Method overloading fails with implicit typecasting and optional parameters

Hi,

I posted an "opening" to this question earlier and thanks to tswalk I realized one very huge weirdness, related to Unity and method overloading.

I encountered an issue while creating method overloads. This compiles without problems (EXAMPLE A):

 void Test(Color v, bool b)
 {
   Debug.Log("color");
 }
 void Test(Vector4 v, bool b)
 {
   Debug.Log("vector4");
 }
 
 void Start()
 {
   Test(Color.red, true);
 }

While this generates an ambiguous call error (EXAMPLE B) (same as above, but the Test methods have optional parameters):

 void Test(Color v, bool b = true)
 {
   Debug.Log("color");
 }
 void Test(Vector4 v, bool b = true)
 {
   Debug.Log("vector4");
 }
 
 void Start()
 {
   Test(Color.red);
 }

This is apparently due to the fact that Vector4 and Color can be implicitly cast to each other (and are part of the UnityEngine DLLs), and the compiler, in case there are added optional parameters, goes crazy. The issue is purely Unity-related (a Unity Assembly generated in Visual Studio doesn't generate any ambiguity, even if it contains calls to methods with implicitly castable types and optional parameters).

In short, the overloads and relative call in EXAMPLE B should totally work, while instead they don't.

Can anyone share some light on this? Or it's just a known bug?

EDIT

Just out of curiosity, tswalk notes that calling Test and including the optional parameter like this:

 Test(Color.red, true);

works both with EXAMPLE A and B