Allow typeof on arbitrary expression

[mwisnicki]

At the moment compiler insists on having valid identifier on right side of typeof so following code is not valid:

var x : typeof (1+ 2);
var y : typeof this;

even though it's possible to assign the expression to variable and then capture the type form that:

var x_ = 1+2;
var x = typeof x;
var y_ = this;
var y = typeof y_;

I propose to allow capturing type of any valid expression. The expression itself wouldn't be evaluated.

[mhegazy]

How is that different from:

var x = typeof (1+2);
var y = typeof this;

[RyanCavanaugh]

@mhegazy we're talking about type-position typeof, not value-position.

typeof this seems useful, but more allowing more complex expressions is trickier. It's not at all apparent to a casual observer that typeof f(x) in type positions only won't evaluate the function call; the restriction to dotted identifiers is a bit of a safeguard in this respect.

Other than typeof this, what kind of uses cases do you want this for?

[mhegazy]

duplicate of #6606