Ensure decoded enum values are valid

lib/xdrgen/generators/go.rb

@@ -551,11 +551,9 @@ def render_union_decode_from_interface(out, union)
         name = name(union)
         out.puts "// DecodeFrom decodes this value using the Decoder."
         out.puts "func (u *#{name}) DecodeFrom(d *xdr.Decoder) (int, error) {"
-        out.puts "  disc, n, err := d.DecodeInt()"
-        out.puts "  if err != nil {"
-        out.puts "    return 0, err"
-        out.puts "  }"
-        out.puts "  u.#{name(union.discriminant)} = #{reference union.discriminant.type}(disc)"
+        out.puts "  var err error"
+        out.puts "  var n, nTmp int"
+        render_decode_from_body(out, "u.#{name(union.discriminant)}", union.discriminant.type, declared_variables: [], self_encode: false)
         switch_for(out, union, "u.#{name(union.discriminant)}") do |arm, kase|
           out2 = StringIO.new
           if arm.void?
@@ -580,16 +578,20 @@ def render_union_decode_from_interface(out, union)
       def render_enum_decode_from_interface(out, typedef)
         name = name(typedef)
         type = typedef
-        out.puts "// DecodeFrom decodes this value using the Decoder."
-        out.puts "func (e *#{name}) DecodeFrom(d *xdr.Decoder) (int, error) {"
-        out.puts "  var err error"
-        out.puts "  var n, nTmp int"
-        out.puts "  var i int32"
-        render_decode_from_body(out, "i", type, declared_variables: [], self_encode: true)
-        out.puts "  *e = #{name}(i)"
-        out.puts "  return n, nil"
-        out.puts "}"
-        out.break
+        out.puts <<-EOS.strip_heredoc
+        // DecodeFrom decodes this value using the Decoder.
+        func (e *#{name}) DecodeFrom(d *xdr.Decoder) (int, error) {
+          v, n, err := d.DecodeInt()
+          if err != nil {
+            return n, err
+          }
+          if _, ok := #{private_name type}Map[v]; !ok {
+            return n, fmt.Errorf("'%d' is not a valid #{name} enum value", v)
+          }
+          *e = #{name}(v)
+          return n, nil
+        }
+        EOS
       end
 
       def render_typedef_decode_from_interface(out, typedef)
@@ -655,7 +657,7 @@ def render_decode_from_body(out, var, type, declared_variables:, self_encode:)
         when AST::Typespecs::UnsignedInt
           out.puts "  #{var}, nTmp, err = d.DecodeUint()"
           out.puts tail
-        when AST::Typespecs::Int, AST::Definitions::Enum
+        when AST::Typespecs::Int
           out.puts "  #{var}, nTmp, err = d.DecodeInt()"
           out.puts tail
         when AST::Typespecs::String