration)
        declarations for an array object can specify
        array types that differ by the presence or absence of a major
        array bound (_dcl.array_).  */
-  if (!d1 || !d2)
-    return allow_redeclaration;
+  if (!d1 && d2)
+    return cb >= bounds_either;
+  else if (d1 && !d2)
+    return cb == bounds_either;
 
   /* Check that the dimensions are the same.  */
 
@@ -1368,7 +1376,9 @@ structural_comptypes (tree t1, tree t2, int strict)
 
     case ARRAY_TYPE:
       /* Target types must match incl. qualifiers.  */
-      if (!comp_array_types (t1, t2, !!(strict & COMPARE_REDECLARATION)))
+      if (!comp_array_types (t1, t2, ((strict & COMPARE_REDECLARATION)
+				      ? bounds_either : bounds_none),
+			     /*strict=*/true))
 	return false;
       break;
 
@@ -1549,10 +1559,10 @@ similar_type_p (tree type1, tree type2)
   if (same_type_ignoring_top_level_qualifiers_p (type1, type2))
     return true;
 
-  /* FIXME This ought to handle ARRAY_TYPEs too.  */
   if ((TYPE_PTR_P (type1) && TYPE_PTR_P (type2))
-      || (TYPE_PTRDATAMEM_P (type1) && TYPE_PTRDATAMEM_P (type2)))
-    return comp_ptr_ttypes_const (type1, type2);
+      || (TYPE_PTRDATAMEM_P (type1) && TYPE_PTRDATAMEM_P (type2))
+      || (TREE_CODE (type1) == ARRAY_TYPE && TREE_CODE (type2) == ARRAY_TYPE))
+    return comp_ptr_ttypes_const (type1, type2, bounds_either);
 
   return false;
 }
@@ -7867,7 +7877,7 @@ build_const_cast_1 (tree dst_type, tree expr, tsubst_flags_t complain,
 
   if (TYPE_PTR_P (src_type) || TYPE_PTRDATAMEM_P (src_type))
     {
-      if (comp_ptr_ttypes_const (dst_type, src_type))
+      if (comp_ptr_ttypes_const (dst_type, src_type, bounds_none))
 	{
 	  if (valid_p)
 	    {
@@ -9909,9 +9919,10 @@ comp_ptr_ttypes_real (tree to, tree from, int constp)
 			   TYPE_OFFSET_BASETYPE (to)))
 	return 0;
 
-      /* Const and volatile mean something different for function types,
-	 so the usual checks are not appropriate.  */
-      if (!FUNC_OR_METHOD_TYPE_P (to))
+      /* Const and volatile mean something different for function and
+	 array types, so the usual checks are not appropriate.  We'll
+	 check the array type elements in further iterations.  */
+      if (!FUNC_OR_METHOD_TYPE_P (to) && TREE_CODE (to) != ARRAY_TYPE)
 	{
 	  if (!at_least_as_qualified_p (to, from))
 	    return 0;
@@ -9930,7 +9941,17 @@ comp_ptr_ttypes_real (tree to, tree from, int constp)
       if (VECTOR_TYPE_P (to))
 	is_opaque_pointer = vector_targets_convertible_p (to, from);
 
-      if (!TYPE_PTR_P (to) && !TYPE_PTRDATAMEM_P (to))
+      /* P0388R4 allows a conversion from int[N] to int[] but not the
+	 other way round.  When both arrays have bounds but they do
+	 not match, then no conversion is possible.  */
+      if (TREE_CODE (to) == ARRAY_TYPE
+	  && !comp_array_types (to, from, bounds_first, /*strict=*/false))
+	return 0;
+
+      if (!TYPE_PTR_P (to)
+	  && !TYPE_PTRDATAMEM_P (to)
+	  /* CWG 330 says we need to look through arrays.  */
+	  && TREE_CODE (to) != ARRAY_TYPE)
 	return ((constp >= 0 || to_more_cv_qualified)
 		&& (is_opaque_pointer
 		    || same_type_ignoring_top_level_qualifiers_p (to, from)));
@@ -10033,10 +10054,10 @@ ptr_reasonably_similar (const_tree to, const_tree from)
 
 /* Return true if TO and FROM (both of which are POINTER_TYPEs or
    pointer-to-member types) are the same, ignoring cv-qualification at
-   all levels.  */
+   all levels.  CB says how we should behave when comparing array bounds.  */
 
 bool
-comp_ptr_ttypes_const (tree to, tree from)
+comp_ptr_ttypes_const (tree to, tree from, compare_bounds_t cb)
 {
   bool is_opaque_pointer = false;
 
@@ -10053,7 +10074,14 @@ comp_ptr_ttypes_const (tree to, tree from)
       if (VECTOR_TYPE_P (to))
 	is_opaque_pointer = vector_targets_convertible_p (to, from);
 
