[go-experimental] Fix nullable support (#5414)

* Fix nullable support in go-experimental client

* Fix support for models with parents and container fields

* Make sure that oneOf interfaces serialize properly even if they're required (non-pointers) on other models

* Spaces => tabs

* Regenerate samples

* Make some methods of nullables pointer-receivers, add tests

* Improve the Get/Set logic to make usage more convenient

* Address review

Author: Slavek Kabrda

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/GoClientExperimentalCodegen.java

@@ -151,6 +151,11 @@ public CodegenProperty fromProperty(String name, Schema p) {
 
     @Override
     public Map<String, Object> postProcessModels(Map<String, Object> objs) {
+        // The superclass determines the list of required golang imports. The actual list of imports
+        // depends on which types are used, some of which are changed in the code below (but then preserved
+        // and used through x-basetype in templates). So super.postProcessModels
+        // must be invoked at the beginning of this method.
+        objs = super.postProcessModels(objs);
 
         List<Map<String, Object>> models = (List<Map<String, Object>>) objs.get("models");
         for (Map<String, Object> m : models) {
@@ -162,6 +167,7 @@ public Map<String, Object> postProcessModels(Map<String, Object> objs) {
                 }
 
                 for (CodegenProperty param : model.vars) {
+                    param.vendorExtensions.put("x-basetype", param.dataType);
                     if (!param.isNullable || param.isMapContainer || param.isListContainer) {
                         continue;
                     }
@@ -178,11 +184,6 @@ public Map<String, Object> postProcessModels(Map<String, Object> objs) {
                 }
             }
         }
-
-        // The superclass determines the list of required golang imports. The actual list of imports
-        // depends on which types are used, which is done in the code above. So super.postProcessModels
-        // must be invoked at the end of this method.
-        objs = super.postProcessModels(objs);
         return objs;
     }