Add nullability support for server parameter delegation

[nomisRev]

Subsystem
Server, core

Motivation
Currently it is not possible to use nullable values with parameter delegation.

get("/nullable") {
    val nameOrNull: String? by call.queryParameters
    val ageOrNull: Int? by parameters
}

Solution

This PR add support by removing the : Any constraint and relying on typeInfo.kotlinType?.isMarkedNullable to check if null is allowed or not.

[nomisRev]

I'm a bit unsure how reliable typeInfo.kotlinType?.isMarkedNullable is across platforms?

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: d828920f-49da-4f84-97ed-67412516833f

📥 Commits

Reviewing files that changed from the base of the PR and between 5710ef9 and 86d7c3f.

📒 Files selected for processing (3)

• ktor-server/ktor-server-core/api/ktor-server-core.klib.api
• ktor-server/ktor-server-core/common/src/io/ktor/server/util/Parameters.kt
• ktor-server/ktor-server-core/common/test/io/ktor/server/http/ParametersTest.kt

---

📝 Walkthrough

Walkthrough

The changes generalize the Parameters.getValue operator and internal getOrFailImpl helper to support nullable type parameters, enabling delegated properties to resolve to null instead of throwing exceptions when the parameter is absent and the target type is nullable.

Changes

Cohort / File(s)	Summary
API Signatures ktor-server/ktor-server-core/api/ktor-server-core.klib.api	Updated type parameter bounds for getOrFailImpl and getValue from A: Any to A: Any?, removing the non-nullability constraint to support nullable delegated properties.
Core Implementation ktor-server/ktor-server-core/common/src/io/ktor/server/util/Parameters.kt	Modified getValue operator to remove the Any bound on generic R and added logic to detect nullable types via typeInfo<R>(), returning null for absent parameters when R is nullable. Updated internal getOrFailImpl helper to match by removing the Any constraint. Updated KDoc to reflect nullable return behavior.
Test Coverage ktor-server/ktor-server-core/common/test/io/ktor/server/http/ParametersTest.kt	Added three new test cases verifying nullable delegated property resolution (Int?, String? returning value or null), nullable type conversion exceptions, and missing non-null delegated value exceptions.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 12.50% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically describes the main change: adding nullability support for parameter delegation in the Ktor server.
Description check	✅ Passed	The description follows the template with all required sections (Subsystem, Motivation, Solution) properly filled with clear, concrete information about the problem and implementation.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch nomisrev/nullable-parameter-delegation

📝 Coding Plan

• Generate coding plan for human review comments

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[bjhham]

Looks like an easy win 👍

I'm pretty sure isMarkedNullable works cross-platform on reified KTypes.

I don't know why we didn't just implement it that way in the first place...

[bjhham]

FYI logged an issue to handle property delegation in the compiler plugin:
KTOR-9422 OpenAPI code inference misses property delegation

[osipxd]

Alternatively we can use null is R if we want to avoid creating TypeInfo instance

[osipxd]

This would probably allow us to keep getOrFailImpl signature untouched because of smart cast. But I'm not sure

[nomisRev]

Yes, I forgot about null is R in reified generics. Seems like a much better solution here.

[osipxd]

Let's move this above [Report a problem] link