-      if (!TYPE_PTR_P (to))
+      if (TREE_CODE (to) == ARRAY_TYPE
+	  /* Ignore cv-qualification, but if we see e.g. int[3] and int[4],
+	     we must fail.  */
+	  && !comp_array_types (to, from, cb, /*strict=*/false))
+	return false;
+
+      /* CWG 330 says we need to look through arrays.  */
+      if (!TYPE_PTR_P (to) && TREE_CODE (to) != ARRAY_TYPE)
 	return (is_opaque_pointer
 		|| same_type_ignoring_top_level_qualifiers_p (to, from));
     }
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index f1e3a99..5ee2a81 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,5 +1,37 @@
 2019-10-09  Marek Polacek  <polacek@redhat.com>
 
+	PR c++/91364 - P0388R4: Permit conversions to arrays of	unknown bound.
+	PR c++/69531 - DR 1307: Differently bounded array parameters.
+	PR c++/88128 - DR 330: Qual convs and pointers to arrays of pointers.
+	* g++.dg/conversion/qual1.C: New test.
+	* g++.dg/conversion/qual2.C: New test.
+	* g++.dg/conversion/qual3.C: New test.
+	* g++.dg/conversion/ref2.C: New test.
+	* g++.dg/conversion/ref3.C: New test.
+	* g++.dg/cpp0x/initlist-array3.C: Remove dg-error.
+	* g++.dg/cpp0x/initlist-array7.C: New test.
+	* g++.dg/cpp0x/initlist-array8.C: New test.
+	* g++.dg/cpp2a/array-conv1.C: New test.
+	* g++.dg/cpp2a/array-conv10.C: New test.
+	* g++.dg/cpp2a/array-conv11.C: New test.
+	* g++.dg/cpp2a/array-conv12.C: New test.
+	* g++.dg/cpp2a/array-conv13.C: New test.
+	* g++.dg/cpp2a/array-conv14.C: New test.
+	* g++.dg/cpp2a/array-conv15.C: New test.
+	* g++.dg/cpp2a/array-conv16.C: New test.
+	* g++.dg/cpp2a/array-conv17.C: New test.
+	* g++.dg/cpp2a/array-conv2.C: New test.
+	* g++.dg/cpp2a/array-conv3.C: New test.
+	* g++.dg/cpp2a/array-conv4.C: New test.
+	* g++.dg/cpp2a/array-conv5.C: New test.
+	* g++.dg/cpp2a/array-conv6.C: New test.
+	* g++.dg/cpp2a/array-conv7.C: New test.
+	* g++.dg/cpp2a/array-conv8.C: New test.
+	* g++.dg/cpp2a/array-conv9.C: New test.
+	* g++.old-deja/g++.bugs/900321_01.C: Adjust dg-error.
+
+2019-10-09  Marek Polacek  <polacek@redhat.com>
+
 	PR c++/92032 - DR 1601: Promotion of enum with fixed underlying	type.
 	* g++.dg/cpp0x/scoped_enum10.C: New test.
 	* g++.dg/cpp0x/scoped_enum11.C: New test.
diff --git a/gcc/testsuite/g++.dg/conversion/qual1.C b/gcc/testsuite/g++.dg/conversion/qual1.C
new file mode 100644
index 0000000..5022da9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/conversion/qual1.C
@@ -0,0 +1,51 @@
+// PR c++/88128 - Implement DR 330: Qualification conversions and pointers to
+// arrays of pointers.
+
+int *a[4];
+const int *const(*ap1)[4] = &a;
+/* if at some level k the P2 is more cv-qualified than P1, then there
+   must be a const at every single level (other than level zero) of P2
+   up until k.  */
+const int *(*ap2)[4] = &a; // { dg-error "cannot convert" }
+int *const(*ap3)[4] = &a;
+int *(*ap4)[4] = &a;
+int *(*const ap5)[4] = &a;
+const int *const(*const ap6)[4] = &a;
+int *const(*const ap7)[4] = &a;
+int *(*const ap8)[4] = &a;
+
+const int *b[4];
+const int *const(*bp1)[4] = &b;
+const int *(*bp2)[4] = &b;
+int *const(*bp3)[4] = &b; // { dg-error "cannot convert" }
+int *(*bp4)[4] = &b; // { dg-error "cannot convert" }
+int *(*const bp5)[4] = &b; // { dg-error "cannot convert" }
+const int *const(*const bp6)[4] = &b;
+int *const(*const bp7)[4] = &b; // { dg-error "cannot convert" }
+int *(*const bp8)[4] = &b; // { dg-error "cannot convert" }
+
+int *c[2][3];
+int const *const (*cp1)[3] = c;
+int const *(*cp2)[3] = c; // { dg-error "cannot convert" }
+int const *const (*const cp3)[3] = c;
+int *const (*cp4)[3] = c;
+int *(*cp5)[3] = c;
+
+double *const (*d)[3];
+double const *const (*e)[3] = d;
+int *(*f)[3];
+const int *const (*g)[3] = f;
+
+// From PR88128.
+int* (*xx)[];
+const int* const(*yy)[] = xx;
+
+// From DR 330.
+int main()
+{
+   double *array2D[2][3];
+
+   double       *       (*array2DPtr1)[3] = array2D;
+   double       * const (*array2DPtr2)[3] = array2DPtr1;
+   double const * const (*array2DPtr3)[3] = array2DPtr2;
+}
diff --git a/gcc/testsuite/g++.dg/conversion/qual2.C b/gcc/testsuite/g++.dg/conversion/qual2.C
new file mode 100644
index 0000000..8a063a0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/conversion/qual2.C
@@ -0,0 +1,14 @@
