Allow defaults in function signatures with syntax (..., p = e, ...).

[m-kurtenacker]

This hijacks the implicit summoning to also allow for defaults to be specified. Type checking infers a type for the supplied expression first, and then checks that the parameter matches. As a consequence, the following variants are all valid:

• p : t = e
• p = e : t
• p = e, if the expression type can be inferred.

[m-kurtenacker]

There is a potential issue when the other parameters are bound to the default expression:

fn test2(l : i32) { l }

fn test(a : i32, b = test2(a)) { a + b }

#[export]
fn main() -> i32 {
    test(4)
}

This code causes the scoping to break, which can be observed by a number of free parameters being found during thorin's internal verification.

[m-kurtenacker]

I changed the way defaults are propagated somewhat. The following example should work:

fn test(a : i32, b : i32, c = 5 : i32) {
    a + b + c
}

#[export]
fn main() -> i32 {
    test(1, 2) + test(1, 2, 3)
}

artic/src/check.cpp

Lines 1190 to 1191 in ed0d2d8

	auto default_expr = default_param->default_expr.get();
	args_tuple->args.push_back(std::unique_ptr<Expr>(default_expr));

I don't know of any good ways to copy an ast::Node the way it would be required here. The way this is currently implemented generated multiple unique pointers to the same address, leading to undefined behavior.