It looks like OverloadResolution.GetParameterType doesn't perform complete check before digging through param array

Description

private static TypeSymbol GetParameterType(int argIndex, MemberAnalysisResult result, ImmutableArray<ParameterSymbol> parameters, out RefKind refKind)
        {
            int paramIndex = result.ParameterFromArgument(argIndex);
            ParameterSymbol parameter = parameters[paramIndex];
            refKind = parameter.RefKind;

            if (result.Kind == MemberResolutionKind.ApplicableInExpandedForm &&
                parameter.IsParams && parameter.Type.IsSZArray())
            {
                return ((ArrayTypeSymbol)parameter.Type).ElementType;
            }
            else
            {
                return parameter.Type;
            }
        }

Contributor guide

Tech stack

csharp

Domain

tooling

Issue type

bug

DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.

4

Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.

1-3 hours

Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.

stale

ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.

clear

Prerequisites

C#Roslynoverload resolution

Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.

30

Research direction

Examine the method OverloadResolution.GetParameterType in the Roslyn source code. Compare it with OverloadResolution.IsValidParams to understand the missing last parameter check. Determine if adding the condition `paramIndex == parameters.Length 1` is sufficient and ensure no other side effects. Look for existing tests related to params overload resolution and add a test if needed.