+// PR c++/88128 - DR 330: Qual convs and pointers to arrays of pointers.
+
+// Make sure we don't accept different bounds.
+
+int *a[4];
+const int *const(*ap1)[5] = &a; // { dg-error "cannot convert" }
+
+int *(*b)[3];
+const int *const (*bp1)[3] = &b; // { dg-error "cannot convert" }
+const int *const (*bp2)[4] = &b; // { dg-error "cannot convert" }
+int *(*bp3)[4] = &b; // { dg-error "cannot convert" }
+
+int *c[2][3];
+int const *const (*cp1)[4] = c; // { dg-error "cannot convert" }
diff --git a/gcc/testsuite/g++.dg/conversion/qual3.C b/gcc/testsuite/g++.dg/conversion/qual3.C
new file mode 100644
index 0000000..4b466d9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/conversion/qual3.C
@@ -0,0 +1,53 @@
+// PR c++/88128 - DR 330: Qual convs and pointers to arrays of pointers.
+// { dg-do compile { target c++17 } }
+
+using P = int *(*)[3];
+using Q = const int *const (*)[3];
+using Qi = const int *[3];
+using Q2 = Qi const *;
+using R = const int *const (*)[4];
+using S = const int *const (*)[];
+using T = const int *(*)[];
+
+void
+f (P p, Q q, Q2 q2, R r, S s, T t)
+{
+  q = p;
+  q2 = p;
+  r = p; // { dg-error "cannot convert" }
+  t = p; // { dg-error "cannot convert" }
+  s = t;
+  t = s; // { dg-error "invalid conversion" }
+
+  // Test const_cast.
+  const_cast<P>(q);
+  const_cast<P>(q2);
+  const_cast<Q>(p);
+  const_cast<Q2>(p);
+  const_cast<S>(p); // { dg-error "invalid .const_cast." }
+  const_cast<P>(s); // { dg-error "invalid .const_cast." }
+  const_cast<S>(q); // { dg-error "invalid .const_cast." }
+  const_cast<S>(q2); // { dg-error "invalid .const_cast." }
+  const_cast<Q>(s); // { dg-error "invalid .const_cast." }
+  const_cast<Q2>(s); // { dg-error "invalid .const_cast." }
+  const_cast<T>(s);
+  const_cast<S>(t);
+  const_cast<T>(q); // { dg-error "invalid .const_cast." }
+  const_cast<Q>(t); // { dg-error "invalid .const_cast." }
+
+  // Test reinterpret_cast.
+  reinterpret_cast<P>(q); // { dg-error "casts away qualifiers" }
+  reinterpret_cast<P>(q2); // { dg-error "casts away qualifiers" }
+  reinterpret_cast<Q>(p);
+  reinterpret_cast<Q2>(p);
+  reinterpret_cast<S>(p);
+  reinterpret_cast<P>(s); // { dg-error "casts away qualifiers" }
+  reinterpret_cast<S>(q);
+  reinterpret_cast<S>(q2);
+  reinterpret_cast<Q>(s);
+  reinterpret_cast<Q2>(s);
+  reinterpret_cast<T>(s); // { dg-error "casts away qualifiers" }
+  reinterpret_cast<S>(t);
+  reinterpret_cast<T>(q); // { dg-error "casts away qualifiers" }
+  reinterpret_cast<Q>(t);
+}
diff --git a/gcc/testsuite/g++.dg/conversion/ref2.C b/gcc/testsuite/g++.dg/conversion/ref2.C
new file mode 100644
index 0000000..418e711
--- /dev/null
+++ b/gcc/testsuite/g++.dg/conversion/ref2.C
@@ -0,0 +1,29 @@
+// PR c++/88128 - Implement DR 330: Qualification conversions and pointers to
+// arrays of pointers.
+
+int *ar[4];
+/* if at some level k the P2 is more cv-qualified than P1, then there
+   must be a const at every single level (other than level zero) of P2
+   up until k.  */
+const int *(&arp)[4] = ar; // { dg-error "discards qualifiers" }
+const int *const(&arp2)[4] = ar;
+int *const(&arp3)[4] = ar;
+int *(&arp4)[4] = ar;
+
+const int *br[4];
+const int *(&brp)[4] = br;
+const int *const(&brp2)[4] = br;
+int *const(&brp3)[4] = br; // { dg-error "discards qualifiers" }
+int *(&brp4)[4] = br; // { dg-error "discards qualifiers" }
+
+int *c[2][3];
+int const *const (&cp1)[3] = *c;
+int const *(&cp2)[3] = *c; // { dg-error "discards qualifiers" }
+int *const (&cp3)[3] = *c;
+int *(&cp4)[3] = *c;
+
+double *const (*d)[3];
+double const *const (&e)[3] = *d;
+
+int *(*f)[3];
+const int *const (&g)[3] = *f;
diff --git a/gcc/testsuite/g++.dg/conversion/ref3.C b/gcc/testsuite/g++.dg/conversion/ref3.C
new file mode 100644
index 0000000..ca5326c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/conversion/ref3.C
@@ -0,0 +1,4 @@
+int a[2];
+const int (&rc)[2] = a;
+volatile int (&rv)[2] = a;
+const volatile int (&rcv)[2] = a;
diff --git a/gcc/testsuite/g++.dg/cpp0x/initlist-array3.C b/gcc/testsuite/g++.dg/cpp0x/initlist-array3.C
index 1a94f4e..4140cd9 100644
--- a/gcc/testsuite/g++.dg/cpp0x/initlist-array3.C
+++ b/gcc/testsuite/g++.dg/cpp0x/initlist-array3.C
@@ -6,5 +6,6 @@ void composite (int const (&) [3]);
 
