extern(C) void function( ref const(Vector2) v0, ref const(Vector2) v1, ref const(Vector2) v2, ref const(Color) color = Color.white, BlendMode blendMode = BlendMode.Disabled ) fillTriangle2D;

If function pointers could be called with fewer than the prototypical number of arguments, and the remaining arguments be always initialized to their .init defaults, you could perhaps make this sort of thing work without the default argument values by using struct defaults.

How would that be deficient?

Color does not .init == Color.white. You're suggesting I define a new type, obscuring the API, every time I want a non-.init default arg?

Also, I think it's correct that functions shouldn't be callable without explicit parameters. Default args are carefully selected, and they are always opt-in.