compiler: add COMPARE_ALIASES flag for type compare and hash

    
    Normally aliases compare as identical to the underlying type.  Add a
    COMPARE_ALIASES flag to let them compare (and hash) differently.  This
    will be used by later patches in this series.
    
    Reviewed-on: https://go-review.googlesource.com/c/143021

From-SVN: r265297

gcc/go/gofrontend/MERGE

-9c985ce6f76dd65b8eb0e4b03c09ad0100712e04
+6f4bce815786ff3803741355f7f280e4e2c89668
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/types.cc

@@ -349,9 +349,16 @@ Type::are_identical(const Type* t1, const Type* t2, int flags,
       return (flags & COMPARE_ERRORS) == 0 ? true : t1 == t2;
     }
 
-  // Skip defined forward declarations.  Ignore aliases.
-  t1 = t1->unalias();
-  t2 = t2->unalias();
+  // Skip defined forward declarations.
+  t1 = t1->forwarded();
+  t2 = t2->forwarded();
+
+  if ((flags & COMPARE_ALIASES) == 0)
+    {
+      // Ignore aliases.
+      t1 = t1->unalias();
+      t2 = t2->unalias();
+    }
 
   if (t1 == t2)
     return true;
@@ -923,12 +930,17 @@ Type::copy_expressions()
 unsigned int
 Type::hash_for_method(Gogo* gogo, int flags) const
 {
-  if (this->named_type() != NULL && this->named_type()->is_alias())
-    return this->named_type()->real_type()->hash_for_method(gogo, flags);
-  unsigned int ret = 0;
-  if (this->classification_ != TYPE_FORWARD)
-    ret += this->classification_;
-  return ret + this->do_hash_for_method(gogo, flags);
+  const Type* t = this->forwarded();
+  if (t->named_type() != NULL && t->named_type()->is_alias())
+    {
+      unsigned int r =
+	t->named_type()->real_type()->hash_for_method(gogo, flags);
+      if ((flags & Type::COMPARE_ALIASES) != 0)
+	r += TYPE_FORWARD;
+      return r;
+    }
+  unsigned int ret = t->classification_;
+  return ret + t->do_hash_for_method(gogo, flags);
 }
 
 // Default implementation of do_hash_for_method.  This is appropriate

gcc/go/gofrontend/types.h

@@ -574,6 +574,9 @@ class Type
   // struct field tags for purposes of type conversion.
   static const int COMPARE_TAGS = 2;
 
+  // Compare aliases: treat an alias to T as distinct from T.
+  static const int COMPARE_ALIASES = 4;
+
   // Return true if two types are identical.  If this returns false,
   // and REASON is not NULL, it may set *REASON.
   static bool