[Issue 19299] string mixins and template mixins should be allowed in function parameter list

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19299

--- Comment #2 from Nick Treleaven <nick at geany.org> ---
Actually std.traits.Parameters does not work, it is just the parameter types.

The pragma above prints:

void(char c = '\0', byte b, int extra)

(Even though the default argument can't be followed by non-default args!).

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