Target of predicate type is not checked

[RyanCavanaugh]

This should be an error (duplicate declarations of a)

function addProp2(x: any): x is { a: string; a: string; } {
	return true;
}

Need to call checkType (or similar) on the operand here

[weswigham]

I've opened #17664 with this repro to add it to our regression suite; but this is already fixed in master.