 int main ()
 {
-  composite({0,1});		// { dg-error "ambiguous" }
+  // Not ambiguous since CWG 1307.
+  composite({0,1});
 }
diff --git a/gcc/testsuite/g++.dg/cpp0x/initlist-array7.C b/gcc/testsuite/g++.dg/cpp0x/initlist-array7.C
new file mode 100644
index 0000000..7a689c66
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/initlist-array7.C
@@ -0,0 +1,21 @@
+// PR c++/69531 - DR 1307, Overload resolution based on size of array init-list.
+// { dg-do run { target c++11 } }
+
+int f(int const(&)[2]) { return 1; }
+int f(int const(&)[3]) { return 2; }
+
+int
+main ()
+{
+   if (f({}) != 1)
+    __builtin_abort ();
+
+   if (f({1}) != 1)
+    __builtin_abort ();
+
+   if (f({1, 2}) != 1)
+    __builtin_abort ();
+
+   if (f({1, 2, 3}) != 2)
+    __builtin_abort ();
+}
diff --git a/gcc/testsuite/g++.dg/cpp0x/initlist-array8.C b/gcc/testsuite/g++.dg/cpp0x/initlist-array8.C
new file mode 100644
index 0000000..ac2774e
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/initlist-array8.C
@@ -0,0 +1,35 @@
+// PR c++/69531 - DR 1307, Overload resolution based on size of array init-list.
+// { dg-do run { target c++2a } }
+
+int f(int (&)[1][1]) { return 1; }
+int f(int (&)[1][2]) { return 2; }
+
+int g(int (&&)[2][1]) { return 1; }
+int g(int (&&)[2][2]) { return 2; }
+
+int h(int (&&)[][1]) { return 1; }
+int h(int (&&)[][2]) { return 2; }
+
+int
+main ()
+{
+  int arr1[1][1];
+  int arr2[1][2];
+
+  if (f(arr1) != 1)
+    __builtin_abort ();
+  if (f(arr2) != 2)
+    __builtin_abort ();
+
+  if (g({ { 1, 2 }, { 3 } }) != 2)
+    __builtin_abort ();
+
+  if (g({ { 1, 2 }, { 3, 4 } }) != 2)
+    __builtin_abort ();
+
+  if (h({ { 1, 2 }, { 3 } }) != 2)
+    __builtin_abort ();
+
+  if (h({ { 1, 2 }, { 3, 4 } }) != 2)
+    __builtin_abort ();
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/array-conv1.C b/gcc/testsuite/g++.dg/cpp2a/array-conv1.C
new file mode 100644
index 0000000..e90b340
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/array-conv1.C
@@ -0,0 +1,33 @@
+// PR c++/91364 - Implement P0388R4: Permit conversions to arrays of unknown bound.
+// { dg-do compile { target c++17 } }
+// { dg-options "-Wpedantic" }
+// C++17, because that has CWG 393.
+
+void f(int(&)[]);
+void fp(int(*)[]);
+void f2(int(&)[][10]);
+void fp2(int(*)[][10]);
+int arr[10];
+int arr2[10][10];
+
+void
+g ()
+{
+  f (arr); // { dg-warning "conversions to arrays of unknown bound are only available" "" { target c++17_down } }
+  fp (&arr); // { dg-warning "conversions to arrays of unknown bound are only available" "" { target c++17_down } }
+  f2 (arr2);// { dg-warning "conversions to arrays of unknown bound are only available" "" { target c++17_down } }
+  fp2 (&arr2);// { dg-warning "conversions to arrays of unknown bound are only available" "" { target c++17_down } }
+}
+
+int(&r1)[] = arr;// { dg-warning "conversions to arrays of unknown bound are only available" "" { target c++17_down } }
+int(&r2)[10] = arr;
+int(&r3)[][10] = arr2;// { dg-warning "conversions to arrays of unknown bound are only available" "" { target c++17_down } }
+/* Note that
+   int (&r)[10][] = arr2;
+   is invalid.  */
+int(&r4)[10][10] = arr2;
+
+int(*p1)[] = &arr;// { dg-warning "conversions to arrays of unknown bound are only available" "" { target c++17_down } }
+int(*p2)[10] = &arr;
+int(*p3)[][10] = &arr2;// { dg-warning "conversions to arrays of unknown bound are only available" "" { target c++17_down } }
+int(*p4)[10][10] = &arr2;
diff --git a/gcc/testsuite/g++.dg/cpp2a/array-conv10.C b/gcc/testsuite/g++.dg/cpp2a/array-conv10.C
new file mode 100644
index 0000000..1ee1a77
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/array-conv10.C
@@ -0,0 +1,22 @@
+// PR c++/91364 - Implement P0388R4: Permit conversions to arrays of unknown bound.
+// { dg-do compile { target c++17 } }
+// { dg-options "-Wpedantic" }
+
+// The other direction: converting from int[] to int(&)[3] is forbidden.
+
+extern int a[];
+extern int (*b)[];
+extern int (&c)[];
+int (&y)[] = a;
+int (&x)[3] = y; // { dg-error "cannot bind reference" }
+int (&z)[3] = a; // { dg-error "cannot bind reference" }
+
+void f(int (*)[3]);
+void f2(int (&)[3]);
+
+void
+test ()
+{
+  f(b); // { dg-error "cannot convert" }
+  f2(c); // { dg-error "cannot bind reference" }
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/array-conv11.C b/gcc/testsuite/g++.dg/cpp2a/array-conv11.C
new file mode 100644
index 0000000..a072b29
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/array-conv11.C
@@ -0,0 +1,23 @@
+// PR c++/91364 - Implement P0388R4: Permit conversions to arrays of unknown bound.
