[c#] laxer extension method matching (#5253)

joern-cli/frontends/csharpsrc2cpg/src/main/scala/io/joern/csharpsrc2cpg/datastructures/CSharpScope.scala

@@ -140,7 +140,10 @@ class CSharpScope(summary: CSharpProgramSummary)
     name: String,
     argTypes: List[String]
   ): CSharpMethod => Boolean = { m =>
-    m.isStatic && m.name == name && m.parameterTypes.map(_._2) == thisType :: argTypes
+    // TODO: we should also compare argTypes, however we first need to account for:
+    //  a) default valued parameters in CSharpMethod, to account for different arities
+    //  b) compatible/sub types, i.e. System.String should unify with System.Object.
+    m.isStatic && m.name == name && m.parameterTypes.map(_._2).headOption.contains(thisType)
   }
 
   /** Tries to find an extension method for [[baseTypeFullName]] with the given [[callName]] and [[argTypes]] in the

joern-cli/frontends/csharpsrc2cpg/src/test/scala/io/joern/csharpsrc2cpg/querying/ast/ExtensionMethodTests.scala

@@ -195,6 +195,22 @@ class ExtensionMethodTests extends CSharpCode2CpgFixture {
 
       cpg.call.nameExact("DoStuff").callee.l shouldBe cpg.literal("2").method.l
     }
+
+    "resolve correctly if the extra argument is type-compatible with the extension method's extra parameter" in {
+      val cpg = code("""
+          |using System.Collections.Generic;
+          |
+          |var x = new List<string>();
+          |x.DoStuff(null);
+          |
+          |static class Extensions
+          |{
+          |    public static int DoStuff(this List<string> myList, object x) { return 2; }
+          |}
+          |""".stripMargin)
+
+      cpg.call.nameExact("DoStuff").callee.l shouldBe cpg.literal("2").method.l
+    }
   }
 
   "consecutive unary extension method calls" should {