Codegen broken since v0.12 using external map scalar types

[fredbi]

What happened?

• I have a schema with imported scalar types.
• works fine with gqlgen =< v0.11.3
• breaks start with v0.12:

codegen: generating core failed: type.gotpl: template: type.gotpl:49:28: executing "type.gotpl" at <$type.Elem.GO>: nil pointer evaluating *config.TypeReference.GO

The faulty template code in codegen/type.gotpl is:

@@ -46,7 +46,21 @@
                                        return v.(map[string]interface{}), nil
                                {{- else if $type.IsMarshaler }}
                                        {{- if $type.IsNilable }}
-                                               var res = new({{ $type.Elem.GO | ref }})

Investigating further, the template fails when looping over my scalar types.

In my case, scalar types are maps, e.g. map[string]interface{}, map[string]string, map[string]float64.
This hits the case of a nullable type with no (known) element type (there is one, but it is not introspected, as only slices are introspected for the type of their element).

What did you expect?

Successful codegen

Minimal graphql.schema and models to reproduce

(TODO)

versions

• gqlgen version? v0.12.x, v0.13.0, current master
• go version? 1.15.3
• dep or go modules? go modules

[fredbi]

Maybe a duplicate (or at least a special case) of #1293

[frederikhors]

@fredbi please tell me if this is still an issue and I'll re-open it.