+// { dg-do compile { target c++2a } }
+// { dg-options "-Wpedantic" }
+
+// Test flexible array member.  Here we're binding int[] to int[].  This worked
+// even before P0388R4.
+
+typedef int T[];
+extern T arr;
+T &t1 = arr;
+
+struct S {
+  int i;
+  int a[]; // { dg-warning "flexible array member" }
+};
+
+void f (int (&)[]);
+
+void
+test (S s)
+{
+  f (s.a);
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/array-conv12.C b/gcc/testsuite/g++.dg/cpp2a/array-conv12.C
new file mode 100644
index 0000000..1156ea3
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/array-conv12.C
@@ -0,0 +1,12 @@
+// PR c++/91364 - Implement P0388R4: Permit conversions to arrays of unknown bound.
+// { dg-do compile { target c++2a } }
+// { dg-options "-Wpedantic" }
+
+int arr[1] = { 42 };
+int(&r)[]{arr};
+int(&r2)[] = {arr};
+int(&&r3)[]{};
+int(&&r4)[]{42};
+int(&&r5)[] = {};
+int(&&r6)[] = {42};
+int(&r7)[](arr); // { dg-warning "conversions to arrays of unknown bound are only available" "" { target c++17_down } }
diff --git a/gcc/testsuite/g++.dg/cpp2a/array-conv13.C b/gcc/testsuite/g++.dg/cpp2a/array-conv13.C
new file mode 100644
index 0000000..9908b7e
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/array-conv13.C
@@ -0,0 +1,17 @@
+// PR c++/91364 - Implement P0388R4: Permit conversions to arrays of unknown bound.
+// { dg-do compile { target c++2a } }
+
+template <typename T> void foo(T);
+
+template <typename F, typename T, typename = decltype(foo<T>(F()))>
+void test(int) { }
+
+// No other overload, so if the above fails because of the conversion,
+// we fail.
+
+void
+fn ()
+{
+  test<int(*)[2], int(*)[]>(0);
+  test<int(*)[], int(*)[]>(0);
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/array-conv14.C b/gcc/testsuite/g++.dg/cpp2a/array-conv14.C
new file mode 100644
index 0000000..793e85d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/array-conv14.C
@@ -0,0 +1,17 @@
+// PR c++/91364 - Implement P0388R4: Permit conversions to arrays of unknown bound.
+// { dg-do compile { target c++2a } }
+
+void f(const int(*)[]);
+void fb(const int(*)[3]);
+void f2(const int(&)[]);
+void fb2(const int(&)[3]);
+
+void
+g ()
+{
+  int arr[3];
+  f(&arr);
+  fb(&arr);
+  f2(arr);
+  fb2(arr);
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/array-conv15.C b/gcc/testsuite/g++.dg/cpp2a/array-conv15.C
new file mode 100644
index 0000000..033a746
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/array-conv15.C
@@ -0,0 +1,23 @@
+// PR c++/69531 - DR 1307, Overload resolution based on size of array init-list.
+// { dg-do run { target c++2a } }
+
+int f(int, int const(&)[2]) { return 1; }
+int f(double, int const(&)[2]) { return 2; }
+
+int f2(int, int const(&)[1]) { return 1; }
+int f2(int, int const(&)[2]) { return 2; }
+
+int f3(int, int const(&)[]) { return 1; }
+int f3(double, int const(&)[]) { return 2; }
+
+int main ()
+{
+  if (f (1, {1}) != 1)
+    __builtin_abort ();
+
+  if (f2 (1, {1}) != 1)
+    __builtin_abort ();
+
+  if (f3 (1, {1}) != 1)
+    __builtin_abort ();
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/array-conv16.C b/gcc/testsuite/g++.dg/cpp2a/array-conv16.C
new file mode 100644
index 0000000..bfb39d1
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/array-conv16.C
@@ -0,0 +1,16 @@
+// PR c++/91364 - P0388R4: Permit conversions to arrays of unknown bound.
+// { dg-do compile { target c++2a } }
+
+using P = int *(*)[3];
+using S = const int *const (*)[];
+using Q = const int *const (*)[3];
+using Qi = const int *[3];
+using Q2 = Qi const *;
+
+void
+f (P p, S s, Q q, Q2 q2)
+{
+  s = p;
+  s = q;
+  s = q2;
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/array-conv17.C b/gcc/testsuite/g++.dg/cpp2a/array-conv17.C
new file mode 100644
index 0000000..3313ed4
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/array-conv17.C
@@ -0,0 +1,39 @@
+// PR c++/91364 - P0388R4: Permit conversions to arrays of unknown bound.
+// { dg-do compile { target c++2a } }
+
+// As conversion/qual1.C, but with [].
+
+int *a[4];
+const int *const(*ap1)[] = &a;
+/* if at some level k the P2 is more cv-qualified than P1, then there
+   must be a const at every single level (other than level zero) of P2
+   up until k.  */
+const int *(*ap2)[] = &a; // { dg-error "cannot convert" }
+int *const(*ap3)[] = &a;
+int *(*ap4)[] = &a;
+int *(*const ap5)[] = &a;
+const int *const(*const ap6)[] = &a;
+int *const(*const ap7)[] = &a;
+int *(*const ap8)[] = &a;
+
+const int *b[4];
+const int *const(*bp1)[] = &b;
+const int *(*bp2)[] = &b;
