[BUG][C] C-client: referenced enum missing in model

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

When a simple enum is referenced in an object properties, it's missing in the model body (_create function) and header (model struct).

openapi-generator version

master branch on 2020-03-17T10:41:00Z.
Don't really know if it's a regression, I tried using the latest stable version too, but then I was mainly stuck with #5477 (this is why I'm using the master branch)

OpenAPI declaration file content or url

enum-bug.yaml

Command line used for generation

./run-in-docker.sh generate -i samples/client/enum-bug.yaml -g c -o /gen/out/enum-c

Steps to reproduce

1. Use the provided "enum-bug.yaml" specification, or any API with a referenced enum inside an object schema
2. Generate C-Libcurl client against the yaml
3. Try to compile it using CMake
4. Multiple errors

Here is the debugModels output:
debug_models.txt

Here is the model header file with error commented:
object_dummy.h

Here is the model body file with error commented:
object_dummy.c

To get the compiler errors, just try to compile the generated code, it will be much simpler.

Related issues/PRs

My last pull request #5604 that fixed a problem that was still present after #5477 was merged

Suggest a fix

A fix could be something like the following fix, as stated in the debugModels output, the referenced enum is considered as "^isModel" and "^isPrimitiveType", so we have to add a third case : an enum that is neither a model nor a primitive type.

Although maybe it should be considered as a model, and that this causes the bug, but I'm not enough familiar with those definitions to be sure.

diff --git a/modules/openapi-generator/src/main/resources/C-libcurl/model-header.mustache b/modules/openapi-generator/src/main/resources/C-libcurl/model-header.mustache
index 03a03276f6..5d492a95ac 100644
--- a/modules/openapi-generator/src/main/resources/C-libcurl/model-header.mustache
+++ b/modules/openapi-generator/src/main/resources/C-libcurl/model-header.mustache
@@ -91,6 +91,9 @@ typedef struct {{classname}}_t {
     {{#isFreeFormObject}}
     {{datatype}}_t *{{name}}; //object
     {{/isFreeFormObject}}
+    {{#isEnum}}
+    {{projectName}}_{{classVarName}}_{{enumName}}_e {{name}}; //enum
+    {{/isEnum}}
     {{/isPrimitiveType}}
     {{#isPrimitiveType}}
     {{#isNumeric}}
@@ -161,6 +164,9 @@ typedef struct {{classname}}_t {
     {{#isFreeFormObject}}
     {{datatype}}_t *{{name}}{{#hasMore}},{{/hasMore}}
     {{/isFreeFormObject}}
+    {{#isEnum}}
+    {{projectName}}_{{classVarName}}_{{enumName}}_e {{name}}{{#hasMore}},{{/hasMore}}
+    {{/isEnum}}
     {{/isPrimitiveType}}
     {{#isPrimitiveType}}
     {{#isNumeric}}

Start with enum-bug.yaml and reproduce generation using the provided run-in-docker.sh command, then inspect the generated object_dummy.h and object_dummy.c alongside debugModels output. Read model-header.mustache and the corresponding C-libcurl model template to trace how referenced enums are classified. Done means the generated client includes the referenced enum in the model and compiles successfully with CMake.