compiler: reject incorrect unsafe.Offsetof expressions.

The x.Field argument to Offsetof may not involve hidden
dereferences of embedded pointer fields.

Also correct uninitialized implicit_ field.

From-SVN: r200221

gcc/go/gofrontend/expressions.cc

@@ -6955,6 +6955,26 @@ Builtin_call_expression::do_lower(Gogo* gogo, Named_object* function,
       return Expression::make_error(loc);
     }
 
+  if (this->code_ == BUILTIN_OFFSETOF)
+    {
+      Expression* arg = this->one_arg();
+      Field_reference_expression* farg = arg->field_reference_expression();
+      while (farg != NULL)
+	{
+	  if (!farg->implicit())
+	    break;
+	  // When the selector refers to an embedded field,
+	  // it must not be reached through pointer indirections.
+	  if (farg->expr()->deref() != farg->expr())
+	    {
+	      this->report_error(_("argument of Offsetof implies indirection of an embedded field"));
+	      return this;
+	    }
+	  // Go up until we reach the original base.
+	  farg = farg->expr()->field_reference_expression();
+	}
+    }
+ 
   if (this->is_constant())
     {
       Numeric_constant nc;

gcc/go/gofrontend/expressions.h

@@ -1872,7 +1872,7 @@ class Field_reference_expression : public Expression
   Field_reference_expression(Expression* expr, unsigned int field_index,
 			     Location location)
     : Expression(EXPRESSION_FIELD_REFERENCE, location),
-      expr_(expr), field_index_(field_index), called_fieldtrack_(false)
+      expr_(expr), field_index_(field_index), implicit_(false), called_fieldtrack_(false)
   { }
 
   // Return the struct expression.