+int *const(*bp3)[] = &b; // { dg-error "cannot convert" }
+int *(*bp4)[] = &b; // { dg-error "cannot convert" }
+int *(*const bp5)[] = &b; // { dg-error "cannot convert" }
+const int *const(*const bp6)[] = &b;
+int *const(*const bp7)[] = &b; // { dg-error "cannot convert" }
+int *(*const bp8)[] = &b; // { dg-error "cannot convert" }
+
+int *c[2][3];
+int const *const (*cp1)[] = c;
+int const *(*cp2)[] = c; // { dg-error "cannot convert" }
+int const *const (*const cp3)[] = c;
+int *const (*cp4)[] = c;
+int *(*cp5)[] = c;
+
+double *const (*d)[3];
+double const *const (*e)[] = d;
+int *(*f)[3];
+const int *const (*g)[] = f;
diff --git a/gcc/testsuite/g++.dg/cpp2a/array-conv2.C b/gcc/testsuite/g++.dg/cpp2a/array-conv2.C
new file mode 100644
index 0000000..5245d83
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/array-conv2.C
@@ -0,0 +1,26 @@
+// PR c++/91364 - Implement P0388R4: Permit conversions to arrays of unknown bound.
+// { dg-do compile { target c++2a } }
+
+struct A {
+  A();
+  A(const A(&)[2]);
+};
+
+using T = A[];
+using U = A[2];
+
+// t binds directly to U{} now.  Before it bound indirectly to a temporary
+// A{U{}}.  ??? But we don't do it now; see reference_binding and the 
+// BRACE_ENCLOSED_INITIALIZER_P block.
+A (&&t)[] = {U{}};
+
+U u{};
+
+T &
+foo ()
+{
+  // This didn't compile before P0388R4: invalid initialization of non-const
+  // reference of type 'A (&)[]' from an rvalue of type
+  // '<brace-enclosed initializer list>'.
+  return {u};
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/array-conv3.C b/gcc/testsuite/g++.dg/cpp2a/array-conv3.C
new file mode 100644
index 0000000..3d92b40
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/array-conv3.C
@@ -0,0 +1,26 @@
+// PR c++/91364 - Implement P0388R4: Permit conversions to arrays of unknown bound.
+// { dg-do run { target c++2a } }
+
+// Ranking of reference initialization conversions
+
+int f(int(&)[]) { return 1; }	    // (1)
+int f(int(&)[1]) { return 2; }	    // (2)
+
+int h(int(*)[]) { return 1; }	    // (a)
+int h(int(*)[1]) { return 2; }	    // (b)
+
+// From P0388R4:
+// (2) and (b) should clearly be better than (1) and (a), respectively,
+// as the former overloads are more restricted. 
+// (a) should be worse than (b), which is implied by (a) necessitating
+// a qualification conversion in that case.
+
+int
+main ()
+{
+  int arr[1];
+  if (f(arr) != 2)
+    __builtin_abort ();
+  if (h(&arr) != 2)
+    __builtin_abort ();
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/array-conv4.C b/gcc/testsuite/g++.dg/cpp2a/array-conv4.C
new file mode 100644
index 0000000..979c69b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/array-conv4.C
@@ -0,0 +1,24 @@
+// PR c++/91364 - Implement P0388R4: Permit conversions to arrays of unknown bound.
+// { dg-do compile { target c++2a } }
+
+// Ranking of reference initialization conversions
+
+void f(int(&)[]) {}	    // (1)
+//void f(int(&)[1]) { }	    // (2)
+void f(int*) { }	    // (3)
+
+//void f2(int(&)[]) { }	    // (1)
+void f2(int(&)[1]) { }	    // (2)
+void f2(int*) { }	    // (3)
+
+// From P0388R4:
+// (3) should be equal to (1) (as it is to (2))
+// Check that we get "ambiguous overload" errors.
+
+void
+doit ()
+{
+  int arr[1];
+  f(arr); // { dg-error "ambiguous" }
+  f2(arr); // { dg-error "ambiguous" }
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/array-conv5.C b/gcc/testsuite/g++.dg/cpp2a/array-conv5.C
new file mode 100644
index 0000000..34678f5
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/array-conv5.C
@@ -0,0 +1,24 @@
+// PR c++/91364 - Implement P0388R4: Permit conversions to arrays of unknown bound.
+// { dg-do run { target c++2a } }
+
+// Ranking of list-initialization sequences
+int b(int   (&&)[] ) { return 1; }   // #1
+int b(long  (&&)[] ) { return 2; }   // #2
+int b(int   (&&)[1]) { return 3; }   // #3
+int b(long  (&&)[1]) { return 4; }   // #4
+int b(int   (&&)[2]) { return 5; }   // #5
+
+/* Here,
+   -- #1, #3 and #5 should rank better than both #2 and #4, as no promotion
+      is necessitated.
+   -- #1 should rank worse than #3, being far less specialized.
+   -- #1 should rank better than #5, as the latter requires a larger array
+      temporary.  (#3 also ranks better than #5 for the same reason--cf. core
+      issue 1307).  */
+
+int
+main ()
+{
+  if (b({1}) != 3)
+    __builtin_abort ();
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/array-conv6.C b/gcc/testsuite/g++.dg/cpp2a/array-conv6.C
new file mode 100644
index 0000000..c2389c8
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/array-conv6.C
@@ -0,0 +1,28 @@
