[Bug]: [GenerateAssertion] emits CallerArgumentExpression after params, producing CS0231

[JohnVerheij]

Description

[GenerateAssertion] emits invalid C# when the source method has a params array preceded by any other parameter. The generated extension method appends [CallerArgumentExpression(...)] diagnostic parameters AFTER the params array, which violates C#'s "params must be the last parameter" rule and produces compile error CS0231.

Expected Behavior

The generator produces an extension method that compiles cleanly, with each [CallerArgumentExpression] parameter placed before the params array.

Actual Behavior

The generated extension method puts [CallerArgumentExpression] parameters after the params array. The C# compiler rejects the file with error CS0231: A params parameter must be the last parameter in a parameter list.

Steps to Reproduce

1. Create a static partial class with a [GenerateAssertion] source method that has at least one non-params parameter followed by params:

using TUnit.Assertions.Attributes;

public static partial class Demo
{
    [GenerateAssertion]
    public static bool ContainsAny(this string value, string label, params string[] candidates)
        => candidates.Length > 0;
}

2. Build the containing project.

3. Observe the build fails with CS0231 in the generated file Demo.GeneratedAssertions.g.cs.

The generator emits:

public static String_ContainsAny_String_StringArray_Assertion ContainsAny(
    this IAssertionSource<string> source,
    string label,
    params string[] candidates,
    [CallerArgumentExpression(nameof(label))] string? labelExpression = null)

The params string[] candidates parameter is followed by another parameter, which is illegal.

TUnit Version

1.44.39 (also confirmed against current main at 420248e)

.NET Version

Reproduces on all targets supported by TUnit.Assertions (net472 / net8.0 / net9.0 / net10.0)

Operating System

Windows

IDE / Test Runner

dotnet CLI (dotnet test / dotnet run)

Error Output / Stack Trace

error CS0231: A params parameter must be the last parameter in a parameter list

Additional Context

Why production [GenerateAssertion] methods do not currently trip this: GenericAssertions.IsIn / IsNotIn are the only [GenerateAssertion] methods in TUnit.Assertions that use params, and both declare ONLY the params parameter (no preceding non-params source parameter). The generator's existing if (!param.IsParams) skip prevents emitting a CallerArgumentExpression for the params parameter itself, so the single-parameter shape stays legal C#. CS0231 fires only when at least one non-params source parameter sits before a params source parameter.

Workaround: drop the params modifier and accept a regular array; consumers then need C# 12 collection-expression syntax (["a", "b"]) at the call site instead of positional "a", "b".

Fix direction: reorder the generator emit so the [CallerArgumentExpression] parameters for non-params source parameters precede the params parameter:

public static String_ContainsAny_String_StringArray_Assertion ContainsAny(
    this IAssertionSource<string> source,
    string label,
    [CallerArgumentExpression(nameof(label))] string? labelExpression = null,
    params string[] candidates)

C# allows optional-with-default parameters to precede params, so the reorder is semantically transparent at every call site.

A PR with this fix and a regression-test fixture is ready to go alongside this issue.

IDE-Specific Issue?

• I've confirmed this issue occurs when running via dotnet test or dotnet run, not just in my IDE