Compiler version

3.7.x

Minimized code

//> using scala 3.7.0

def fun(x: Int)(using p: Int, q: Int = 0): Int = 
  if x <= 0 then p * q
  else fun(x - 1)(using p = p + x)

@main def test = 
  println(fun(3)(using p = 0, q = 1))

Output

We would expect that the computation outputs (3 + 2 + 1) * 1 = 6, but
it's actually (3 + 2 + 1) * 0 = 0, because in the recursive call to fun, the default value p = 0 takes precedence over the enclosing p (which we set to 1 initially).

Expectation

That's extremely sneaky and feels wrong. I've seen instances of this interaction of defaults and implicits in the Scaladoc codebase (where implicit parameters keep getting added by people in AST traversals, in itself a problem for another time).

At least we should get a big fat warning that something surprising is going on,
but ideally, we change the behavior so that the default only applies if implicit resolution cannot find a binding in context.