+// PR c++/91364 - Implement P0388R4: Permit conversions to arrays of unknown bound.
+// { dg-do run { target c++2a } }
+
+// Ranking of reference initialization conversions
+
+int f1(const int(&)[]) { return 1; }
+int f1(const int(&)[1]) { return 2; }
+
+int f2(const int(&)[]) { return 1; }
+int f2(int(&)[1]) { return 2; }
+
+int f3(int(&)[]) { return 1; }
+int f3(const int(&)[1]) { return 2; }
+
+const int arr[1] = { 42 };
+
+int
+main ()
+{
+  if (f1(arr) != 2)
+    __builtin_abort ();
+
+  if (f2(arr) != 1)
+    __builtin_abort ();
+
+  if (f3(arr) != 2)
+    __builtin_abort ();
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/array-conv7.C b/gcc/testsuite/g++.dg/cpp2a/array-conv7.C
new file mode 100644
index 0000000..07c709f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/array-conv7.C
@@ -0,0 +1,34 @@
+// PR c++/69531 - DR 1307, Overload resolution based on size of array init-list.
+// { dg-do run { target c++2a } }
+
+int f(int const(&)[]) { return 1; }
+int f(int const(&)[2]) { return 2; }
+
+int f2(int const(&)[]) { return 1; }
+int f2(int const(&)[1]) { return 2; }
+
+int f3(int const(&)[]) { return 1; }
+int f3(int const(&)[1]) { return 2; }
+int f3(int const(&)[2]) { return 3; }
+
+int main ()
+{
+  if (f ({}) != 1)
+    __builtin_abort ();
+  if (f ({1}) != 1)
+    __builtin_abort ();
+  if (f ({1, 2}) != 2)
+    __builtin_abort ();
+
+  if (f2 ({}) != 1)
+    __builtin_abort ();
+  if (f2 ({1}) != 2)
+    __builtin_abort ();
+
+  if (f3 ({}) != 1)
+    __builtin_abort ();
+  if (f3 ({1}) != 2)
+    __builtin_abort ();
+  if (f3 ({1, 2}) != 3)
+    __builtin_abort ();
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/array-conv8.C b/gcc/testsuite/g++.dg/cpp2a/array-conv8.C
new file mode 100644
index 0000000..635c767
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/array-conv8.C
@@ -0,0 +1,26 @@
+// PR c++/69531 - DR 1307, Overload resolution based on size of array init-list.
+// { dg-do run { target c++2a } }
+// Example from [over.ics.rank].
+
+int f(int    (&&)[] ) { return 1; }    // #1
+int f(double (&&)[] ) { return 2; }    // #2
+int f(int    (&&)[2]) { return 3; }    // #3
+
+int
+main ()
+{
+  // Calls #1: Better than #2 due to conversion, better than #3 due to bounds.
+  if (f({1}) != 1)
+     __builtin_abort ();
+  // Calls #2: Identity conversion is better than floating-integral conversion.
+  if (f({1.0}) != 2)
+     __builtin_abort ();
+  // Calls #2: Identity conversion is better than floating-integral conversion.
+  if (f({1.0, 2.0}) != 2)
+     __builtin_abort ();
+  // Calls #3: Converting to array of known bound is better than to unknown
+  // bound, and an identity conversion is better than floating-integral
+  // conversion.
+  if (f({1, 2}) != 3)
+     __builtin_abort ();
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/array-conv9.C b/gcc/testsuite/g++.dg/cpp2a/array-conv9.C
new file mode 100644
index 0000000..82f615d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/array-conv9.C
@@ -0,0 +1,27 @@
+// PR c++/91364 - Implement P0388R4: Permit conversions to arrays of unknown bound.
+// { dg-do compile { target c++2a } }
+
+int arr[1];
+extern int arr2[];
+
+void
+test ()
+{
+  int (&r)[1] = const_cast<int(&)[1]>(arr);
+  int (&r2)[] = const_cast<int(&)[]>(arr); // { dg-error "invalid" }
+  int (&r3)[1] = (int(&)[1]) arr;
+  int (&r4)[] = (int(&)[]) arr;
+  int (&r5)[1] = static_cast<int(&)[1]>(arr);
+  int (&r6)[] = static_cast<int(&)[]>(arr);
+
+  // Try c_cast_p.
+  int(*p1)[] = (int(*)[]) &arr;
+  int(*p2)[1] = (int(*)[]) &arr; // { dg-error "cannot convert" }
+  int(*p3)[] = (int(*)[1]) &arr;
+  int(*p4)[] = (int(*)[1]) &arr2;
+  int(*p5)[] = (int(*)[]) (int(*)[1]) &arr;
+  int(*p6)[] = (int(*)[1]) (int(*)[]) &arr;
+  int(*p7)[] = static_cast<int(*)[]>(&arr);
+  int(*p8)[] = static_cast<int(*)[1]>(&arr);
+  int(*p9)[] = static_cast<int(*)[1]>(&arr2); // { dg-error "invalid" }
+}
diff --git a/gcc/testsuite/g++.old-deja/g++.bugs/900321_01.C b/gcc/testsuite/g++.old-deja/g++.bugs/900321_01.C
index 6b52783..c3b1ab5 100644
--- a/gcc/testsuite/g++.old-deja/g++.bugs/900321_01.C
+++ b/gcc/testsuite/g++.old-deja/g++.bugs/900321_01.C
@@ -20,7 +20,7 @@ void function_0 ()
 {
   // we miss the first two because typeck.c (comp_array_types) deems
   // it okay if one of the sizes is null
-  ptr_to_array_of_ints = ptr_to_array_of_3_ints;	// { dg-error "" } 
+  ptr_to_array_of_ints = ptr_to_array_of_3_ints;	// { dg-error "conversions to arrays" "" { target c++17_down } }
   ptr_to_array_of_3_ints = ptr_to_array_of_ints;	// { dg-error "" } 
 
