c++: Allow subobject references in C++20.

The last new thing allowed by P1907R1: subobject addresses as template
arguments.  The ABI group has discussed mangling for this; there has been
some talk of a compressed subobject mangling, but it hasn't been finalized,
so for now I'm using normal expression mangling.  In order for two array
subobject references to compare as equal template arguments, the offsets
need to have the same type, so I convert them to always be the same type,
currently ptrdiff_t.  Base class conversions are represented as a cast to
reference type, only if necessary to resolve an ambiguity.

This patch also updates the value of __cpp_nontype_template_args, since
the paper is fully implemented.

gcc/cp/ChangeLog:

	* mangle.c (write_base_ref): New.
	(write_expression): Use it for base field COMPONENT_REFs.
	* pt.c (invalid_tparm_referent_p): Canonicalize the type
	of array offsets.  Allow subobjects.

gcc/c-family/ChangeLog:

	* c-cppbuiltin.c (c_cpp_builtins): Update
	__cpp_nontype_template_args for C++20.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/nontype2.C: No error in C++20.
	* g++.dg/template/nontype25.C: No error in C++20.
	* g++.dg/template/nontype8.C: No error in C++20.
	* g++.dg/cpp2a/nontype-subob1.C: New test.
	* g++.dg/cpp2a/nontype-subob2.C: New test.
	* g++.dg/cpp1z/nontype3.C: Now C++17-only.
	* g++.dg/cpp2a/feat-cxx2a.C: Adjust expected value.

2 files changed, 76 insertions, 4 deletions

diff --git a/gcc/cp/mangle.c b/gcc/cp/mangle.c
index ab2d8ec..43ff2e8 100644
--- a/gcc/cp/mangle.c
+++ b/gcc/cp/mangle.c
@@ -2850,6 +2850,60 @@ write_member_name (tree member)
     write_expression (member);
 }
 
+/* EXPR is a base COMPONENT_REF; write the minimized base conversion path for
+   converting to BASE, or just the conversion of EXPR if BASE is null.
+
+   "Given a fully explicit base path P := C_n -> ... -> C_0, the minimized base
+   path Min(P) is defined as follows: let C_i be the last element for which the
+   conversion to C_0 is unambiguous; if that element is C_n, the minimized path
+   is C_n -> C_0; otherwise, the minimized path is Min(C_n -> ... -> C_i) ->
+   C_0."
+
+   We mangle the conversion to C_i if it's different from C_n.  */
+
+static bool
+write_base_ref (tree expr, tree base = NULL_TREE)
+{
+  if (TREE_CODE (expr) != COMPONENT_REF)
+    return false;
+
+  tree field = TREE_OPERAND (expr, 1);
+
+  if (TREE_CODE (field) != FIELD_DECL || !DECL_FIELD_IS_BASE (field))
+    return false;
+
+  tree object = TREE_OPERAND (expr, 0);
+
+  tree binfo = NULL_TREE;
+  if (base)
+    {
+      tree cur = TREE_TYPE (object);
+      binfo = lookup_base (cur, base, ba_unique, NULL, tf_none);
+    }
+  else
+    /* We're at the end of the base conversion chain, so it can't be
+       ambiguous.  */
+    base = TREE_TYPE (field);
+
+  if (binfo == error_mark_node)
+    {
+      /* cur->base is ambiguous, so make the conversion to
+	 last explicit, expressed as a cast (last&)object.  */
+      tree last = TREE_TYPE (expr);
+      write_string (OVL_OP_INFO (false, CAST_EXPR)->mangled_name);
+      write_type (build_reference_type (last));
+      write_expression (object);
+    }
+  else if (write_base_ref (object, base))
+    /* cur->base is unambiguous, but we had another base conversion
+       underneath and wrote it out.  */;
+  else
+    /* No more base conversions, just write out the object.  */
+    write_expression (object);
+
+  return true;
+}
+
 /* <expression> ::= <unary operator-name> <expression>
 		::= <binary operator-name> <expression> <expression>
 		::= <expr-primary>
@@ -3268,6 +3322,8 @@ write_expression (tree expr)
 	      ob = TREE_OPERAND (ob, 0);
 	      write_expression (ob);
 	    }
+	  else if (write_base_ref (expr))
+	    return;
 	  else if (!is_dummy_object (ob))
 	    {
 	      write_string ("dt");
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 5f43e9c..cfe5dcd 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -6907,7 +6907,9 @@ template_parm_object_p (const_tree t)
 }
 
 /* Subroutine of convert_nontype_argument, to check whether EXPR, as an
-   argument for TYPE, points to an unsuitable object.  */
+   argument for TYPE, points to an unsuitable object.
+
+   Also adjust the type of the index in C++20 array subobject references.  */
 
 static bool
 invalid_tparm_referent_p (tree type, tree expr, tsubst_flags_t complain)
@@ -6935,6 +6937,19 @@ invalid_tparm_referent_p (tree type, tree expr, tsubst_flags_t complain)
       {
 	tree decl = TREE_OPERAND (expr, 0);
 
+	if (cxx_dialect >= cxx20)
+	  while (TREE_CODE (decl) == COMPONENT_REF
+		 || TREE_CODE (decl) == ARRAY_REF)
+	    {
+	      tree &op = TREE_OPERAND (decl, 1);
+	      if (TREE_CODE (decl) == ARRAY_REF
+		  && TREE_CODE (op) == INTEGER_CST)
+		/* Canonicalize array offsets to ptrdiff_t; how they were
+		   written doesn't matter for subobject identity.  */
+		op = fold_convert (ptrdiff_type_node, op);
+	      decl = TREE_OPERAND (decl, 0);
+	    }
+
 	if (!VAR_P (decl))
 	  {
 	    if (complain & tf_error)
@@ -6976,9 +6991,10 @@ invalid_tparm_referent_p (tree type, tree expr, tsubst_flags_t complain)
 		     decl);
 	    return true;
 	  }
-	else if (!same_type_ignoring_top_level_qualifiers_p
-		 (strip_array_types (TREE_TYPE (type)),
-		  strip_array_types (TREE_TYPE (decl))))
+	else if (cxx_dialect < cxx20
+		 && !(same_type_ignoring_top_level_qualifiers_p
+		      (strip_array_types (TREE_TYPE (type)),
+		       strip_array_types (TREE_TYPE (decl)))))
 	  {
 	    if (complain & tf_error)
 	      error ("the address of the %qT subobject of %qD is not a "