C# 12: Support for lambda param parameter and parameter defaults.

[michaelnebel]

It turns out that the extractor "almost" works out of the box for params and default parameters for lambda expressions.
The implementation of "ordinary" lambdas are based on System.Func whereas the implementation of lambdas with parameter defaults or the use of params parameters are based on compiler generated delegates (as these features are already support by delegates).

Below is an example of some C# code that is compiled and de-compiled to illustrate the difference.

class LambdaArgumentsTest
{
    void M1()
    {
        var l1 = (int x) => x + 1;
        var l2 = (int x, int y = 1) => x + y;
    }
}

Compiling and de-compiling yields

[CompilerGenerated]
internal delegate TResult <>f__AnonymousDelegate0<T1, T2, TResult>(T1 arg1, T2 arg2 = 1);
internal class LambdaArgumentsTest
{
    [Serializable]
    [CompilerGenerated]
    private sealed class <>c
    {
        public static readonly <>c <>9 = new <>c();

        public static Func<int, int> <>9__0_0;

        public static <>f__AnonymousDelegate0<int, int, int> <>9__0_1;

        internal int <M1>b__0_0(int x)
        {
            return x + 1;
        }

        internal int <M1>b__0_1(int x, int y = 1)
        {
            return x + y;
        }
    }

    private void M1()
    {
        Func<int, int> func = <>c.<>9__0_0 ?? (<>c.<>9__0_0 = new Func<int, int>(<>c.<>9.<M1>b__0_0));
        <>f__AnonymousDelegate0<int, int, int> <>f__AnonymousDelegate = <>c.<>9__0_1 ?? (<>c.<>9__0_1 = new <>f__AnonymousDelegate0<int, int, int>(<>c.<>9.<M1>b__0_1));
    }
}

In this PR

• Add extractor support for creating methods from compiler generated delegates. The compiler generated code isn't identified as source code by the extractor and thus it won't be extracted correctly. A method IsCompilerGeneratedDelegate has been introduced to catch "exactly" these use-cases. As the Invoke method doesn't have any location, we use the containing types location (which appears to re-use the source location of the lambda even though itself is compiler generated).
• Add Code QL library support for getting the arguments provided by params for delegate like types (these are not considered static call targets and thus the implementation for getRuntimeArgumentFromParameter has been updated to handle this case).
• Add test for lambda arguments.
• Add test for lambda default parameters.

[tamasvajk]

LGTM, added some questions.

[tamasvajk]

Does this predicate consider named arguments? (This question is not related to delegates.) It might be worth doing this change in a separate PR.

[michaelnebel]

It does not. I also noted that difference, but didn't want to make any changes related to that as it is not possible to use named arguments for lambdas.

[tamasvajk]

Is the Symbol itself IsImplicitlyDeclared for the cases that we're interested in? There's probably no point in checking the ContainingType.TypeKind, we already know that Symbol is a DelegateInvoke, so its container can't be anything else then a delegate, can it?

[michaelnebel]

The Symbol itself is implicitly declared, but the same also holds for the Invoke method call symbol "ordinary" lambdas. I tried to be as narrow and conservative as possible in terms of detecting when we deal with a compiler generated delegate type.
Good point about the typekind check. I will fix that!

[hvitved]

LGTM. Did you run DCA already?