Optional pattern behaves differently in version 8, 9, and 10+

Question

We recently ran into an issue when testing Rubi on older versions. It boils down to this simple example which I didn't dare to simplify further as I cannot test it in versions < 10

Test[XXX[expr_, x_], n_Integer : Infinity, OptionsPattern[]] := True /;
  Head[x] === Symbol && n > 0;
Test[___] := False;
XXX[blub, x] // Test

The important part is n in the pattern and it should say

• n is an integer
• n is optional an Infinity per default
• n must be greater zero

Above code gives False in versions 8 and 9, but True in version 10 and 11. I'm not looking for a workaround, but I'd like to understand why it returns False in the older versions.

Maybe someone who still has access can give me some insight.

Answer 1

In M9 and earlier, the default value must match any pattern restrictions. In your example, the head of the default is not Integer, and so the default value could never be triggered. In either 10.2 or 10.3 the pattern matcher was relaxed so that the default value no longer needed to match pattern restrictions.