Add regression test: #13519, C# optional parameters from F# (#19473)

* Add regression test for #13519: C# optional parameters from F#

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Add ParamArray + optional params regression test for #13519

Addresses reviewer feedback: the original issue was specifically about
the intersection of omitted optional arguments and ParamArray arguments,
as identified by Don Syme. This adds a test covering that exact scenario.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: T-Gro

tests/FSharp.Compiler.ComponentTests/Interop/SimpleInteropTests.fs

@@ -114,3 +114,112 @@ Holder.Label <- "nope"
         fsLib
         |> compile
         |> shouldFail
+
+    // https://github.com/dotnet/fsharp/issues/13519
+    [<FSharp.Test.FactForNETCOREAPP>]
+    let ``Issue 13519 - C# method with optional parameters can be called from F# without specifying defaults`` () =
+        let csLib =
+            CSharp
+                """
+using System;
+
+namespace CSharpLib
+{
+    public class MyClass
+    {
+        public string DoSomething(string required, string optional1 = "default1", string optional2 = "default2")
+        {
+            return required + "|" + optional1 + "|" + optional2;
+        }
+    }
+}
+            """
+            |> withName "CSharpOptionalParams"
+
+        let fsApp =
+            FSharp
+                """
+module TestApp
+
+open CSharpLib
+
+[<EntryPoint>]
+let main _ =
+    let c = MyClass()
+    let result = c.DoSomething("hello")
+    if result <> "hello|default1|default2" then
+        failwithf "Expected 'hello|default1|default2' but got '%s'" result
+    let result2 = c.DoSomething("hello", "custom1")
+    if result2 <> "hello|custom1|default2" then
+        failwithf "Expected 'hello|custom1|default2' but got '%s'" result2
+    0
+                """
+            |> withReferences [ csLib ]
+
+        fsApp
+        |> asExe
+        |> compileExeAndRun
+        |> shouldSucceed
+
+    // https://github.com/dotnet/fsharp/issues/13519
+    // Specifically testing the intersection of optional parameters and ParamArray
+    // as identified by Don Syme: https://github.com/dotnet/fsharp/issues/13519#issuecomment-1253808416
+    [<FSharp.Test.FactForNETCOREAPP>]
+    let ``Issue 13519 - C# method with optional parameters and ParamArray can be called from F# without specifying defaults`` () =
+        let csLib =
+            CSharp
+                """
+using System;
+
+namespace CSharpParamArrayLib
+{
+    public class Assertions
+    {
+        public string BeEquivalentTo(string expected, string because = "", params object[] becauseArgs)
+        {
+            string formatted = because;
+            if (becauseArgs != null && becauseArgs.Length > 0)
+            {
+                formatted = string.Format(because, becauseArgs);
+            }
+            return expected + "|" + formatted + "|" + becauseArgs.Length;
+        }
+    }
+}
+                """
+            |> withName "CSharpParamArrayLib"
+
+        let fsApp =
+            FSharp
+                """
+module TestParamArrayApp
+
+open CSharpParamArrayLib
+
+[<EntryPoint>]
+let main _ =
+    let a = Assertions()
+
+    // Call with only the required argument - omitting both optional and params
+    let result1 = a.BeEquivalentTo("test")
+    if result1 <> "test||0" then
+        failwithf "Test 1 failed: Expected 'test||0' but got '%s'" result1
+
+    // Call with required + optional, omitting params
+    let result2 = a.BeEquivalentTo("test", "because {0}")
+    if result2 <> "test|because {0}|0" then
+        failwithf "Test 2 failed: Expected 'test|because {0}|0' but got '%s'" result2
+
+    // Call with all arguments including params values
+    let result3 = a.BeEquivalentTo("test", "because {0}", "reason")
+    if result3 <> "test|because reason|1" then
+        failwithf "Test 3 failed: Expected 'test|because reason|1' but got '%s'" result3
+
+    0
+                """
+            |> withReferences [ csLib ]
+
+        fsApp
+        |> asExe
+        |> compileExeAndRun
+        |> shouldSucceed