Introduce Parameter Should Not Try to Introduce on a Parameter

[akhera99]

Fixes #58893

[ryzngard]

does this also inhibit things like args[0] and args + "My String"?

[Youssef1313]

Is the new behavior expected for this case?

public class C
{
    public void M(string s)
    {
        localFunction();

        void localFunction()
        {
            _ = s.ToString();
        }
    }
}

Or do we need to tighten the condition to make sure the parameter symbol corresponds to the most inner containing method?

[Joe4evr]

Wonder if it's that particular scenario that's the motivation for making the suggestion in the first place.

[Youssef1313]

@Joe4evr The original scenario looks different to me. See the added test.

[sharwell]

For this scenario, it should still be possible to explicitly pass the parameter instead of implicit capture. Whether that's part of the current refactoring or some other is of less importance to me.

[Youssef1313]

With this check, do we still need the below condition? (line 72 to 76)

            var parameterNode = expression.FirstAncestorOrSelf<SyntaxNode>(node => syntaxFacts.IsParameter(node));
            if (parameterNode is not null)
            {
                return;
            }

Edit: Answer is we need both. The checks have completely different purposes.

[CyrusNajmabadi]

i think the original fix was corrrect. i don't see why we'd want to special case local functions.

note: we should have extra tests here. for example if the user is on a local-func parameter, we shouldn't add that a second time either.

[Youssef1313]

IMO the correct fix is none of the two, but:

            if (expressionSymbol is IParameterSymbol parameterSymbol && parameterSymbol.ContainingSymbol.Equals(containingSymbol))
            {
                return;
            }

[CyrusNajmabadi]

i'm ok not adding one parameter to another container. but i don't feel strongly about that.

[Youssef1313]

@CyrusNajmabadi This scenario is important IMO if I want to move from:

public class C
{
    public void M(string s)
    {
        localFunction();

        void localFunction()
        {
            _ = s.ToString();
        }
    }
}

to:

public class C
{
    public void M(string s)
    {
        localFunction(s);

        localFunction("Another string");

        void localFunction(string s)
        {
            _ = s.ToString();
        }
    }
}

[akhera99]

@Youssef1313 I agree, I like that example.

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