   ptr_to_array_of_3_ints = ptr_to_array_of_5_ints;	// { dg-error "" } 
diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index 045fbe3..8b7443c 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,3 +1,11 @@
+2019-10-09  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/91364 - P0388R4: Permit conversions to arrays of	unknown bound.
+	PR c++/69531 - DR 1307: Differently bounded array parameters.
+	PR c++/88128 - DR 330: Qual convs and pointers to arrays of pointers.
+	* testsuite/23_containers/span/lwg3255.cc: Adjust test to match the
+	post-P0388R4 behavior.
+
 2019-10-09  Jonathan Wakely  <jwakely@redhat.com>
 
 	PR libstdc++/91057
diff --git a/libstdc++-v3/testsuite/23_containers/span/lwg3255.cc b/libstdc++-v3/testsuite/23_containers/span/lwg3255.cc
index 638c881..bab7da3 100644
--- a/libstdc++-v3/testsuite/23_containers/span/lwg3255.cc
+++ b/libstdc++-v3/testsuite/23_containers/span/lwg3255.cc
@@ -28,8 +28,7 @@ using std::is_constructible_v;
 
 // LWG 3255 span's array constructor is too strict
 
-// FIXME: remove '!' from next line when P0388R4 is implemented:
-static_assert( ! is_constructible_v<span<const int* const>, array<int*, 2>> );
+static_assert( is_constructible_v<span<const int* const>, array<int*, 2>> );
 static_assert( is_constructible_v<span<const int>, array<const int, 4>> );
 
 static_assert( is_constructible_v<span<int, 1>, int(&)[1]> );