Go patch committedd: Don't do implicit double pointer indirection

[Ian Lance Taylor]

The Go frontend code that lowers field references could sometimes
introduce a double pointer indirection in cases where it is not/safe
appropriate.  For example, in

        var p **struct { f int }
        p.f = 0

the assignment LHS was being incorrectly lowered to (*(*p)).f.  This
patch by Than McIntosh detects this situation and issues an error
message.

This fixes https://golang.org/issue/21770.  A test will be committed
to the master testsuite in https://golang.org/cl/62331.

Bootstrapped and ran Go testsuite on x86_64-pc-linux-gnu.  Committed
to mainline.

Ian

Index: gcc/go/gofrontend/MERGE
===================================================================
--- gcc/go/gofrontend/MERGE     (revision 251574)
+++ gcc/go/gofrontend/MERGE     (working copy)
@@ -1,4 +1,4 @@
-9d0d5c03a8086f5dd3a23e910abd6e470196973c
+52ebad939927e6cbfb48dd277cef8db451e36533
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.
Index: gcc/go/gofrontend/types.cc
===================================================================
--- gcc/go/gofrontend/types.cc  (revision 251533)
+++ gcc/go/gofrontend/types.cc  (working copy)
@@ -11829,6 +11829,12 @@ Type::bind_field_or_method(Gogo* gogo, c
          go_assert(st != NULL);
          if (type->struct_type() == NULL)
            {
+              if (dereferenced)
+                {
+                  go_error_at(location, "pointer type has no field %qs",
+                              Gogo::message_name(name).c_str());
+                  return Expression::make_error(location);
+                }
              go_assert(type->points_to() != NULL);
              expr = Expression::make_unary(OPERATOR_MULT, expr,
                                            location);