Promote jump statements syntactically to expressions, similar to how throw expressions work.

[Kile-Asmussen]

This is, in my experience a commonly-used pattern:

var foo = bar.Baz() ?? throw new Exception("message"):

throw is a jump statement, and therefore its use as an expression is well-founded: it will never reach the end-point, and therefore whatever value evaluations may come after, are irrelevant.

My proposal is that we allow the same of at least return.

It would maybe be beneficial to allow it of yield break as well, due to similar usecase for early termination. The utility of allowing it for yield return is quiestionable, as is allowing it for break and continue. The less said about goto the better.

The following is another extremely common pattern:

if (arg is null)
  return default;

or the more terse version that explicitly matches arg into a non-null val to appease the type checker

if (arg is not {} val)
  return default;

which could then be abbreviated to

var val = arg ?? return default;

and even things like

arg ??= return default;

would be possible.

[333fred]

Duplicate of #867.