Rest Client @RestForm does not serialize enums like other parameter types

[krnhotwings]

Describe the bug

If you create a custom enum, you can implement fromString to deserialize a string to its appropriate enum object. Conversely, you can override toString to serialize the enum to a custom string rather than the enum's name.

When implementing resource endpoints in RestEASY, deserialization is respected for @RestQuery and @RestForm where input strings are converted to appropriate enums. Serialization from enum to string is also respected when outputting an enum as plain text, in json, etc.

When implementing Rest Client endpoints that take an enum parameter, the underlying HTTP requests should contain the enum's toString serialization for query params, form params, etc.

Serialization is respected for Rest Client query params via @RestQuery.

However, serialization is not respected for Rest Client form params via @RestForm and exhibits behavior inconsistent to the other prior examples. Form params will use the enum's name rather than toString output.

Perhaps the code path that Client form params follow may be calling Enum#name() rather than toString?

Expected behavior

In form bodies, enums should be serialized by toString. For example, if you had an enum:

public enum MyEnum {
    BLAH("blah");

    private final String value;

    MyEnum(final String value) { ... }

    @Override
    public String toString() { 
        return value;
    }
}

A rest client endpoint that takes @RestForm MyEnum myEnum should pass the following to the HTTP request's form body:

myEnum=blah

Actual behavior

Form bodies will end up with the name representation of the enum:

myEnum=BLAH

How to Reproduce?

Demo repo is here:

https://github.com/krnhotwings/quarkus-rest-client-enum

Reproduction steps are in the README:

1. Start server with quarkus dev
2. Run curl commands

Expected output

curl -X GET "http://localhost:8080/resteasy?myEnum=test"
curl -X POST "http://localhost:8080/resteasy" -d "myEnum=test"
curl -X GET "http://localhost:8080/restclient?myEnum=test"

Note in the log output that the 3rd curl will make an HTTP request to httpbin with the lowercase enum serialization:

Request: GET https://httpbin.org/get?myEnum=test Headers[User-Agent=Quarkus REST Client], Empty body

Unexpected output

curl -X POST "http://localhost:8080/restclient" -d "myEnum=test"

However, form params will be uppercased:

Request: POST https://httpbin.org/post Headers[Content-Type=application/x-www-form-urlencoded User-Agent=Quarkus REST Client content-length=11], Body:
myEnum=TEST

Output of uname -a or ver

Linux 6.8.6-200.fc39.x86_64

Output of java -version

openjdk version "21.0.2" 2024-01-16

Quarkus version or git rev

3.9.4

Build tool (ie. output of mvnw --version or gradlew --version)

Gradle 8.6

Additional information

No response

[quarkus-bot]

/cc @cescoffier (rest-client), @geoand (rest-client)

[krnhotwings]

In light of this comment, I updated the demo repo and added a test case to cover @RestForm enums inside of @BeanParam:

curl -X POST "http://localhost:8080/restclient/bean" -d "myEnum=test"

which outputs the same uppercase enum name:

Request: POST https://httpbin.org/post Headers[Content-Type=application/x-www-form-urlencoded User-Agent=Quarkus REST Client content-length=11], Body:
myEnum=TEST